Hewlett Packard 8640B
Signal Generator
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This is what the seller
termed a "barn find" and the description isn't bad.
I've had a few pieces of equipment that have suffered from being
in a damp garage for donkeys' years and this is pretty typical.
The main problem, at least exterior-wise is aluminium corrosion
and the worst aspect of this is damage to lettering on dials.
The front panel isn't too bad as it's mainly pin-hole corrosion
that should clean off.
Another problem though, that
can be serious, is rust affecting things like grub screws tapped
into brass and expansion/contraction of fittings using plastic/metal
combinations especially if plastic goes brittle and disintegrates.
After a lot of trouble I finally
removed the knobs using 0.05 inch hex grub screws, but the larger
screws are not yet removed. Only one knob looks really bad and
may need a printed paper scale replacement. I find that it's
only after investment has been made that one discovers life-threatening
problems.. whether it be test equipment or cars! Still at £45
the gamble here is not too risky.
Below, the 8640B with the top
cover detached and the upper panel removed and the lower panel
partially cleaned up. |
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The seller said he'd plugged
it in and nothing had happened, but before I tried to power it
up myself I thought I'd check the mains setting. Being made in
the USA it will have a 120/240 volt setting. This is to be found
hidden away, to be seen only in good lighting or using a torch.
Adjacent to the IEC connector you'll see a fuse and a small lever
for removing it. Remove the fuse and you'll see a tiny printed
circuit board and on this in gold-plating are numbers, which
in my case included "240". Apparently you can pull
out the board using a special tool and turn it over to be then
equipped for 120 volt operation.
I plugged in a kettle lead and
pushed the on/off button. I should mention that I'd already checked
and seen about 20 ohms across the live/neutral IEC connector
pins so was fairly confident of something interesting happening.
There was a sort of coughing noise and the fan came on... it
then intermittently went off for a fraction of a second before
coming back on so something is not quite right. The front-panel
display lit with various similar red characters but flashing
at a regular rate... usually with other things this means that
there's a (not too serious) problem preventing normal operation.
This might be an under or over voltage somewhere in the power
supply, but until I've studied the user manual I can't yet tell.
After gazing at the display
for about a minute hoping it would suddenly say 500MHz there
were cracking noises like a firework and a little flame appeared
at the rear of the chassis, followed by volumes of smoke. Fortunately,
this is not an unknown occurrence here, so I just waggled the
mains lead free and what might have been panic.. went away. The
culprit was a mains suppression condenser. One of those chubby
semi-transparent gold coloured things.
Exactly the same thing happened
when I plugged in my BBC B computer after a score of years being
hidden away (and on lots of circuit boards for lifts.. my day
job is repairing such things).
I have several 8640B documents
downloaded from the Net and in one I (eventually) found the mains
input circuit diagram revealing C6 as the failed condenser. This
is wired across the rear of a filter and before the mains switch
so the designers must have put their money on the mains fuse
to prevent a conflagration? |
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Why these things fail,
I don't know, but they do fairly regularly. Maybe it's something
to do with their rating as this example says "250VAC"
on the side.
As most electrical engineers
know, the UK does not have the widely published 230v mains but
240v mains which can rise a lot higher than 250 volts. Of course
it's the tolerances that matter not the actual number.
I just cut the legs and removed
it for now.
Below, the initial bad components. |
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This board (covered by Service
Sheet A22) whose circuit diagram is shown above as "A12
rectifier assembly 08640-60003", is located between the
mains transformer and the set of large smoothing capacitors and
carries five bridge rectifiers each using four identical Motorola
diodes coded 14-18 with 2 legs, in cans not unlike TO5 (cathode
to can). Three were open circuit (unusual for a diode as these
generally fail short-circuit) and two measured with Vf about
5 to 10% higher than the remainder of the good diodes using a
diode tester. My guess is that these two had been stressed by
failure of the other diodes and likely to be unreliable.
The diodes are 1901-0418 (Motorola SR1846-12)
probably rated at 1200 volts (unnecessarily) and mechanically
equivalent to the 1N4999 rated at 3A and 200 volts. The HP documentation
mentions both 1.5A and 3A so take your pick but the higher current
rating is better.
Modern replacements could be 1N5402,
BY255 or BYW95.
The last being in a SOD64 package which
can fit better as the board sits very close to the smoothing
capacitors. However the hole sizes for the diodes vary with CR9/CR11
being a lot smaller than typical 1.3mm 3A diode legs with SOD64
having 1.35mm legs. |
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I decided to use a set of 1N5408
because I have several of these. The anode pin sits centrally
in the original diode so the cathode of the new diodes is bent
over as close as possible to the body to minimise height. Before
bending the cathode legs I filed these down to fit the mounting
holes. The two new diodes on the left however are mounted differently
because the mounting hole was only around 0.75mm diameter. The
cathodes are soldered directly to the gold plated areas. In one
case I added a through wire to carry current rather than relying
on the hole plating.
The board is tricky to fit back in place
because of the limited access but having done this I powered
the thing up. This time the fan didn't come on, which it had
done previously (although intermittently) and since switching
the counter mode lock to OFF the display came on without flashing,
however it read all zeroes rather than displaying a frequency.
Pressing a few buttons (those that are no longer seized) caused
lamps to come on. Pressing the lock button caused the display
to flash so it seems the equipment is at least alive. Some switches
are still seized including the RF ON/OFF and the leftmost modulation
switch which has a broken lever (and suggests a serious fault).
Why does the display show all zeroes?
Several possibilities including the obvious one that the oscillator
isn't working or that the various frequency dividers are faulty.
There are other reasons, more likely.. a power supply problem
or a wrong switch setting. This equipment is actually two instruments
in one.. a signal generator AND a frequency counter (my Marconi
TF2002 for example, because the tuning arrangement is just a
linear scale aided by a crystal reference requires an external
frequency counter before one can see exactly what the output
is) so obviously all zeroes may indicate the counter circuitry
isn't working. There are some basic tests to enable one to work
out the reason for the problem, but initially I'll just check
the power supply voltages, in particular the 5 volt rails. After
the power had been applied I had noticed one of the set of red
LEDs was dimmer than the others... so armed with a voltmeter
I again plugged in the mains lead.
I was just about to press the on/off
switch when there was a flash and a bang and smoke so I quickly
removed the mains cable. The 1.25A fuse was very black. That
meant a catastrophic fault. Looking at the circuit diagram for
the second time I realised the problem must be in the mains filter
area because this is before the on/off switch (see below). The mains filter is integral with the IEC connector
assembly which is a fairly small, sealed box fitted into a vertical
slot in the rear panel. |
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Access to the filter is
awkward. It's secured by a pair of spring steel plates that grip
the inside of the rear panel, and to remove the assembly you
have to press down first on the front edges of the lower plate
whilst levering the lower edge of the mains connector. Ideally
the power supply motherboard should be detached but, by using
a couple of thin screwdrivers through convenient apertures, the
lower edge can be freed. Once this has unclipped the same method
is used to release the easier top clip and the assembly pulled
back until it's free from the rear panel. A really tight fit.
The cable prevents the filter
assembly from being pulled out completely, but sufficient to
get a drill in position to remove the four pop-rivets holding
the external metal shield.
Once the top of the shield has
been bent back you can see a ferrite filter coil wrapped in yellow
tape. Unsure of the actual failure I unwrapped the coil but found
it was OK. In the front edge of the filter are two long orange/brown
coloured feed-through capacitors and tracking could be seen near
one of these. I pulled off the wires (which go to the on/off
switch) and broke away the capacitors. One was clean but the
second was blackened.
My first thought was to replace
the whole assembly with a small metal plate holding an IEC connector,
but repair was better because the filter assembly also includes
mains voltage selection (with the tiny PC board) and all the
transformer primary wiring (below left). I found a couple of
ceramic feed-throughs which I soldered in place of the originals
and added a pair of 400VAC capacitors as filters. I could only
use one self tapping screw in place of the old pop-rivets because
of the tight fit of the filter so I used solder to hold the screen
together. The cut red wires went to C6.
Below, connections to the rear
of the IEC connector. |
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Above.. the repaired filter
assembly being forced back in place. It's an interference fit.
A new 1A fuse. |
Above... The filter back
in place with the mains switch wires soldered to the live sides
of the capacitors. |
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The next task was to continue
testing as I'd intended immediately before the mains filter failed.
Before plugging in and switching on, I removed the series pass
power transistor fitted underneath the chassis as this is reported
to develop poor contacts with its mounting socket. Sure enough
the 2N3055 legs had traces of corrosion and after cleaning I
refitted the transistor, plugged in and switched on. The red
power supply LEDs all lit and the display came on with all zeroes
as before. I checked the various voltages at the test points
on the plug-in boards and all looked reasonable.
I then proceeded to press the various
buttons and switches. Some carried no markings and as the top
front panel was detached I hadn't checked to see what these were
for. To my surprise, the lower left button of the group of six
in the centre resulted in a display. Checking the front panel
revealed the answer. One can use the frequency counter to read
either the internal signal frequency OR the frequency of a signal
at the BNC counter input. The reading could be varied with the
tuning knob from 3352 to 4168 although I can't say in what position
is the range switch because the knob is detached and the spindle
doesn't want to turn easily. The HP8640B uses lots of plastic
gears which over the years have degraded. My example has several
gear wheels with cracks in their hubs and in a couple of cases
extending to the teeth.
As gear problems are so common with
this series of equipments there are several repair solutions
which I need to investigate further. There's also the tuning
problem. The tuning knob rotates through only around 450 degrees...
from a solid-feeling end-stop to a springy stop so maybe a damaged
gearwheel is the root cause. The tuning mechanism not only drives
the oscillator cavity but also includes links to a couple of
ancillary parts so maybe one of these hitting its endstop is
the reason for the fault? |
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I removed the cover from the
pcb carrying the display because that circuit board also carries
the set of six push buttons and the x10 is completely
frozen. Detaching the pcb was slightly puzzling until I noticed
you don't pull the board outwards but lift the riser into which
it's plugged. There was discolouration around the seized switch
but after lots of switch cleaner and gentle persuasion the thing
started to work and after lots of presses worked perfectly. With
that working I found the x10 and x100 buttons did
exactly what they should.
