This equipment provides coverage
from 10KHz to 550MHz via step up and step down push buttons or
direct key entry.
It was relatively cheap to buy
(under £200 in 2002 when it was 11 years old) considering
the build quality and performance.
Below is a key to the controls.
It's quite user friendly but it needed a handbook to let me know
most of the functions. When the power is turned on (button 1)
the generator runs through a self test routine which gets more
worrying as the equipment ages. I've had two problems so far.
One, immediately after buying it and turning it on for the first
time, which turned out to be a short-circuit protection diode
and the second quite recently which was again the loss of an
internal power rail. This was easily fixed although the self
test gave a strange fault report which I imagine was because
the guys writing the test firmware must have imagined the power
supplies would never fail?
The two most frequently used functions
are setting the frequency and setting its level.
Direct entry via the keypad "3"
is the most useful way of doing either but the up and down arrows
"4" can make incremental changes, for example to determine
the tuning peak of a circuit.
For example: pressing Freq, 465 then
KHz will set the generator to 465KHz. Pressing Lvl, 5 then uV
will set the output to 5 microVolts. Once this has been done
you can set the output to CW "Mod Off", AM, or FM setting
the modulation depth as required. The button marked "5"
switches the output on or off and "Rate" sets the modulation
to either 400Hz or 1KHz.
All perfectly logical. Also, there's
a menu feature which lets you set global parameters such as the
A store feature lets you keep routinely
used output settings.
Although synthesised generators
are jolly stable and accurate they are not always as useful as
an old fashioned analogue equipment when aligning a radio.
In the case where one needs
to tune a generator to a radio for example an analogue type is
just twiddled until a signal appears but a synthesised type often
is tricky to use. I've often listened for clicking noises which
is the generator locking to its frequency and increasing or decreasing
the settings until the clicking gets louder and louder. Once
you're hearing fairly loud clicks you can increase the frequency
resolution until the clicking changes to a signal, a bit messy.
I was testing my DST100 receiver the Wavetech broke down. It
took me a little time to discover the fact because the receiver
has a few intermittent faults. When AM was selected the received
signal kept cutting out for brief moments before re-appearing
almost instantly, but connecting the generator to an oscilloscope
showed it was the Wavetech not the receiver. RF output was OK
but switching on modulation resulted in an intermittent RF signal.
The manual points to modulation problems being in a specific
area, that of the AM/DIV module so that's where I started. After
removing two dozen screws the top cover can be detached revealing
at the side of the unit the view below.
The AM/DIV module has two circuit
boards mounted in an aluminium sleeve which can be removed after
unplugging various RF plumbing, including one at the rear of
the unit connecting to a power amplifier module. At each end
of the sleeve you need to remove the end plates by taking out
their four fixing screws plus unscrewing the nuts securing the
Below, pictures of the circuit
board carrying a lot of the final RF circuitry. Modulation is
carried out using a mathematical process controlled by one of
the units two microprocessors, but as with all the circuit boards
in the signal generator local power supplies are fitted in order
to provide accurate voltages. The top view does have a clue to
the fault I'm investigating but the underside view reveals clearly
Here's the fault: three
dry joints at the pins of a power regulator transistor Q905 an
MJE253. Although the designers included a hole for fitting a
heatsink, this wasn't fitted so after replacing the solder I
fitted a small brass bush which should help keep the transistor
temperature down a little. In fact, as I tracked down faults
later on I found the overheating was due to chemistry. Everyone
knows about Mullard AF117 problems but I for one hadn't heard
about Motorola transistor problems. Read on....
Back in business after
the Wavetech had gone successfully through its diagnostic tests.
After using the Wavetech on and off for a week or so I noticed
a sizzling noise on the output. I looked again at the repaired
output board and found another dry joint and a discoloured resistor.
