Looking at the AR88LF
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This very fine example
of an AR88LF was given to me by my pal, and ex-Plessey colleague,
John McGowan many years ago. Since then I'm sure it's doubled
in weight, and having finished the refurbishment (or as much
as I wanted to do) of an R109A, I decided to investigate this
receiver. My XYL and I struggled to get it from the rack in my
workshop into our conservatory then after another struggle removed
the receiver from its case. Before the case came off I'd plugged
it into the mains supply and connected a loudspeaker (I have
the proper AR88 one) and switched on. A short aerial wire was
already fastened to it and I was quite amazed when Radio 4 on
198KHz boomed from the loudspeaker, presumably as it had been
doing at the moment it was last switched off at least ten years
ago? |
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Having had previous experience
repairing an AR88, I'd been somewhat apprehensive turning
on the receiver on, but having removed the case, I found I needn't
have worried because the flat brown condenser between anode and
ground at the output valve had been snipped off. I wonder how
many deaf AR88s are out there with open circuit output transformers?
It's reported that failure of that specific condenser results
from stress when the receiver is used without a loudspeaker.
There's sufficient power developed in the output circuit that
speech can be clearly audible from transformer vibration.
The AR88 models have a good
selection of black metal valves and all these cleaned up nicely.
Sometimes left in the damp these valves can get very rusty but
all were gleaming after a rub down. In fact all the valves, including
the glass 6V6GT, 5Y3 and the 0D3, have very clear markings and
seem to be either very lightly used or have been replaced. I
noticed the output valve is the correct 6V6GT instead of a 6K6GT
(fitted in the AR88D) but the former although consuming a heater
current 50mA more and having an anode load slightly higher can
provide an extra watt of power (5.5W) at 3% less distortion (12%)
so is a bonus... but it does get extremely hot.
Recently, I was given a couple
of boxes of surplus meters and one of these will make a good
S-Meter for the AR88 as this example was never fitted with such
a thing. As I twiddled the dial on the latest receiver to be
receiving some TLC I noticed that the tuning, although mechanically
very precise, the same couldn't be said for Radio 4, whose signal
seemed to be spread over far too much of the spectrum. It was
hard to define the best place to listen to the broadcast so I
pondered over fitting an S-Meter that would help. |
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When I first tested the
AR88LF it was very crackly and the audio kept dropping to a low
level and back.. really annoying.
The RF gain control has a newish
6.8Kohm resistor grounding one end of the pot. Here you can see
a lot of solder and nicely wetted steel chassis but unfortunately
the solder hadn't quite reached the resistor wire so the joint
is dry and hence intermittent.. |
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The
commonly available document on fitting an S-Meter to an AR88
can be simplified by adding extra detail and some pictures for
the LF version. The wiring is already in place but the hole through
which the new sensitivity pot should be mounted is not
drilled in my chassis.. Why is this I wonder? Before starting
I'd recommend anyone planning to add a meter to first identify
V5 (1st IF amplifier) valveholder and familiarise with the connections
to the 6SG7.
Many AR88s are not fitted with
an S-Meter and its position is occupied instead by a yellowed
plastic panel screwed in place by 4 short US-threaded screws
(coarser than 6BA) which penetrate some way into the rear of
the front panel but not all the way through. The top two are
slightly longer because they hold in place a bracket holding
a dial lamp whose holder is pushed through a hole in the metal
bracket. The first step is to remove this bracket and the lower
screws... the latter are tricky due to limited access but can
be removed with a bit of fiddling and perhaps with the help of
pliers to loosen them. The yellow plastic panel comes off leaving
a clear plastic aperture. To fit a meter you need to make a panel
on which to mount it, then fasten the panel using the 4 original
tapped holes. My meter had mounting studs facing rearwards and,
as I didn't want to modify the meter case, had to be sandwiched
between the mounting plate and front panel. This required a set
of 4 new screws having the correct thread and long enough to
penetrate through the new mounting panel into the front panel.