Next the meter. This records
precisely zero reading with any button or switch position except
for a tiny reading in one of the FM settings.. Could the RF output
be knackered? Lack of voltage depends on not only the condition
of the RF stages, but also a fault in the area from where the
meter circuit gets its information I suppose. A couple of signs
make me think the situation is bad. Firstly the digital readout
seems stuck in the range 3352 to 4168. If this is in KHz it equates
to the 80m amateur band, and if a transceiver has been tested...
has the user inadvertently switched to transmit and blown up
the output circuitry of the signal generator? This example appears
to have little or no protection, and rumour has it that lots
of examples of the HP8640B are short of an astronomically priced
RF power hybrid for said reason.
I decided to bite the bullet
and see if there's any RF output so carried the thing to the
workshop and connected it to a scope and turned it on. The display
went a solid yellow. I pushed the Auto button and a nice sinewave
was revealed. "Measure" gave me around 4.1MHz at over
5 volts RMS. I twiddled the attenuator and the trace responded
so all is well.. the RF output is fine. Now I have to work out
why the tuning range is foreshortened and why the meter fails
to work... and fix the AM switch which is seized. |
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I made a fair bit of progress without
achieving much. It seems, because the HP8640B was so popular
it was manufactured for ages, and over the years various improvements
were made. You have to keep your design engineers employed after
all... and lots of cash must have rolled in so wages weren't
a problem (especially underpaid engineers). The various electronic
modules look much the same but careful inspection revealed some
significant differences. I'm trying to track down the lack of
any meter readings.. that goes for AM or FM modulation and RF
output. After printing out circuits showing the path from the
RF amplifier to the meter and looking for problems I found that
reality and the documents didn't line up so I looked in a different
manual and again, printed out the same path. This is much better
as it looks almost right. For example, the actual meter is connected
to an op-amp in one version of the manual and a transistor in
the second. Give the job to an engineer and he'll come up with
a much more complicated solution than necessary so changing from
a transistor to an op-amp was an excellent idea.. not to mention
adding a raft of auto-ranging circuitry. "Keep it simple",
sounds great but won't keep your team of engineers busy.
Using the second manual I can now follow
the RF meter input backwards from the meter board to its local
motherboard, then a small riser connecting to a second, smaller
motherboard where the AGC board resides. At this point the track
disappears. I expected it to make its way to the AGC board on
which are located some processing circuits dealing with a rectified
DC level from the RF output board, but does the track go via
the RF board or is there a break or perhaps a small diode under
the motherboard which cannot be detached without loads of effort?
At this point I wondered if the AGC
board was fitted the right way around. None of the plug in boards
carries a polarising key, but instead is fitted with coloured
levers which match coloured guides, but in some cases the plastic
handles had gone brittle.. hence no handles and therefore no
immediate clue to orientation. I looked further and noticed numbers
on the motherboard indicating the plug numbering. To my surprise
the AGC board was the wrong way round. Maybe this explains the
lack of a meter reading? I switched it around but still no meter
reading. There's a strong possibility the reversal has blown
up a transistor or something vital, but checking the circuitry
showed damage is unlikely except perhaps a transistor or two
or maybe an element in a 74 series package (7406 and 7402). However,
I don't really see that any damage could have affected the metering
circuits. The correct circuit diagram (which I can't find foc
on the Net) for this specific equipment should reveal why there's
a discontinuity in the meter circuitry. I got bored so moved
onto the tuning problem... |
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The good news is that
I found the cause of limited tuning and it's got nothing to do
with the oscillator cavity as such. In fact I've seen the problem
before in my DST100
receiver and it's almost exactly the same problem for a different
solution to the mechanical design problem of counting rotations
of a tuning mechanism. Basically you need to achieve acceptable
mechanical tolerancing down to a single knob rotation to a linear
shift in the mechanism. Say you need to count 10 turns.. then
you can allocate one tenth of an inch for a mechanism depth of
say one inch, plus an allowance for a (slightly wobbly) gearbox.
So if a depth of 15mm is available you can allow 5mm for a gearbox
or frame and 1mm for each turn of the tuning knob. During manufacturing
you can plan to adjust things so that the mechanism works perfectly,
but you're dealing with inaccuracies such as operator setting
up in manufacture and manufacturing tolerances in the different
parts. Given an expected equipment life of say 20 years you'll
get wear and tear which will eventually result in the mechanism
failing.
So what exactly went wrong?
The plastic disks each have an oblong pip with square corners
and as wear increases the position of the pips will move from
optimum to only just working. Then, given a spot of rough handling,
the square corner of a pip might get worn and, instead of locking
against an end stop, slightly more rotational force than normal
will allow an extra turn of the knob. This is what had happened
and at this point I'll refer to the cavity tuner to now be in
no-mans-land. The piston has moved beyond its design stroke and
the indicated frequency will be higher or lower than expected,
depending on whether the forced turn was clockwise or anti-clockwise.
Now, if the knob is turned back, instead of freely moving the
piston all the way back, the wrong side of the next pip will
hit the endstop. You cannot exit from no-mans-land and the tuning
range will be about a tenth of the normal range with the frequency
coverage extending out-of- range. In my case 3352 to 4168KHz,
being part of the 2-4MHz range because the piston has pushed
too far into the cavity. Is the fact that this range of frequencies
covers the 80m amateur band a clue to the demise of this example
of the HP8640B?
Anyway...no-mans-land can be
exited, at least temporarily, by gently pressing on the first
locking pip (below left), then the next and so on.. but turn
the knob the other way and the worn pip (below right), instead
of waiting for several to back up, will slip past the stop and
you're back in no-mans land... The solution is either to re-align
the mechanism or to repair the worn pip..... below, you can see
about 1mm is available for adjustment to overcome the damage
from the peg wearing away more of the leading edge of the last
pip. Possibly a small sliver could be cut from the pip to square
its leading edge, but access is tricky? |
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Returning to the lack
of a meter reading and armed with prints from the three manuals
I'd downloaded (none of which is quite right) I was determined
to track down the meter fault. Part of the problem is understanding
the way the diagrams are coded and another is having to keep
turning 25kgm of chassis upside down and back again. I'll summarise
what I found over the period of a day interspersed with spells
of gardening. Working backwards from the meter. Two wires clip
to pins on the metering motherboard where tracks go to the meter
pcb. Here, various switches and op amps develop a sensible meter
current together with an indication on lamps as to which meter
scale is relevant. The meter circuit is driven from a voltage
arriving on a meandering printed circuit track heading forward
from the rear of the equipment. Having checked that no damage
would follow, I connected an AA cell negative to the chassis
and positive to this track. The meter swung to full scale and
the lowest lamp came on. The meter pcb seems to be OK.
The long track, visible from
the top of the chassis, goes to a short riser which connects
to another motherboard underneath the chassis. Here it goes to
another riser to yet another motherboard on which the AGC board
is fitted. This is accessed, like the meter pcb, from the top
of the chassis and carries an op amp which handles the voltage
from the circuit used for detecting the RF. Again, checking that
no damage could occur, I connected the AA cell, but this time
to the input of the AGC pcb. The meter failed to move, so I reversed
the polarity, feeding a negative 1.5 volts to the input.. and
the meter swung to two-thirds scale. The AGC board is working.
Next, I traced the track backwards from the input to the AGC
pcb. A track goes across the motherboard back to the riser, but
from there, instead of more track, there was a bunch of wires
heading off to the rear of the attenuator switch where a rotary
printed circuit switch (I refer to this type as a mode switch
because these were commonly termed as such in VCRs for selecting
circuit changes associated with record and play). There's also
a potentiometer whose shaft goes through the attenuator case
to the centre of the attenuator switch. The mode switch meter
circuit has two wires and a pair of resistors used to interface
between the AGC input and the RF amplifier. The opposite side
of the riser, to that connecting to the AGC motherboard, connects
to the RF power motherboard which is sandwiched between pcbs
in the RF box. On the underside of this box, under a lid secured
by a set of screws, is the pcb carrying the RF power output circuit.
Included on this pcb is a circuit for processing the rectified
RF signal and its output voltage is the one that passes through
the attenuator mode switch to the AGC pcb.
The voltage at the output is
an unvarying 0.186 volts, so there must be a fault in the rectifying
and switching circuit or in one or more of the several signals
from the front panel controls, which includes the RF on/off switch.
To investigate exactly what's happening to the meter signal I've
shown below its path starting from the RF power pcb and ending
at the meter. |
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Left, the RF output pcb,
26A1. The RF power hybrid circuit carries an RF detector diode
and filter which feeds circuitry (lower left) where it's processed
in accordance with signals from the various equipment controls
(the group of coloured wires) and outputs via the small transistor
top right of the pcb (Q1). The tiny pot above the transistor
sets the DC level. From the wiper of this pot it passes via the
grey wire to the motherboard P26A6, below the pcb, and from there
to the connector and the riser at the side of the RF box, below
left, connecting to the PSU motherboard. |
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Below, before heading
off from the meter amplifiers the DC signal passes through the
dark yellow wire, via a mode switch coupled to the main RF attenuator.
Depending on the attenuator setting one of two resistors is inserted
into the path before it returns to the riser via the blue wire.
Later, I found that one of these resistors defines the normal
power output of 10dBm and the second a higher value of 19dBm
used for the +20dB attenuator range. |
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Top right you can see
the corner of the PSU motherboard where it connects to the RF
power riser. Connected to this riser are several wires going
to the mode switch coupled to the RF Attenuator (bottom left).