I resoldered the joint and fitted a new resistor. After assembly
the sizzling noise was still present. The equipment cannot be
tested by usual means so I decided to detach all the circuit
boards and apply 18 volts to each. This is the standard supply
to all the boards and on each one a 15 volt regulator provided
on-board power. Starting with the output board I noticed the
new 68 ohm resistor I'd fitted was very dark in colour and sure
enough, powering the board showed the resistor was getting very
warm. The 18 volt current was over 400mA and across the 68 ohm
resistor I measured about 11 volts. This represents a power dissipation
of about 1.75watts although the handbook states it's a half watt
component so something is wrong. The chief feed from the resistor
is the collector of an MRF571 RF transistor and this measured
as a pair of diodes on my transistor tester. I substituted a
BFR90 for the MRF571 and the resistor ran cool with 11 volts
on the transistor collector intead of the previous 4 volts, but
after reassembling everything there was still a sizzling noise
on the RF output. Maybe the transistor had failed due to a problem
with its components and this is still giving trouble?
A little more about this
saga. Having diagnosed a faulty drive transistor I eventually
decided to splash out and buy a proper MRF571 (see above) because
the output wasn't quite right. It was difficult to put one's
finger on the problem, but using a scope I determined the RF
was jumping around amplitude-wise and frequency-wise. I found
a source of the rare device and ordered it. In fact it came as
a set of six transistors from Poland although I'll probably never
use the other five. After dismantling the assemblies I fitted
the new transistor and the RF problem seemed to have cleared
up... or had it?
Above right to left...driver
transistor Q900, with Q901 bias set for Q900, Q905 current regulator
for Q902, and RF output Q902. Q903 sets the bias voltage for
Q902. Below Q905 (centre) is a voltage reference diode LM336Z-2.5.
The MRF839F is a variant of an MRF839
fitted to a Type 319-07 base. This is a view of the underside
showing the heatsink.
After replacing the temporary
BFR90 with a new MRF571 it turned
out the problem was still present albeit not as bad as before.
Why was it that sometimes the RF was as clean as a whistle and
at other times infuratingly crackly? By experimenting I discovered
that any RF output greater than 137.5Mz was faultless but below
that I found the RF was sort of wobbly. Time to re-read the repair
manual and study the circuit diagrams. This explained the reason
for the transition frequency. Above the frequency of 137.5MHz
the RF amplifiers, feeding the output socket via a programmable
attenuator, are driven directly from a VFO but below that a mixer
is involved. This mixer combines a second RF signal of precisely
512MHz with the VFO to derive any frequency from 10KHz to 137.5MHz.
This being so, any problem in the mixer circuit or in the generation
of the fixed frequency of 512MHz could be responsible for the
As the board carrying the mixer
was already on the bench I checked this first. Looking at the
mixer circuitry (below with shield removed) showed that it was
possible for a faulty component in the selection process to result
in a bad RF output from the mixer. For example a bad capacitor
in the filtered connections from the microprocessor outputs might
be responsible. For example if the mixer was being turned on/off
by noise on the selector line this might show up as a crackle
on the mixer output. With difficulty I removed a metal cover
from the mixer circuitry but everything seemed to be OK.
Time to study the repair manual
circuits once more. These are poorly scanned in my copy making
it a time consuming job to identify the parts (not to mention
the use of a crossed zero making a nought look like an eight).
Before tackling the 512MHz oscillator which is on another circuit
board I looked at the RF amplifier transistors. I'd already replaced
the driver transistor because it had a leak of a couple of hundred
ohms between its collector and base so I looked at the output
device. This is Q902, an MRF839F fitted on a heatsink. It has
an odd-looking base listed as Type 319-07 and I'd noticed several
years ago when I had a problem with crackly RF output that the
bias circuitry had been running very hot leaving the Q905 power
transistor dry-jointed. I'd resoldered it and for a short time
all seemed well, but why had this happened?
The designers of the equipment
surely would not make the basic error of not fitting a heatsink
to the transistor? Maybe this overheating is related to my current
RF instability problem? My circuit schematic is pretty poor but
I eventually reproduced the circuit on a sheet of paper. The
manual tells me the bias circuit performs two functions... to
set the operating point for the MRF839F and to set its collector
I applied an 18 volt supply
to the board and measured the various points in the circuit.
The RF power transistor collector was sitting at 5.5 volts and
I couldn't figure out why, so began to unsolder components. All
the capacitors were fine and all the resistors were OK. Maybe
the bias setting transistor was duff? I lifted off its connections
to base and emitter and found absolutely no change... very odd.