The fitting instructions suggest removal of IF cans (they must
be joking) but an easier method, if you have correct nuts, is
to fit two locked nuts to the shafts of the lower screws so that
a suitable spanner can be used to drive the threads into the
front panel. To do this press your finger on the head of each
lower screw, position the start of the thread in the hole with
tweezers then gently use a spanner to rotate the screw until
its secure.
Once the meter is fitted fasten
the leads provided to the meter terminals, including any shunt
required to give you 5mA full scale deflection. The leads may
be darkened through age but the black/red coloured lead goes
to meter negative (see later for advice).
There are now three jobs remaining.
Firstly, fit a 100 ohm potentiometer to the rear panel so it's
accessible once the case is in place. Secondly, identify the
grounded lead intended for connection to the pot. This is to
be found soldered to the ground pin of V13, the 150 volt regulator
valve. Lift this off and connect to the pot. In my chassis a
spare hole, which I suspect was added by a previous owner, for
a now-missing coax socket was located on the chassis rear apron.
This proved suitable for mounting the pot, and a few inches of
wire soldered to the freed wire enabled me to reach the pot centre
pin. A second wire to one end of the pot track returned to the
regulator ground pin completed pot wiring.
The final task is to identify
two wires soldered to the first IF amplifier valve base ground
connection. One of these wires goes to the positive terminal
on the meter and the second routed to the rear apron where it's
soldered to the regulator ground pin. Lift off these two wires
and tidy up their ends prior to resoldering. Cut the ground connection
of the 100 ohm resistor fitted to the 1st IF amplifier cathode
pin, pull the resistor so it's sitting vertically then tin the
free end ready for soldering. Find a suitably-sized decoupling
capacitor (the official screed says 4700pF but I chose a 47nF
x 250v capacitor because it was a perfect physical size for the
job) and solder one end to a suitable ground connection leaving
the capacitor mounted vertically. Solder together the free end
of the 100 ohm resistor, the free end of the new capacitor and
the two wires freed from their grounding point.
You now have in place the S-Meter
and its wiring. If you're not confident or if you're not able
to recognise or identify the correct wires involved in the exercise
(for example if they're discoloured with age), don't connect
the meter until the wires are checked out. Once you've unsoldered
the three wires that were in the original harness you can buzz
out their ends. One freed from V5 Pin 1 goes to the meter positive
terminal, whilst the other goes to the end of the wire freed
from V13 Pin which should be connected to the wiper of the new
pot. The remaining wire at the meter is grounded and connects
to the meter negative terminal. Turning on the receiver should
result in a meter deflection but it's a good precaution to have
previously wound the new pot to minimum resistance before switching
on. Gradually increasing the shunting resistance should give
you a mid-scale reading before testing with a tuned signal. |
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Above.. fitting a new
S-Meter with a replica off-white tinted scale. The fixing plate
was made from a piece of scrap fairly flexible non-brittle plastic
which has the benefit of flexing and giving more leeway in fitting.
Positioning this non-official meter was tricky and I might move
its position downwards slightly by enlarging its mounting hole
to make it more symmetrical at a later date. The published scale
(thanks to N3FRQ) was made to fit the meter by measuring the
needle pivot point to the scale edge (=25.4mm) then setting the
printing size factor to make the print match this measurement.
I glued the paper to the reverse of the original meter plate
using a very thin coating of spray mount adhesive to keep it
dead flat and to avoid contact with the needle.
My choice of meter was one from
my collection. It was a centre-zero 400uA fsd instrument having
sufficient pointer adjustment, by sliding both upper and lower
positioning levers, to make it line up with the right hand scale
limit. I added a 68 ohm shunt resistor to make the fsd roughly
equal to about 5mA to match the official AR88 meter. |
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Left... the wiring around
V5 after modifications and above... the new potentiometer fitted
in a surplus hole. Avoid using any hole provided for reaching
trimmers etc. |
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Now that I've fitted a working
S-Meter I can clearly see several major problems with the receiver.
Tuning across a strong broadcast it occupies far too much bandwidth with
several woolly peaks due to poor IF alignment and meter deflection
seems to have little bearing on tuning. The RF gain control setting
has an odd effect on both recovered audio and the S-Meter reading.