On the right is the rear of
the main cavity oscillator assembly |
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This is 26A6, the motherboard in
the RF box. The signal from the attenuator mode switch passes
to the AGC amplifier pcb. You can
just see the socket for the AGC amplifier pcb at the lower edge
of the picture. Directly underneath this motherboard is the RF
output pcb A26A1. |
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Above is PSU motherboard
and on its left, against the RF power box are the connections
to the riser connecting through to the meter motherboard. |
Above, the AGC amplifier pcb
(lower) and the AM Offset pcb, 26A2 which handles the interface
between the front panel controls and the meter switching on P26A1.
The meter signal enters the AGC pcb 26A4 at Pin 7 from motherboard
26A6 where it enters above left from the riser carrying the wiring
from the attenuator mode switch.
The riser connects the meter
signal from P26A6 to the power supply motherboard. |
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Above, the signal enters
the meter motherboard at Pin 30 from the riser.
Right, the amplifier on the
meter pcb is fed by the rightmost gold track at Pin 9, and outputs
back to the Meter motherboard to the red and white wires connecting
directly to the meter. The Meter pcb carries three push switches,
the lowest for the RF voltage/power level. In addition to the
meter voltage circuitry, another circuit evaluates the appropriate
meter scale lamp to be lit. |
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Having worked out the
path of the meter signal representing RF output I had already
checked the voltages, so re-capping...Clearly the meter was reading
zero volts but what was the input to the meter pcb? It was less
than 100mV so after checking that no damage would result I connected
an AA cell and saw more or less full scale deflection for +1.5
volts. I then repeated the test, but this time at the input to
the AGC pcb. No result with 1.5 volts but, again a good meter
deflection with the AA cell reversed and putting -1.5 volts into
the meter amplifier. So, all things considered both the AGC and
meter pcb's are working. What then is the output from the RF
power amplifier pcb? It should be something like, at least minus
1.5 volts, but in fact it read +0.186 volts , hence a zero meter
deflection.
By now, I'd decided there were
too many differences between the free manuals so I bit the bullet
and purchased a download for the manual describing my own HP8640B
(1625U). Looking at the RF amplifier pcb (see below) there are
differences between most of the circuits so maybe there had been
problems in earlier models (or of course a problem with 1625U?).
A key piece of information is the correct voltage emerging from
26A1, which is around minus 3 volts. Mine, as I said was plus
186mV. The manual tells you to check Q1 and Q2 plus associated
circuitry, but of course the fault could be as far back as the
basic RF output detector diode or even low RF output.
Sticking to the recommendation,
I looked at the emitter of Q1. This was sitting at about 9.5
volts, and the base of Q1 about 9 volts. The emitter of Q1 goes
via a 5.1Kohm resistor to +20 volts so the emitter current should
be (20-9)volts divided by 5.1Kohm = 2.1mA. The collector of Q1
connects through a 10Kohm resistor plus around 130 ohms, representing
the 200 ohm pot plus about 400 ohms from R16 and R17. Ignoring
any current drawn through CR6 and any Q1 base current, the voltage
at the collector of Q1 should be determined by the emitter current
of 2.2mA. The resistance is 10.13Kohm so the voltage is 10.13Kohm
x 2.2mA. = 21.2 volts. This sets the collector at around plus
1.2 volts with the pot supplying a max change of only 0.3 volts.
Therefore the meter will read zero volts because the voltage
at the wiper of the pot should be less than zero to produce a
meter deflection. Allowing say a Q1 base current of say 0.25mA
the collector will be 10.13Kohm x 1.95ma = 20-19.75 = minus 250mV
The measured voltage was +186mV making the base current a bit
less. The reason for the low voltage must be something amiss
in the circuitry either between the power amplifier RF detector
and the base of Q1, otherwise a low RF output. |
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Working backwards from
something said to be correct. . If the test point DET is minus
3 volts (Note: re-reading this later I realised that this figure
relates not to +10dBm but to the maximum output where around
+19dBm was the target.. a loop based on +10dBm would make DET
around minus 1 volt) the collector current of Q1 should be about
17 volts divided by 10.13Kohm = 1.68mA. This is roughly reflected
into Q1 emitter current making the emitter voltage about 20-(5.1Kohm
x 1.68mA) = 11.4 volts. It's actually 9 volts, so either the
rectified voltage from the power amplifier is low (0.55 volts)
or this is being reduced by loading from Q6, Q7 or Q3. Otherwise
the sample & hold circuit is incorrect and not allowing Q3
source to rise to 3 volts. I noticed the detected voltage doesn't
vary when the AGC switch is set to on or off. I imagined the
RF output should be held to a precise level so the attenuator
settings will be correct so maybe the RF output is stuck without
any AGC action, or the AGC circuit isn't working? |
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Next, I'll investigate the AGC system
and ensure this is working correctly before moving to the RF
amplifiers. AGC is carried out on the A26A4 pcb. Checking the
output of the feedback loop, which maintains the RF output into
the attenuator at a precise figure, I found this to be about
15 volts. This represents the maximum drive level and will force
the RF output amplifier to its maximum output. This is really to be expected because the AGC
input is driven from much the same sensors as the meter drive.
In other words, if the detected RF output is too low to drive
the meter it will drive the AGC circuit to increase the output
until it reaches the correct level. If the AGC loop is inoperative
the drive to the RF output stage will be the maximum of 15 volts.
We can test the AGC loop quite simply by setting the RF on/off
switch to off. This reduces the output from +15 to -1.7 volts
so, what then is the detected RF output? I checked the voltage
at R22 and found this to be 580mV and this was unchanged whether
the RF switch was set to on or off. Could the fault lie in the
RF amplifier? This is a hybrid device (used in both the 8640A
and 8640B) for which I found the circuit below..
The detector output appears to be fixed
at a low level. The HP manual tells me the hybrid amplifier has
a gain of 16dB and I've already established a decent RF power
level is present. Could the detector be faulty? The detector
diode is driven via a 200 ohm resistor with a 33pF shunt capacitor
perhaps used to eliminate variations in the self capacitance
of the diode as well as reducing the diode current with RF filtering
provided by a 100pF capacitor. I soon discovered the circuit
below which I understand has been drawn from an examination of
the inside of the hybrid. |
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Looking at the circuit
above, one could test the detector diode using a multimeter as
R17, R15 and R16 offer a path to ground. In fact the diode circuit
measured 1.095 volts one way and infinite the other. To confirm
this is about right I connected an OA90 diode in series with
a 1.2Kohm resistor and then a GEX54 diode. The reading I got
with the former was 1.2 volts and the latter was 1.0 volts with
infinite in reverse. Ostensibly the path which includes R15 proves
that if RF output is present the detector circuit should work
normally. This means that unless the hybrid has a very peculiar
fault the problem must be on the 26A1 pcb. Candidates are C4
and Q6, C5 and Q7, or Q3. However, the HP8640B has fooled me
a few times before, so thinking a little more... the penny dropped.
I measured the voltage at the junction of R22 and R23 as 580mV
but a quick calculation showed me that this is more or less the
voltage, using Ohm's Law, expected at the junction of R22/R23,
and it's a positive voltage NOT a negative voltage which would
be the case if the detector diode was working because the diode
anode connects to R22/R23 junction (not it's cathode as shown
in the drawing above!).
When you're dealing with a faulty
circuit things are rarely clear cut especially if you can't see
the parts. If I was prepared to remove the hybrid and grind off
its top all may be revealed, but in doing this there's a fair
risk the hybrid would end up (more?) damaged so some experimentation
is called for. Looking at the above hybrid circuit it would appear
that RF current flowing through R15 is sensed by D3 which is
forward biased via R22/R23 and R17/R15/R16 with C8 and C9 used
merely to isolate any DC relating to D3. C8 and C9 also provide
the possibility to use a DC connection to the diode making performance
much less variable than it would have been using a DC blocking
capacitor.
During the fault-finding exercise
I'd discovered the last setting had probably been in the 80m
amateur band. This poses an interesting question which is why
this equipment had seemingly been dumped in a damp garage to
moulder away. One answer is that during its last use an 80m transceiver
had been accidentally switched to transmit. What followed might
have been catastrophic, and no doubt had been accepted as such,
when the meter displaying RF output had dropped to zero. Hybrid
RF amplifiers are hard to source and if a replacement was found,
it would cost a lot more than the price of a cheap modern signal
generator.
What might have happened is not as bad
as it could have been because, with normal RF output present,
but uncontrollable via the AGC circuit, the damage seems to have
been isolated to only the detector circuit. My guess is a high
voltage passed through the attenuator and zapped the diode, D3.
This failed short-circuit and immediately the 200 ohm resistor
had gone open circuit, leaving R15 intact. The overload probably
wasn't severe enough to kill the RF amplifier.. but all this
remains to be verified. Read on. |
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Above is my experimental
detector circuit (in retrospect this was the Mk1). I'm using
a small Schottky VHF/UHF diode, type 1N5711 connected via a 33pF
Suflex capacitor and 200 ohm resistor to the RF output with ground
return for the diode bias a 1Kohm resistor. A 120pF Suflex capacitor
decouples RF present at the diode anode where it's soldered to
the live end of R22. I switched on the equipment and instantly
the meter read two-thirds scale, and the voltage at the test
point (top right) read -5 volts (the manual tells me it should
be -3 volts, but of course lots of pot twiddling has probably
left the AGC loop miles out of adjustment and the parts I've
used are probably not optimum).
Checking the meter reading as
the attenuator was rotated gave me the following results with
reference to the 0-10 meter scale:
Max attenuation up to 0dBm =
2, +10dBm = 3.4 and +20dBm = 6.8
Having worked out the likely
key reason for this equipment to have been relegated to a damp
garage, I need to consider if any other fault could have been
introduced at the same time as the damage to the RF output hybrid.