At this point the penny dropped. The MRF839F was faulty. With
some difficulty I removed the transistor and discovered it had
the same fault as the driver transistor. There was a leak between
the base and collector. It measured 284 ohms and this resistance
didn't look like a diode because its value was exactly the same
in both directions when checked with an ohm-meter. The 284 ohm
leak was turning on the transistor through self-biasing and pulling
down its collector voltage. Q902's resulting collector current
of around 300mA through Q905 together with the latter's increased
emitter-collector voltage was resulting in a dissipation in Q905
of around 3 watts and because there isn't a heatsink Q905 was
getting really hot.
Even though Q902 had a bad leak,
the balancing circuitry in the Wavetek was managing to maintain
the correct RF output to the attenuator, but was the leak responsible
for the crackle in the RF. Perhaps the output from the mixer
was lower than that from the straight-through VFO thus pushing
up the demands on Q902?
Before proceeding further, I
fitted a 2N3866 in place of the MRF839F to test the theory. The
collector voltage correctly measured as 12 volts but that type
of transistor doesn't fit easily due to space considerations
in the metal case for the circuit board and a 2N3866 is anyway
not capable of running enough power output.
A search of the Internet revealed
the MNRF839F was a pretty rare beast with only two far-east suppliers
with stock and a UK source quoting over £55 plus postage
and VAT. I commenced the ordering process for one of the cheaper
options but found that delivery was quoted at 20 to 40 days by
airmail. Maybe it's pigeon post? I didn't place the order as
I would not be home at the delivery date and I didn't fancy supplying
credit card information to a Chinese company.
Vce 16V, Vcb 36V, Veb 3.5V, Ic 0.6A
Dissipation Pd 10W @ 110 deg C
RF out 3W 806 to 960MHz
This and its stablemate the MRF371 both
degraded during their lifetimes by developing a leak between
base and collector. Maybe this is due to tin-whiskers which grow
from the material inside the case over a long period eventually
Back in the workshop I
carefully removed the MRF839F and checked it with a transistor
tester... it read "two diodes". Now.. many years ago
back in the 1970s I had a really expensive RF power transistor
with a similar fault. Our Plessey rep (also a radio ham) had
given me a sample of their latest VHF power transistor... a huge
device worth a huge sum of money. I made a 2m linear amplifier
from it but during experiments I'd killed it. To cut a long story
short I mended it.
Can I repair my MRF839F? As
far as measurements are concerned there's a fixed value resistor
between the base and collector. This had been the exact same
fault with my exotic free sample. My reasoning was that being
an NPN device I could connect a negative voltage between base
and collector and fuse open the short because the device can
tolerate quite a high Vcb without damage. I set up my power supply
and connected the positive output to the collector pin and the
negative supply to the base... set the current limit to 50mA
and gradually increased the supply voltage. All that happened
was an increasing current then constant drain of 49mA from 13
to 18 volts. I adjusted the current limit to 500mA and tried
again. This time, once past about 12 volts the current dropped
then rose again then as I increased the voltage it suddenly dropped
Maybe it worked or maybe I'd
fused a connection to an electrode? I connected my transistor
tester and it said "NPN transistor", "gain 17"...
so I removed the 2N3866 and soldered back the MRF839F. I applied
the 18 volt supply and the collector voltage measured 12 volts.
Feeling fairly confident I reassembled
the Wavetek and turned it on. After a major repair job it's very
comforting to see the self-diagnosis to report all is well. I
turned on my monitor receiver (Icom IC7000) and set the Wavetek
to 137.5MHz. Perfect.. a rock solid carrier at 137.500MHz.
I detuned the Wavetek to 137.4MHz
and tuned the receiver... drat.. the rock steady carrier was
now warbling. Turning from SSB to AM, I could hear a sort of
crackly sound so the duff transistor wasn't the reason for the
fault after all. I suppose the Wavetek checks the RF level and
cranks up the internal RF levels to compensate for a ropey amplifier
transistor so even with a failing part the output is guaranteed..
obviously up to a point.
The noise on the received signal
seems slightly less than before, but then again there was never
any noise above 137.5MHz, but I'd assumed that the mixing process
increased the stress on the MRF879F maybe because of reduced
RF input level, but no.. the problem must be in the circuitry
associated with the incoming heterodyne frequency (the local
oscillator feeding the mixer)? Checking the manual, I see this
oscillator runs on a fixed frequency of 512MHz and is derived
on the LO/REF board so that's next on the list...