One consideration is the output attenuator. Whatever it's setting,
0dBm, +10dBm or whatever could have resulted in damage to that
specific range. I measured the input impedance to ground and
found it was basically 50 ohms from max attenuation to 0dBm,
when it rose to 60 ohms. In the +10dBm and +20dBm settings it
was infinite. As there's a 100nF capacitor linking the attenuator
output to the RF output socket, any 50 ohm load there wouldn't
show up. |
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I've not yet seen any of
the lamps to the right of the meter illuminate so I'll need to
check them. It seems these are all warning lamps but after checking
their continuity I think all are OK and after some experimenting
I noticed the top right (warning) lamp came on so the circuitry
for these is probably OK.
Looking now at the three lamps
(DS4, DS5 and DS6) on the left side of the meter. The lower lamp,
DS6, is always on which seems wrong as these lamps are supposed
to indicate the specific scale relating to the RF voltage which,
since adding the external diode detector has varied sufficiently
to swap over the lower/centre lamps (DS6/DS5). In fact the logic
behind lamp selection is not complicated, being any odd numbered
attenuator setting (= any voltage range starting with 3) should
flip illumination from the lower (= any voltage range starting
with 1) to the centre lamp (= any voltage range starting with
3). The control connection indicating odd/even passes from the
mode switch A1A1 at the rear of the attenuator via pin 8 of A2
to a set of three transistors on the pcb. When the control line
is high (= any voltage range stating with 1) the lower lamp (0-10)
illuminates and when a ground is subtended from the mode switch
to Pin 8 the lamp for the 0-3 scale should come on. The mode
switch on the rear of the attenuator is coupled to the attenuator
selection switch shaft and has three separate functions, one
of which is to forward a ground connection whenever an odd range
(= a voltage range starting with 3) is selected.
I'd looked at this mode switch
previously when tracing another of its functions (to do with
meter readings) and it looked OK, but having successfully traced
the connection from A2 Pin 8 all the way the this mode switch,
I realised the wiper responsible for carrying ground for odd
attenuator settings was missing. There are two ways to get at
the mode switch. The obvious one is to detach the attenuator,
but as the knob refuses to be parted from
the attenuator shaft I decided to try the most awkward which
is to unscrew the A1A1 assembly from the back of the attenuator
left in-situ. From the messy soldering at that blue potentiometer
I'm not the first to investigate this area. As you can see below
the end wiper was missing, and in fact one of the plastic pips
securing the centre wiper had broken also. The plastic disk has
two sets of pips so I moved the good wiper and the wobbly centre
one to the spare set of pips. Finding a suitable new wiper could
have been easier but I found that a contact from an edge connector
provided something suitable so glued that in place. I then used
an epoxy resin to complete the job. |
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Once I'd assembled the mode
switch I found the new spring contact wasn't flexible enough
and the pair of adjacent tracks was not being shorted by the
new spring contact. The problem was that the ends are joined
and either one side or the other made contact with the track
but the contact needed to be able to twist slightly for the second
track to be touched. I suppose I could have used a second piece
of the material and reversed it with the join under the glue
but I removed the spring and instead found a flexible relay contact
of the right shape (below) and which is joined at the base allowing
the two springs to work independently. |
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Left, the Mk2 repair: I searched
through lots of relays until I found a miniature type having
a low enough current capacity for the spring to be small and
flexible enough for the job. I used one of the pair above which
looked OK. Note, on the left there are two original wipers not
four. Each has two prongs and pairs are joined at their base.
Having twin prongs reduces the tension required to bring them
into contact with the fixed plate carrying the tracking. I used
one of the above but I needed to separate the two prongs slightly
so these would make contact with adjacent printed circuit tracks.
By allowing enough length in the spring the operating tension
was lessened. The pencil mark tells me the centre line. |
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If anyone needs to carry out
this repair, or a similar repair to other mode switches, you'll
find a circlip mounted behind a tensioning spring and washer.
The circlip has very small holes for using circlip pliers. The
holes are so tiny I had to spend half an hour carefully filing
down the ones you see below left. I eventually removed the
pot to gain better access to the securing screws for the circuit
board and access to the shaft of the switch. Note that the shaft
of the pot has an open slot so you don't have to bend the mating
spring joining the switch to the pot. The plain side of the spring
slides through the hole in the centre shaft and the hooked side
of the spring fits into the slot in the shaft of the pot. If
you fit it the other way it will eventually slip out. |
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One step forward and two
back! I started to assemble the repaired mode switch and
the adjacent plumbed-in coax feed to the oscillator came adrift.
I'd noticed it was very wobbly when I first removed the bottom
cover and the reason turned out that the coax bulkhead socket
had sheared off. The coax complete with plug and socket came
away from the back of the oscillator so I'll need to remove the
whole oscillator assembly, remove parts of the rear enclosure
and fit a new socket. Looking into the hole I could see the loose
nut and a small loop to which the socket had been soldered. I
suppose I would have needed to remove it anyway because I
need to fix the tuning mechanism. |
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There was a slight possibility
I could access these eight screws and repair the broken socket
without having to remove the whole assembly but the screws were
too tight so not feasible...
At first sight I'd have to unsolder
lots of wiring to remove the whole thing completely, but this
wasn't necessary. A better solution would have been to pull out
the riser to which the cabling is attached, but it seems one
can sufficiently loosen the whole oscillator assembly without
needing to remove it.
Below, it was possible to detach
the second solid coax lead remove the four fixing screws, and
rotate the whole assembly to access the side plate and gain access
to the damaged output connection. |
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The oscillator turned out to
be a very tight fit and I had to unscrew the two coax plugs at
the RF driver module to gain a few more mm of wiggle room. No
unsoldering because the cables allowed enough movement to access
the side inspection panel. Once the oscillator was loose I upended
the chassis so the inspection panel was horizontal, removed the
eight long screws, lid and two RF gaskets, to expose the damage.
The right-angled bulkhead socket fixing thread had broken off
and subsequent movement had detached the RF connection to the
circuit board allowing the nut and a star washer to fall into
the box carrying the RF amplifier circuit board. I initially
looked for a new socket but was unsuccessful. I noticed the remains
of the thread had a flat (this is not used as the mounting hole
is circular and provides room for the added wires). I used a
couple of stiff wires from a CAT5 socket which I'd cut open looking
for flexible gold-plated springs to earlier repair the mode switch.
These are made of gold flashed brass and once soldered in place
made the old connector a force-fit in its mounting hole. I added
superglue to prevent the thing moving and soldered the ground
wires to the circuit board ground plane, then soldered the RF
output wire to the centre conductor (see the pictures below).
Then came the job of refitting
the oscillator (I'd forgotten to fix the multi-turn mechanism..
but that can be done later because I'm not happy yet about what
exactly is wrong with it). The various solid coax cables screwed
back into place including the repaired cable which now feels
nice and secure.
Slightly puzzling is that there
are two sizes of RF connector nuts. Either this is an HP manufacturing
arrangement or some solid coax leads have been replaced during
the life of the signal generator. |
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After refitting the repaired
oscillator I continued with the repairs to the circuit board
fitted to the rear of the attenuator. I'll describe these as
progress unfurled... With the repaired mode switch and its pcb
in place I added the potentiometer, having fitted three new connecting
wires because these were pretty fragile and it would have been
virtually impossible to re-connect these in-situ if one or more
had broken off. I then re-connected the set of push-on wires
from a sketch I'd made, which completed the overhaul, and tentatively
turned on the equipment. Immediately I noticed a new lamp was
lit and switching the attenuator backwards and forwards proved
the repair to the scale indicator lamps actually worked.
My satisfaction at seeing the
centre scale lamp illuminate for the first time was short-lived
because the RF output meter showed zero RF output. Maybe I'd
inadvertently pressed one of the switches, but the RF on/off
switch said ON and none of the push buttons resulted in any meter
movement, although I did notice a slight kick upwards with the
attenuator turned to +10dBm. I couldn't believe the repaired
mode switch area was bad so could it be a random failure? But
no, I ruled this out as being too much of a coincidence.. more
likely I'd disturbed an RF connection.. perhaps one of the rigid
coax cables was now bad? The frequency display indicated a frequency
in the 4-8MHz range, and twiddling the tuning knob altered the
reading, so at least the oscillator was working and the repaired
coax connection was fine.
I checked the RF output at the
test point in the corner of the RF output pcb. Nothing.. this
lines up with the meter reading so the metering circuit itself
was probably not to blame. Time to use an oscilloscope and see
exactly what was going on. I found there was RF at the output
from the hybrid, but only around 180mV RMS instead of something
like +20dBm which is around 2 volts. Could the rigid coax between
the driver and the output stage be bad? The driver pcb output
into the coax measured 20mV, so maybe a short circuit in the
rigid coax? I also checked the auxiliary RF output and this was
OK as was the RF output directly from the driver hybrid which
measured about 500mV RMS. The problem was in the circuitry between
the driver hybrid and the output into the filter box which seemed
to be around 20mV RMS. Not thinking too clearly, and being interested
in looking at the filter pcb for the first time, I opened up
the screened box, detached the pcb above the filter assembly
and made some resistance checks. The filter pcb was very interesting
and I realised that all the slide switches and relays might have
a bearing on the stuck 4-8MHz range I'm working with. But...
I gave this area a clean bill of health in respect of zero RF
output and put everything back.... but see later.
I decided to think logically
and came to the conclusion that the reason for the lack of RF
was connected with the modulator on the RF driver pcb which is
built from lots of diodes. The control voltage to the modulator
(which is described in the documentation as an attenuator) was
probably the culprit. I reckoned the control voltage was attenuating
the RF from 500mV to 20mV. Ordinarily the modulator, at least
in CW mode, should be driven by the AGC circuitry to produce
the correct RF level into the main attenuator.
I measured the control voltage
and saw it was around 1.5 volts, but somewhere I'd read that
circa 15 volts should be expected. Back to the schematics.. and
I found the vernier control was the primary driver in producing
the control voltage. Of course the vernier control was the potentiometer
on the back of the main attenuator that I'd fitted not too long
ago. |
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Making measurements around the
pcb at the back of the attenuator is tricky because some connections
are hidden and pin numbering is slightly odd. Also, resistance
measurements can be misleading because of various external circuit
connections and it was at this point I found the vernier potentiometer
had a dead spot at one extremity. I also discovered the pot was
possibly wrongly wired (some time ago I noticed the wires at
the pot were roughly soldered as if someone was messing with
the circuit... perhaps looking for a fault?).
After 30 minutes of checking I found
the pot was indeed wrongly wired and corrected this. I turned
on the equipment, confident that all would be well.. but no..
the RF output although slightly greater (not quite as close to
zero as it had been) was still miles too low, being down in the
first few meter graduations. In fact.. the wrongly wired pot
hadn't previously resulted in zero RF output.. but it hadn't
actually altered the meter reading as it should have done...
so that had been a fault I'd overlooked after restoring the RF
meter readings. |
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Back to that A1A1 pcb.. and
after comparing the circuit diagram with the wiring, and the
pcb tracking, decided all was now well as everything matched.
Could the problem be the wiper springs? I could clearly see the
springs were in solid contact with the gold tracks and I'd already
squirted switch cleaner on them just to make sure, so I convinced
myself it must be an open circuit.. maybe corrosion had opened
a gold flashed copper track... so reluctantly removed the whole
assembly for the second time. Having got decent circlip pliers
it was much easier this time, and I'd anyway decided to fit a
new pot without a dead spot...
I carefully checked the pcb.
It looked OK. I looked at the plastic disk with the three springy
contacts and all looked fine. I could see nothing wrong with
either the circuit board or the mode switch, so what on earth
was the fault? I connected my multimeter with croc clips to the
vernier tracks, and set to buzzer held the plastic disk against
the tracks. Nothing heard.. so I jiggled it and examined the
thing using my magnifying goggles. The spring contacts were solidly
shorting the tracks but continuity was absent. Very odd indeed
so maybe there was a break between the two prongs... surely not?
I placed the multimeter
against the prongs and found no continuity. I carefully placed
the multimeter against the pair of springs on the same prong.
Again no continuity. I looked at the prongs with my magnifying
goggles.. perfectly shiny and undamaged, but then I realised
what had happened... To secure the springs on the opposite side
of the plastic disk I'd used a spot of superglue before using
epoxy cement to finish the job. Superglue has a very small surface
tension and flows across anything on which it drops. Gold flashed
springs provide no resistance to flow and the glue had formed
an incredibly thin insulation layer. I have a tube of superglue
remover and applied a little to the springs then cleaned it off
to be rewarded by continuity between the prongs and, after holding
the disk against the track, I could see continuity across the
vernier circuit.
I reassembled the pcb then found
a decent 1Kohm linear pot and fitted this in place of the bad
one (which not only had a dead spot, but whose resistance readings
I later discovered were very intermittent). After a quick check
that everything was connected correctly I switched on and RF
output was restored and for the first time.. twiddling the vernier
control made the output go up and down. Now.. what's the next
problem? |
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One of the trickiest jobs
on the list is to overhaul the A9 assembly. This is a complicated
thing that would have benefited from microprocessor control,
but thankfully (for repairers) early microprocessors complete
with complicated custom chips and (now long obsolete) special
logic chips were still a gleam in the eyes of designers when
the HP8640B was under development. I suspect so much cash had
been invested in it that, although a digital design was possible,
HP continued to manufacture the analogue version for ages. Below... A9 in-situ. The black markings are
to aid reassembly later. |
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A9 deals chiefly with
switching the frequency range, and handling modulation... a very
clever mechanical design which allows the switching of one set
of circuits by two separate control knobs forms the basis of
the assembly. In addition to gear wheels, no less than three
mode switches are involved. Alas, the clever design is let down
by deterioration of parts due to ageing. Firstly, in my example,
the main gearwheels which have brass centres onto which are pressed
plastic gears, are pretty fragile. As I'd spotted the damage
I didn't attempt to force anything, so touch wood, I can repair
rather than renew. As with most old equipment, certain parts
are critical.. and the whole of A9 falls into this category.
Removal looks straightforward,
but you do need to know in advance that it's quite possible,
because there's not much wiggle room. One important matter is
the link between A9 and the filter cam shaft.. the metal linkage
needs to be in a vertical position for A9 to slip out (the
coupling was very hard to turn). Once out the gears and mode
switches can be examined properly. Clearly, the gearwheel plastic
material has shrunk over the years, but this appears to be random
because in my equipment shrinkage varies even between identical
gears. I've heard it said that gear cracking is aggravated by
expansion of the brass centre hubs and looking at a couple of
cracked gears.. one is cracked at the hub and not in the gear
itself and another has cracked at the hub with the crack extending
to the teeth which implies the brass centre probably is involved
in the start of the cracking. |
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I tackled the two gears
differently. In one case (the leftmost of the set of gears above),
I ran superglue into the crack, into which I added small pieces
of metal as a filler (see below). The reasoning was that the
plastic is unlikely to expand and adding material would be better
than relying solely on a large thickness of glue. The metal washer
adds significant strength and will not interfere with operation. |
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The worst damage was to one of
the two bevel gears. One crack extended through to the teeth
making it unusable because the gap must have been the best part
of a mm in width at the teeth. A second crack wasn't too bad.
The repair solution was to press out the brass centre bush after
removing both grub screws. This was a risky business with the
severe cracking but I initially left the gear overnight in the
freezer hoping the brass would shrink a little and possibly relax
its adherence to the plastic.
If anyone would like to copy the method
I'll explain it here. I found a metal collar (this can be a large
nut) whose inside diameter was slightly larger then the diameter
of the brass bush, plus an M5 hex bolt whose circular head was
just smaller in diameter than the diameter of the brass bush
and whose length was short enough to allow the brass bush to
press out. I used a small engineering vise as the press. The
collar, gearwheel and bolt were positioned centrally and the
vise very carefully tightened. Once the assembly was under pressure
I waited and as the pressure slackened, gradually increased it
to the previous amount. The end result was the brass bush dropped
out leaving the plastic gear unscathed... in fact as soon as
the brass bush was out the main crack began to slowly close up,
and after 30 minutes the main crack was almost closed. I placed
the shoulder of the bevel in the side of the vise and gently
fully closed the crack whilst applying superglue. I also applied
superglue to the second less obvious crack which had closed by
itself.
Leaving the glue to set I measured the
brass bush. This was knurled and the overall diameter measured
11.37mm. I also checked the size of the hole in the glued gear
to find it was 10.77mm, some 0.6mm less than the bush. The next
task was to reduce the diameter of the brass bush to something
close to 10.8mm. I used a digital vernier tool to make the measurements
but I suppose trial and error would be possible.
As the reduced brass bush needs to be
as circular as possible and not having a lathe, I clamped my
bench drill horizontally in a large bench vise. The bush was
held on a bolt, having a diameter very close to that of the hole
in the bush, with washers either side. Using a fine flat engineering
file I reduced the size of the bush until it was about 10.8mm,
not quite small enough to fit the gear. The inside of the hole
in the gear was slightly rough because of the knurling on the
bush so I carefully scraped the roughness with a scalpel (you
could use a round file) until the bush fitted the hole. It's
important not to apply force otherwise the superglue might fail.
Once the bush has slid into the gear you need to double check
the position of the grub screw holes which must line up perfectly
with the tapped holes in the bush, then superglue the plastic
to the bush.
At this point I was very pleased to
have completed the repair... but alas, I hadn't appreciated one
important point. To continue... because the brass bush was now
smaller by over 0.5mm the threaded section of the gearwheel and
the threaded hole in the bush were no longer continuous, neither
was the position of one hole in the gear perfectly lined up with
that in the bush. This next step is pretty critical. If you forced
the "0.05 inch" grub screw into the brass bush it would
almost certainly cross-thread and be likely to split the plastic
or jam in the bush. The aim therefore is to remove the thread
from the two holes in the plastic part of the gearwheel. If you
try to do this without care the plastic might split. I used a
sharp drill, turning it carefully by hand with the minimum stress
because the repaired crack was centred on the grub screw hole
in the gear (the weakest point).
Once the hole was slightly enlarged
I used a mini drill fitted with a round burr tool to grind away
the thread immediately adjacent to the brass bush. This is less
risky than trying to remove the thread completely. By trial and
error I was able to fit both grub screws backwards from the centre
of the bush so that both were clear of the centre hole. Of course
there's a risk that you'll forget and sometime in the future
try and remove the grub screw completely and crack the plastic,
but you could place a note to this effect inside the case? |
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The first job is
to remove the centre bush and reduce its diameter to almost fit
the plastic gear
There are two of these gearwheels
with the bevel but the other, which is not cracked, has no securing
grub screws because it's designed to freewheel on its axle. |
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The second job is to clean up
the centre of the plastic gear and re-fit the bush without cracking
the repaired plastic.
Finally, clean up the thread
in the plastic and fit the pair of grub screws (from the inside
of the brass bush not as normal) ie. Insert the 0.05 inch
Allen key through the hole in the plastic gear and attach the
grub screw with tweezers inside the bush and unscrew it into
the thread. |
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I suspended A9 overhaul
at this point so that glue can set, and delayed fixing the two
damaged mode switches, but this should be easy because I already
repaired the one at the rear of the attenuator...