At least the much more complex
VFO RF signal is OK becauseanything above 137.5MHz, where this
is used without the 512MHz local oscillator, is rock steady.
The local oscillator is not
just a simple crystal oscillator multiplied up because this might
result in lots of spurious images (harmonics etc), but instead
a 512MHz phase-locked loop oscillator whose output is divided
down and referenced to an internal stable source of 10MHz. However,
being a phase-locked loop means any solder joint or dodgy component
in the loop (or its power supply) might result in an intermittent
lock problem. The scratchy crackly noise when listening on AM
suggests a dry solder joint or perhaps a bad capacitor. I might
add that the problem isn't severe enough to result in loss of
lock for long enough for this to be detected, but of course it's
not nice to hear a scratchy crackly test signal when aligning
There seems to be a couple of
options available before removing the circuit board. First I
can unscrew the RF cable carrying the 512MHz local oscillator
and see if what emerges is crackly and secondly check the internal
reference output which is 10MHz and see if that is crackly. So
Well I pulled down the Wavetek
from its shelf and before pulling the LO/REF board, I first checked
the 10MHz reference signal which is supplied via a BNC connector
on the rear panel and found it was rock solid (in fact it is
derived from a 10MHz crystal although it might have been a bad
component in its circuitry) so next I'll check the LO signal
itself. I unscrewed the coaxial copper link carrying the LO signal
and decided first to inject a 512MHz signal from my other decent
signal generator into the mixer board. I turned on my Marconi
TF2008 but I noticed the dial was marked up to only 510MHz or
2MHz short (Sod's law), so instead I set the TF2008 initially
to 170.666MHz and then to 256MHz. Both resulted in a cleanish
512MHz AM signal on my Icom monitor receiver although I suspect
this receiver similarly uses 512MHz in its circuitry so although
I got a clash and some heterodynes the RF output signal was crackle-free.
This then clears the output board with its mixer and points definitely
to a noisy 512MHz local oscillator. I connected the 512MHz output
from the LO/REF board to my monitor receiver and heard similar
heterodynes as before, but I could also hear the dreaded background
crackling so I switched off, unplugged the module and extracted
the LO/REF board shown below. This is neatly laid out but a quick
examination revealed no obvious dry joints or burnt parts.
Above, the LO/REF board:
Top right is the 512MHz RF output and below this the reference
oscillator circuitry. On the left of the board are the phase
lock loop logic chips with some microprocessor-driven control
circuitry. Below these chips are the three on-board power supply
regulators. Turning the board over reveals a large selection
of surface-mounted parts, some quite exotic but including a pair
of tantalum chip electrolytics.
I noticed some discolouration
around the RF output socket.. shown below. Soldering can often
look messy when seen close-up but dry joints, apart from really
bad ones, can only be seen with a strong magnifier. That discolouration
around the 512MHz output socket looks like a candidate for noise
if it's conductive.... but nothing looked unusual when I checked
with an ohm meter so I just cleaned it off.. another red herring.
The next step was to power
the board and see if there were any clues to crackling. I connected
three power supplies, 18V, 8V and -18V. which are stabilized
by three regulator chips to 15V, 5V and -15V. Each supply line
drew a nominal amount and some parts were warm but not hot. I
looked for the phase locked loop and found this on the underside
of the board in the form of a number of tiny chip transistors
of which the oscillator is marked "R33", a microwave
oscillator transistor type NE68133-T1B rated in the upper GHz
plus two others (BSS88C) which are used for biasing and switching
the oscillator on and off. The R33 uses striplines formed by
the printed circuit and various chip components plus a pair of
10uF 25V electrolytics paralleled with low inductance chip capacitors,
presumably for bypassing local noise on the +/- 15 volt power
The 512MHz phase-locked oscillator transistor
NE68133-T1B which is equivalent to a
Pt dissipation 200mW
Marked as "R33"
Listening on 512MHz indicated
the oscillator was turned off, but by adding a 1K resistor connecting
the OSC on/off switch line going to pin 13 of IC604 (a 74HC574N
latch) to 5 volts I was able to turn it on. Again, I had a problem
because the monitor receiver uses an internal 512MHz oscillator,
but at least I was able to detect that the oscillator could be
turned on and off and, more importantly, I could hear that the
signal wasn't too clean so I looked for anything that would give
me a clue to the crackly output.
I tried an oscilloscope at various
points but its bandwidth was miles too poor to see 512MHz although
I could see some intermittency as the trace kept intermittently
bobbing around. Before I fired up my spectrum analyser which
goes up to 1.5GHz I tried my multimeter turned to volts. Nothing
much except I found both emitter and collector connections to
the R33 (at the marked ends of two 10uF capacitors) each intermittently
varied by up to ten millivolts. Measured volts would jump around
say 3.72V.. 3.68V...3.81V.. etc so I tried moving outwards to
the other ends of the emitter and collector feed resistors (R626
and R618) and found these voltages were rock solid. Dabbing R33
with my finger produced much the same effects only worse as the
circuit attempted to maintain lock. During testing the emitter
voltage was about minus 4 volts and the collector about plus
9 volts although the two capacitors, C656 and C649 (the other
ends of which are grounded) were each connected the same way
round. This is a bit odd. As the capacitors are surface-mounted
tantalum types they carry a stripe to indicate polarity (similar
to that on a surface-mounted diode cathode) however, the capacitor
stripe marks the positive connection, so clearly, although C649
is OK, C656 is back-to-front.
Tantalum capacitors are marked
with a stripe for their anode connection so this has to be connected
to a positive voltage.
I measured the capacitor with an ESR
tester which gave 9.4uF with a resistance of around 0.5 ohm although
this figure kept changing by up to 0.2 ohms or so each time I
Next I connected a variable current-limited
voltage across it monitored by my multi-meter switched to mA.
With the striped end positive and a voltage of 1 volt the current
through it read at the limit set by the power supply which is
wrong for a good capacitor. As the voltage and current limit
were increased the capacitor failed (changing to a resistor)
at around 5 volts. Rechecking on the ESR meter showed the capacitor
was now defunct.
This example (C656 on the schematic)
degraded because it was decoupling a negative voltage connected
to its anode.
Checking with a multimeter, with
the oscillator off its emitter is some 12 volts negative and
when on is about 4 volts negative. This means that when the output
frequency is greater than 137.5MHz, although current limited
to an extremely low value, C656 carries 12 volts with the supply
negative connected to the capacitor anode. I removed C656 and
fitted a 10uF electrolytic. The emitter and collector voltages
were now rock steady. Was the back-to-front tantalum capacitor
a fault in design or assembly and for how long did the capacitor
last before starting to fail? Alternatively, could the capacitor
have degraded to the extent it drew DC current resulting in a
change in the operation of the bias transistor circuitry and
polarity reversal across itself?
Because I wasn't sure exactly
what the true voltages would be when the equipment was reassembled
I decided to use two tiny 22uF 16volt electrolytics series-connected
as a non-polarised pair in place of C656. This solves the problem
of the voltage changing from positive to negative and back once
the equipment is fully reassembled and with the mixer in/out/in
service. The clearance between the metal sleeve and the circuit
board dictated the maximum diameter of the new capacitors as
no greater than 4mm.
I fitted the LO/REF board in
its sleeve and reassembled the rest of the Wavetek and turned
it on. I could hear the 512MHz signal turning on when 137MHz
was selected then off when 138MHz was selected. The good news
was that when I listened to both the 137 and the 138MHz signals
at 1mV output they registered at exactly the same level on the
monitor receiver S-meter and both were perfectly stable with
Above you can see from
the button markings the various features. Readings can be swapped
between volts and dB. Changing frequency can be done either by
direct entry eg. Freq, then 110, then a press of the KHz button.
Output by pressing Lvl, then 1000, then pressing the mV button.
Altering frequency is achieved
by pressing Freq which activates a cursor under the frequency
display. The cursor can then be moved by the left and right Cursor
buttons to rest under the desired significant figure which can
then be increased or decreased using the up and down cursor buttons.
The same facility is available for output level or modulation.
Minimum output is 0.1uV and the equipment is screened enough
for this level to be usable.