The next major task on the list
is to repair the AM switch. When the HP8640B was removed from
the barn (ie it's a "barn find") I guess the owner
tried the various knobs and switches. He'd mentioned he'd plugged
it in and seen no display.... The front controls had suffered
from damp and aluminium corrosion had seized many of them. Pressing
the AM slide switch had broken off the lever which is quite small
and, being plastic, had limited strength. Fortunately the broken
lever was still present in the slot. |
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The AM slide switch is
not a discrete component. The top part carries three wipers,
just like the ones in the mode switches that connect across gold
flashed printed circuit track on one of the motherboards. Although
the similar FM slide switch is riveted to the motherboard, the
AM switch top is held in place by six nuts and bolts, enabling
the top to be detached. Five of the nuts are readily accessible
once the mains on/off switch and the modulation control plus
the adjacent BNC socket have been removed, with the sixth a little
bit awkward but possible. Slightly tricky was detaching the BNC
connector because the securing nut was seized, also from aluminium
corrosion, but by fitting a spanner to its rear flats I was able
to jiggle it until freed.
Once the slide switch top was
detached I cleaned it and glued the lever back in place. This
however is only the beginning of the repair because superglue
will not have sufficient strength to keep the lever attached.
The key to this repair is to minimise the overall size of the
lever because there's very little clearance in the way it's fitted. |
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Pausing for a moment... there's
a couple of alternative solutions for a bad AM slide switch.
One is to dispense with the original lever and glue an alternative
metal one in its place. The second, which will need to be thought
out in detail, is to procure a discrete slide switch having five
positions and mount it as close to the motherboard as possible
but this will mean either modifying the lever on the new switch
or cutting the front panel. A warning here.. not all slide switches
have the same switch configuration. The HP8640B needs an ON-ON-ON-ON-ON
switch. |
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Once the six securing nuts and
bolts have been removed three main pieces lift off. The centre
part carrying the spring contacts is shown in its assembled position.
The two edge pieces are identical** and carry two slots. The
wide slot mates with the centre piece and the narrow one is a
guide for a springy wire with a raised pip (clipped to the front
edge of the centre part) which locks into holes (the dark areas)
to provide an ident for the five positions of the slide. The
broken lever is shown before glueing back in place at the front
edge of the centre part.
** They differ slightly because the
fixing screws, which have different spacing, are countersunk
so that the lever can pass over them, and the edge pieces can't
be swapped around. |
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My solution was to strengthen the
existing lever. Because of the tiny allowable clearances I needed
a material having the minimum thickness combined with strength.
After some searching in my collection of "useful" stuff,
I found several candidates each of which I measured with a digital
vernier. By far the best was a sheet of thin steel having a mere
0.15mm thickness. Because it's so thin it was easy to cut with
scissors and, using my small vise, I bent a small piece into
shape. It looks complicated but it was quite easy to do. Once
I was happy with the fit I glued the metal over the top and along
the lever. Pictures below... |
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The metal has a thickness
of 0.15mm (about 5 thou inch).
From memory I think it's a scrap
piece of mu-metal which is sold in very thin sheets and very
easy to cut and shape.
A nice touch is the side of
the lever carrying the marking is the visible side once the switch
is reassembled and the repair is hardly visible. |
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The A9 module connects
to this cam assembly on the left via a coupler. It in turn mechanically
links via six cams to the filter pcb, below.
I'd already noticed that an
inordinate amount of force was needed to turn the operating shaft
when positioning the linkage vertically to enable A9 to be extracted.
I was now concerned that the coupler could be damaged if I didn't
investigate and correct the stiffness in operation. Of course
this stiffness could be quite normal, but a check on the Net
suggested something had probably seized. |
|
|
 |
|
If you look closely at
the filter pcb you can see six slide switches operated by black
plastic pegs more or less in a vertical line to the right of
the metal casting. These pegs are part of the cam assembly mounted
vertically above the filter pcb and should be free to move sideways,
however the whole cam is seized. Note the tiny relays at the
top of the pcb. These are used to select High-Low filters (see
later). |
 |
Left and right are the
bearings at the end of the cam shaft. You can see a dark residue
(left) which is dried grease. This had turned into a glue which
had locked the bearing causing the whole thing to rotate in its
mount between the base and lid of the box in which it fits.
Switch cleaner dissolved the
grease which I need to remove and replace with fresh grease.
A suggestion is to also reduce the pressure from the metal plate
acting as an ident for the cams as this normally introduces too
much force on the teeth of the mating gear. Note the hairline
crack close to the cam ident gear. |
 |
|
|
After applying fresh grease
to the bearings and the cams and slackening the two screws holding
the ident plate I added a little superglue to the latter screws
to prevent them dropping out. After refitting the lid which holds
the camshaft in place the shaft turned smoothly so I returned
to refurbishing the A9 module. The main task here is to complete
work on the three mode switches. One has two wipers, the second
three wipers and the one on the opposite side has four wipers.
The first couple had one wiper each still fastened to the transparent
disks and the other had all four still present. I fashioned new
wipers and stuck these in place and decided then to remove the
third to check its condition. I found that all four wipers were
variously detached so I added a spot of superglue to each. The
paper was to ensure the wipers were lined up as the glue set
and to absorb any excess. The wipers were
made from relay parts as before. |
|
|
|
Once the mode switch disks
had been repaired I reassembled the A9 unit. Of course you need
to ensure the correct switch positions are selected and each
gear is secured primarily on the flat of its shaft so that misalignment
can't occur. With both main shafts fully anti-clockwise the pair
of mode switch wipers should be vertical looked at from the underside
with the rear mode switch at 180 degrees to these. The pair of
bevel gears should mesh closely and the spur gear should be in
line with the other three.
The action of the main shafts
now feels nice and positive, but I need to fit the repaired module,
taking care to line it up with the cam shaft to see if it works
correctly. Oops.. see the results below.. could be better? |
|
Range |
Low |
High |
Low |
High |
Comment |
A8A3 logic |
|
512 |
460 |
1100 |
4.584 |
11.015 |
Decimal point? |
position 3 |
|
256 |
230 |
550 |
2.292 |
5.5 |
Decimal point? |
position 3 |
|
128 |
115 |
275 |
4585 |
11017 |
?? |
position 3 |
|
64 |
57.5 |
137 |
5732 |
13772 |
Decimal point? |
position 4 |
|
32 |
28.8 |
68.7 |
2865 |
68681 |
Decimal point? |
position 4 |
|
16 |
14.4 |
34.3 |
1432 |
03441 |
Decimal point? |
position 4 |
|
8 |
7.20 |
17.1 |
7.159 |
17.208 |
OK |
position 5 |
|
4 |
3.60 |
8.59 |
3.570 |
8.607 |
OK |
position 5 |
|
2 |
1.80 |
4.29 |
1.790 |
4.303 |
OK |
position 5 |
|
1 |
0.90 |
2.14 |
0.895 |
2.153 |
OK |
position 5 |
|
0.5 |
0.45 |
1.07 |
4.476 |
10.759 |
Decimal point? |
position 6 |
|
|
Without a decimal point
the display was a bit mystifying but having noted the "overlap"
range frequencies given in the user manual, things became a lot
clearer. It seems the HP8640B is as complicated in its logic
circuits as in its mechanical bits and pieces. The Counter Time
Base Assembly, A8A3 is responsible for handling the decimal point
position, which it does by reading the range setting (=band code),
a set of data inputs A-E (Counter Mode). Complications arise
when the push buttons for giving additional accuracy are pressed
(ie x10 and x100). For each range only a single data input (A
to E) can be at logic LOW. Assume the x10 and x100 buttons give
the correct output, both are HIGH. The calculation on the position
of the decimal point isn't a static logic level because a new
calculation is performed at a regular rate set by the Time Base.
I removed the upper board of the two mounted in the box at the
right of the display. This is the one carrying the display, A8A3,
and underneath is A8A2 which carries the decimal point logic.
Noteworthy is the fact that some of the i/c's have rust on their
legs, but otherwise, being gold flashed, the printed circuit
looks OK. Measurement of voltages around the legs of i/c's is
not straightforward when things are powered up because much of
the circuit is either driven by a "clocking" source,
or perhaps asynchronous signals, rather than being static voltages.
The former will be averaged by a DC voltmeter and therefore pretty
meaningless when it comes to checking logic states.
I'm looking for unusual static
voltages. Basic 74 series logic, on which the HP8640B, mainly
relies, generally has two possible DC levels, being 0 or 1 logic
states. These levels are roughly 0.5 volt and 3.5 volts, give
or take, and once versed in circuit checking one can readily
see an error due to a faulty chip. Whilst not a clear cut way
of detecting a problem, it does point you to further analysis.
Below, I've shown the decimal point decoder on pcb A8A3 and one
of the display chips on pcb A8A2. |
|
|
|
Tracing back from the
decimal point pin on each of the seven display i/c's I found
that printed circuit track is routed to Pins 11 to 15 on the
riser carrying the display board (eg Pin 12 for DP4 of Display
U4 above). These are connected to U3 (Pin 14 for DP4 above),
an SN74LS96N which is a rather uncommon i/c described as a "5-bit
serial-parallel, parallel-serial storage register". Checking
U3 dynamically is tricky but, because the chip has four control
inputs handling its five identical "totem-pole" output
circuits, and its outputs connect either to five identical displays,
or with the display board unplugged, free from other connections,
all outputs should test identically when unpowered. Using a multimeter
set to diode or ohms I would expect to see each output to be
roughly the same.. for example if one showed a forward-biased
diode so should the remainder. In fact I found anomalies both
in resistance and with a diode test.
I found a supplier offering
a new i/c for 99 pence, so rather than continue testing with
a major unknown, I'll just swap the 74LS96 and if this doesn't
cure the problem, I'll continue testing. I have an i/c tester
so before the new chip arrives I can remove and test the old
one. This is not an easy job however. The lower pcb in the housing
is held in place by five soldered connections to feed-through
capacitors plus three rigid coax connections. The coax leads
must all be disconnected before the casing can be removed. Connection
to the adjacent motherboard is via a loose riser held in place
by pressure once the casing is screwed back. Once the casing
was free I cut the five feed-through leads and the board lifted
out. I removed U3 and tested it. |
 |
 |
|
Left, A8A3 pcb less U3
at top right corner, removed from its case (above) with a view
of the main chassis (below left) showing the three rigid coax
leads (the right lead just pulls off whilst the others unscrew)
and, right, the loose riser. |
|
 |
 |
 |
|
Static tests showed the
outputs were faulty and these only connect to the display so
other i/c's on the board should not have been affected by this
failure.
The chip is really complex so
should I rely 100% on the test meter? |
. |
|
|
 |
|
I believe this item is
Option 001. |
|
|
Whilst I'm waiting for
the new 74LS96 chip I'll tackle any remaining problems, one of
which I spotted only when refixing some of the control knobs.
This is another split gearwheel hidden away on a shaft protruding
from this Modulation Frequency Oscillator box, however.. when
I looked at this it seems quite difficult to get at and when
I rotated the control knob to which it's attached the crack was
equally spaced between the limits of operation and never actually
engaged, remaining clear of the driving gear so I just left it
in place. |
 |
|
|
|
Finally, the new 74LS96N
arrived.. I say "new" but the date code told me it
was made in Week 25 of 1982. Anyway popping it into the tester
proved it was fine but, because a week had taken place, I found
it tricky getting all the parts back together. Eventually I'd
used up all the screws and it looked OK, at least mechanically
and I'd taken the trouble to remove dust from the red display
filter. I switched on and the decimal point was....... OK. |
|
|
|
A little difficulty then ensued because,
once I'd hooked up my scope to the RF output I discovered the
RF didn't match the counter display, but after correctly re-aligning
the filter cam to the range switch all was fine... except the
top three ranges displayed incorrectly. The RF output was OK,
because I could see a 128MHz trace on my 100MHz scope, but the
counter display thought it was only 1.000MHz. So I checked further... |
|
Range |
Tuning MHz |
Comment |
RF Output Volts RMS |
|
512 |
4.000 |
wrong display |
NA |
|
256 |
2.000 |
wrong display |
NA |
|
128 |
1.000 |
wrong display |
9.5 |
|
64 |
64.000 |
OK |
7.5 |
|
32 |
32.000 |
OK |
8.4 |
|
16 |
16.000 |
OK |
11.1 |
|
8 |
8.000 |
OK |
12.0 |
|
4 |
4.000 |
OK |
15.6 |
|
2 |
2.000 |
OK |
15.8 |
|
1 |
1.000 |
OK |
14.0 |
|
0.5 |
0.500 |
OK |
13.4 |
|
The GDS1102 scope runs
out of steam at something over 128MHz so I've put NA in the RF
output column.
Once everything gets sorted
out I'll use my DSA815TG to make accurate tests.
Clearly, there's a problem,
though not with the signal generator.. but maybe the frequency
counter? |
|
|
Reading the repair manual
for the HP6840B I spotted a truth table relating to the logic
for selecting the right set of dividers for the counter. This
suggested that the reason for the bad display was Pin 9 of U6
on A8A3 was stuck at logic one. This pin is tied via 10Kohm to
the 5.2V rail but is grounded via S2 on A9A1 in the top three
ranges. I traced the signal where it exits from the track on
plug P2 at the top of the front range switch pcb via a flat cable
to the local motherboard. It then passes via a loose riser to
a 1mH choke on A8A3 to Pin 9 of U6. Initially I found the path
to be open circuit, but after re-plugging P2 into the motherboard
and re-checking, the path measured OK, so I turned on the power
and was pleasantly surprised to see 512/256/128MHz all present
on the display. For anyone interested I've reproduced the relevant
section of the truth table below.
The fault is that 128, 256 and
512MHz are displaying as 1, 2 and 4MHz respectively and from
the table you can see that this can occur if U6C9 sticks at logic
high. The codes for the higher frequency ranges are then the
same as those for 1, 2 and 4MHz. This can be due to one of a
number of different reasons.. an open circuit, a faulty mode
switch (S2 on A9A1), a faulty U6 i/c or even a bad ground connection.
The various i/c's U6, U7 and U11, in addition to U3 and U5, are
used to drive the dividers which develop the appropriate RF frequency
from the main oscillator to drive the counter. If there's a fault
within this area you can get a true RF output but a false display
(which was exactly the case here). |
|
RANGE |
U6B4 |
U6B5 |
U7B5 |
U11B4 |
U6C9 |
U11A2 |
U6C10 |
U6B6 |
U11B6 |
|
1MHz |
H |
H |
L |
H |
H |
H |
H |
L |
H |
|
128MHz |
H |
H |
L |
H |
L |
H |
H |
L |
H |
|
2MHz |
H |
H |
H |
L |
H |
H |
H |
L |
L |
|
256MHz |
H |
H |
H |
L |
L |
H |
H |
L |
L |
|
4MHz |
H |
H |
H |
H |
H |
H |
H |
L |
H |
|
512MHz |
H |
H |
H |
H |
L |
H |
H |
L |
H |
|
|
The next task is to measure
the RF output and see if it matches the attenuator figures and
to see if it maintains the correct amplitude across the whole
frequency range. I might see if I can improve the range switch
before I start. Mine is in a dreadful condition because it has
been forced and the lever has broken off. Apparently the knob
design varies because I found a picture of a pristine example..
below. The range knob is completely different to mine which used
to be a similar shape to the peak deviation knob to the left.
Can you spot something odd, presumably
overlooked by the last owner of this model? click to see the
answer |
 |
|
|
|
Here's before and after
pictures... so far. |
 |
 |
|
A new knob to replace
the broken one. |
|
HP designed the knobs
to include a thin layer of aluminium covering the plastic surface
before adding a stencilled coloured background with black printing.
Maybe this was done to improve the life of the knob but alas,
damp results in corrosion to the aluminium and the oxide destroys
the finish. The modulation frequency knob doesn't use the same
finish and has outlasted the others.
You can also see that damp attacks
the frame of the equipment resulting in a very scruffy appearance.
I used emery paper on the frame and cleaned up the front. I found
the best method of finishing the cleaning of the front panel
was to use a rubber on the end of a pencil which nicely removed
blemishes. I need to slightly increase the font size and use
bold to make the numbers more legible. The method I used for
the range dial was to draw a circle (PhotoShop), then type each
number, rotate by 30 degrees and type the next. I used a definition
of 600dpi and had to keep cropping the drawing as it increased
in size every time I flattened the result between numbering.
Printing was straightforward. I measured the size of the knob
surround, noted the auto-print size and then applied a percentage
reduction to match the print to the size of the knob. A thin
black surround plus a deepening of the colour to match the other
knobs might be an idea for the Mk2 version... |
|
|
I was finally at the stage where
I can test the 8640B using a spectrum analyser. I did the tests
with the lid off the RF compartment, but using a level of 0dBm
or thereabouts this doesn't matter too much. I checked each range
in turn from low to high and found a couple of interesting facts.
Firstly the three lowest ranges were poor in terms of harmonics
(I'd previously noted the sinewaves for the lowest ranges were
distorted somewhat) and the remainder pretty good, although switching
from 256MHz to 512MHz failed to make any difference I now realise
that it won't make any difference because I'd need a doubler..
Option 002). Amplitude-wise the signals were in the same ballpark,
although I haven't yet attempted to set up or check this aspect. |
 |
All the measurements were much
the same. This shows a 256MHz CW signal over a spread of 200-300MHz
but extending the scan to 1GHz showed no discernable harmonics.
Below is a picture of the front of the
8640B for the output shown here.
The output meter indicates 0dBm which
doesn't line up with the picture. |
|
 |
|
Below, harmonics were
only obvious in ranges up to and including the 2Mhz range. Is
this a fault? Well, I looked at the circuit diagrams of the filter
module and the four filters for the lowest ranges all looked
similar, and each is followed by a second (common) filter to
handle high numbered harmonics. I then looked at the specification
which made bold statements regarding spurii, particularly sub-harmonics
and non-harmonically related signals. Of course, because the
8640B doesn't use mixing processes it won't be subject to these.
Instead it relies on a single high stability UHF oscillator and
to obtain suitable output frequency ranges, the oscillator is
divided by ecl (high speed.. emitter coupled logic) circuitry
whose outputs are square waves. Square waves are rich in harmonics
so will need converting back to sinewaves. This is achieved through
a set of filters, so the harmonic performance will be directly
governed by these filters. HP specify a harmonic output of only
better than -30dB at 1 volt (RMS). This latter voltage equates
in a 50 ohm system to +13dBm so -30dB represents -17dBm which
doesn't sound great.
The pictures below are not ideal
because the metal cover over the RF amplifier is not fitted,
neither is the outer case... and the spectrum analyser bandwidth
is set quite high at 30KHz and more. The 2nd harmonic at 500KHz
looks like it's 25dB down and the 3rd is 36dB down. The 1-2MHz
range is better as both 2nd and 3rd harmonics are almost 31dB
down.. not brilliant but just within spec. The good news is harmonics
are very much lower in ranges greater than 4MHz. Thinking about
filter design... the designers provided a decent overlap between
ranges so it's actually not just very difficult to design a filter
having a good performance within a specific range, it's impossible
to design such a filter. For example the filter for the Range
covering 16 to 32MHz cannot remove the 2nd harmonic of 16MHz
at the low end of the range without removing the desired 32MHz
element when tuning to the high end. To get around this each
range carries two filters which are selected by relays driven
by circuitry governed by a pots coupled to the main oscillator
tuning gears. In this way either a low band or a high band (and
even both) is appropriately switched into circuit. If you listen
carefully when tuning you can hear a faint click from the relays
at roughly centre frequency. This isn't the case though for the
ranges up to 8MHz where HP designers reckoned the 2nd harmonic
would be at a sufficiently low level to not bother with the added
complication. In essence, they reckoned the sinewave shape up
to 8MHz would be good enough to result in the 2nd harmonic being
better than 30dB down. |
 |
 |
 |
|
RF Output, Range 0.5 to 1MHz
at 500KHz
Oscillator tuned to 256MHz &
divided by 512 |
RF Output, Range 1 to
2MHZ, at 1MHz
Oscillator tuned to 256MHz &
divided by 256 |
RF Output, Range 2 to
4MHZ, at 2MHz
Oscillator tuned to 256MHz &
divided by 128 |
|
|
Reading the Hewlett Packard
documentation regarding the performance, but between the lines,
an interesting comment is that after changing the modulator or
its pre-amp you should check for harmonics "particularly
at 4MHz in the 4-8MHz range". The indication may be that
you shouldn't be upset to see harmonics below this range... which
is exactly the findings above. Fortunately the 2nd harmonic in
the higher ranges was not really noticeable, but if it had been
a problem the HP solution is to swap resistors to reduce signal
level in the divider chain. This applies also to the lower ranges,
but I'm not going to bother about this...
The next stage in fault-finding
is to look at the loop carrying the meter amplifiers with regard
to the RF levels. Hopefully this will
explain why the 256MHz signal set at 0dBm has a level of over
6dBm, above.
I'll just go over the way the
ALC (automatic level control) loop operates. The output amplifier,
driven by the modulator (these are the gold hybrids in the RF
compartment) puts a well defined RF level into the attenuator.
This level is +10dBm so that, for example, when the attenuator
is in the 0dBm setting and the vernier control is adjusted to
make the RF level meter read exactly 0dBm, you'll get 0dBm RF
output. That is excepting when an attenuator range of +20dBm
is selected. In that case a (different) resistor is switched
into the AGC loop that pushes the RF amplifier output up to +19dBm
giving an extra boost of almost 9dB to the RF output over that
achieved from the +10dB setting with 0dBm on the meter. |
 |
|
The ALC loop is controlled
by two voltages, Vdet and Vref, which together control the modulator
(essentially a voltage controlled attenuator) to form a steady
state closed circuit. Twiddling the Output Level Vernier or switching
the extra 10dB control (actually 9dBm), or indeed turning RF
off, will manually change the steady state. Changes in RF output
due to varying conditions (resulting for example from small changes
in RF levels whilst tuning) will automatically maintain the steady
state. Preset controls will determine the relationship between
the manual or automatic loop changes and the loop steady state.Click here or the schematic above
to read the PDF for power/meter adjustments. |
|
After thinking about the
possible reasons for the slightly high output, I decided to first
tackle the non-linearity. That is the poor frequency response
of the home-brew detector. The one embedded in the hybrid uses
DC coupling to the diode detector so will be much less frequency
sensitive than my external detector which is coupled to the output
via a 33pF capacitor. Below I've shown the effective resistance
of the 33pF capacitor compared with one of 15nF. The diode is
in series with a 200 ohm resistor which has the effect of reducing
the effect of the capacitor perhaps up to a few hundred ohms,
but once the frequency has dropped to 4MHz the capacitor has
a significant effect. The rectified voltage will be less so the
loop will then bump up the RF level into the attenuator. Choosing
a 15nF capacitor shows that the 200 ohm resistor significantly
damps out the variation. Inside the hybrid a 10nF coupling capacitor
is used to block the diode DC bias so 15nF should be a good choice
for the external detector (this was the Mk2 version). |
|
Freq MHz |
500 |
250 |
125 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
0.5 |
MHz |
|
33pF |
9.6 |
19 |
38.6 |
73 |
151 |
301 |
603 |
1206 |
2411 |
4823 |
9646 |
Ohms |
|
15nF |
0.02 |
0.04 |
0.08 |
0.17 |
0.3 |
0.6 |
1.3 |
2.6 |
5.5 |
11 |
21 |
Ohms |
|
|
With the new coupling capacitor
in place you can see a significant improvement in the RF output
level. The highest test frequency is nearly 4dB in error and
is almost certainly due to stray inductance effects which increase
the detector resistance, reducing the voltage fed into the AGC
loop and bumping up the power slightly. A chip capacitor in place
of the wire-ended type should improve this. Also notable from
the reduction in output level is a reduction in the level of
the second harmonic output. I tried a second experiment and the
output level could be brought closer to 0dBm by slightly changing
the 200 ohm series resistor. |
|
Freq MHz |
550 |
256 |
128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
0.5 |
|
O/P Level dBm |
-2.0 |
+1.75 |
+1.88 |
+1.77 |
+1.72 |
+1.75 |
+1.62 |
+1.74 |
+1.8 |
+1.8 |
+1.86 |
|
Meter dBm |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
2nd Harmonic |
-48 |
-35 |
-38 |
-42 |
-46 |
-47 |
-44 |
-41 |
-47 |
-37 |
-38 |
|
2nd H attn. |
46 |
37 |
40 |
44 |
48 |
49 |
46 |
43 |
49 |
39 |
40 |
|
|
I tried modifying my home-brew
detector to see if it could be improved. I used two 10nF chip
capacitors in parallel in place of the 15nF discrete capacitor
to reduce inductive reactance from its leads at 500MHz, and experimented
with the series resistor, settling on a 30 ohm chip because that
value fitted exactly between the new chip capacitor and the diode
detector. Initially I tried two 100 ohm chips which brought down
the level to 1.5dB then having put 30 ohms in place the difference
across the whole RF output range was less than 1dB, reading typically
+0.8dBm with the attenuator at 0dB and the vernier adjusted to
a meter value of zero dBm. Because there must be some RF leakage
I expect the discrepancy to fall slightly once the lids are back
in place. I could have tried an sm schottky diode in place of
the 1N5711 but the end result was good enough already. |
 |
The modified detector
circuit (Mk2) aimed at better frequency linearity and slightly
higher DC output to get the AGC loop output voltage to drive
the modulator hybrid such that the RF amplifier produces exactly
+10dBm at all frequencies from 450KHz to 550MHz.
The AGC loop sets the RF output
to +10dBm for all attenuator settings other than +20dBm, where
the ALC circuit is modified by changing resistors on the A1A1
pcb (bolted to the rear of the attenuator).
For the maximum output of around
+19dBm, a 10K resistor, R2 is bypassed and R3 of 28.7K replaces
a 1K resistor R4.
Below a view of the underside
of the 8640B (less the RF compartment cover)
|
|
 |
|
I was determined to get the
RF detector working as well as the original so embarked on the
Mk3 design. I'd managed to reduce the error between the meter
indication of 0dBm and RF output to within 1.8 to 2dB and decided
to obtain a more sensitive detector diode. At +10dBm the RF voltage
is about 700mV and as the highest frequency is 550MHz the diode
(CR1 below)must have a very low self capacitance and a very low
forward voltage drop (circa say 140mV). The bias voltage is set
by R22 and R23 from the 20 volt supply (see below). This works
out at about 613mV. The diode bias resistor is 1Kohm and the
swamping resistor which averages the detected voltage across
the whole frequency range is 30 ohms so the DC component of the
diode current is about 470uA. Loss through the swamping resistor
is negligible at about 14uV. The peak RF voltage is 990mV so
there isn't a lot to play with to drive the AGC loop.
A detector diode having a capacitance
of say 3pF has an impedance at 550MHz of about 100 ohms so in
terms of a 50 ohm system will be pretty leaky. I
chose a new diode having a capacitance of only 0.3pF (=1000
ohms at 550MHz). The old diode is a 1N5711 whose 2pF works out
at about 150 ohms so the new one is much better. Once in
place it's pretty obvious from its tiny size and fragility you
can't transmit into it without ruining the diode. I wonder
how many hybrids have been written off when only the detector
is damaged, because the loss of the diode=immediate loss of meter
reading which might have been assumed to be loss of RF output? |
|
|
 |
|
Left: the diode uses an
SC79 package (body 1.3mm x 0.8mm x 0.6mm thick with 0.15mm legs).It
may be OK for a machine to stick it onto a circuit board but
not easy for me to solder in place as shown below! I used the
giant 30 ohm resistor to give mechanical stability. |
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The perfect diode for
the job isn't exactly the easiest diode to fit in place. The
size of the body (SC79) in inches is 28 thou x 48 thou and the
legs 6 thou each. You can only just see the cathode band and
soldering it needs a steady hand. I used a thin silver plated
wire to connect the anode to the bias point and hopefully this
will absorb any mechanical shock. I wired it in place and switched
on... the meter read zero. Well, I wasn't too surprised because
seeing the cathode band was next to impossible even with my magnifying
goggles. After unsoldering it no less than twice and failing
twice to get the circuit to work I wondered if the cathode band
was printed on the wrong end... so soldered it in place for the
fourth time.. and success, the meter read 0dBm..... Below is
the Mk3 detector. |
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I checked the RF output
at 5MHz. With the attenuator set at 0dBm and the vernier producing
0dBm on the meter, the spectrum analyser read 0dBm. Switching
up and down proved the same power output within less than +/-0.5dB
was achieved all the way from 450KHz to 550MHz.
I screwed the RF module lid
back in place and looked again at the spec for the SMS7630-079LF.
In small print it said "Marking:anode". |
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I decided to check the
various controls now that the basic RF area is working. Much
to my surprise everything worked. I monitored output on my Icom
R7000 tuned to 120MHz. First, a nice clean carrier, then switching
the 8640B to AM I checked the two fixed modulation frequencies
followed by the variable controls and all were OK. Next FM and
this worked on all the relevant settings. I then switched the
receiver to SSB and checked the carrier. This was nice and steady
and was fine with both the Lock and the Half Digit switches doing
as they should. The meter produced lots of readings in all three
selector switch settings. I recorded lots of pictures, even checking
the second harmonic of 500MHz at 1GHz.
Time to put the top and bottom
covers in place and fit in my test gear racks for use on my next
project. |
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Answer: The Range
switch should show 16-32 not 8-16 because the knob is fitted
wrongly.
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