With a long wire aerial, broadcasts are roughly on the correct
dial markings but, as the control is rotated to increase gain,
an initial increase in volume is followed by a quietening then
blanking out of audio. The S-Meter reading rises then reverses,
dropping back again. Clearly AVC action is almost completely
absent on one or more stages and the huge gain from the AR88
circuitry massively overloads the later stages. First I'll examine
resistor values, then replace most if not all decoupling condensers.
A pound to a penny most of the bathtub condensers (eg above left)
are duff! |
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Hopefully.. now that the S-Meter
is present I shall be able to monitor the results of repairs
and tweaks. Using my high impedance meter and tuned to Radio
4 on 198KHz and looking at the AVC line.. starting at V8 where
the incoming IF signal is fed into a diode cathode and rectified
at the diode anode Pin 8, I can see minus 15.3 volts, which is
decoupled at C48 and routed through a pair of 560Kohm resistors,
R23 & R27 to the 1st and 2nd IF amplifiers V5 & V6. The
AVC bias is also routed via a 100Kohm resistor R9 to the pair
of RF amplifier stages V1 & V2, where it's further decoupled
by C47, 4700pF. Checking the grid bias voltages at V1 & V2
showed around minus 13 volts which isn't too bad. However, checking
the control grids of V5 & V6 indicated all was not well as
these were sitting at minus 3.9 volts and minus 2.9 volts respectively
showing a leak equivalent to some 12 volts for both.V5 &
V6 grids are decoupled by C76 and C93, which oddly are the top
and bottom condensers in the bathtub in the picture above left,
and only millimeters from the new S-Meter wiring.
I unsoldered C76 & C93 and
temporarily soldered in place a pair of new 0.1uF plastic capacitors
then switched on the receiver. Radio 4 came on smoothly and remained
perfectly clean no matter what the RF gain setting. With the
RF gain at maximum, the S-Meter moved smoothly up to a maximum
of almost 80dB (I am using a 350 foot long wire) and back down
again as I tuned across the station and the bandwidth control
roughly worked when I switched it to its different settings.
The receiver was transformed, with the extra wide bandwidth,
multiple tuning humps, backwards reading S-Meter and audio distortion/blanking
gone. I again tested the AVC voltage and found it's now within
about a volt at all the points in its circuit and the S-Meter
works perfectly. But.. there are a further 16 bathtub condensers
left.... |
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This is a picture showing
the main components in the AVC circuitry, missing out RF components
etc.
V1 and V2 are RF amplifiers,
V5 and V6 IF amplifiers and V8 the AVC rectifier.
The condensers are located within
two 3-section bathtubs. Resistor values are not particularly
critical, neither are condenser values as long as these do not
leak current.
Note that original AR88
schematics use the letter "M" for Kohm with "Meg"
for Mohms. |
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Now for a computer problem.
I decided to tackle the IF alignment of the AR88LF so moved my
Wavetek signal generator into the conservatory where I'd decided
to work during the spell of nice weather and not shut myself
away in the workshop and my XYL. I'll just check the IF before
I get underway... but my XYL is busy using our computer and my
battery charger for the laptop disappeared after a tidy up of
the office... but no worries as I have the AR88D instruction
manual and 455KHz looks fine. I connected the Wavetek to the
aerial tag and set it to 10mV so it would get through any input
rejector. I heard a slight hiss but no sign of the modulation
tone so I upped the input to 100mV and the hiss increased in
volume and a slight tone in Waveband 2 which seemed to tune which
didn't feel so good as the IF input signal might change slightly
but shouldn't tune sharply as this did, so I switched to Waveband
1, the LF range... the hiss was now present and no tone so I
increased the input to 1000mV.. then tuned the Wavetek up and
down. This takes ages because you really need to push the up/down
buttons a lot and moving in my TF2008 would be awkward as space
is limited.. No luck so I moved the input lead to the mixer grid.
I could now hear a tone but nothing like as strong as it should
be. Eventually my XYL vacated our computer and I checked my own
website in the listing for IFs which I uploaded back in 2000
and discovered the IF was 735KHz. http://www.radiomuseum.co.uk/commsifs.html
Reconnecting the Wavetek to
the aerial and reducing the input to 1mV at 735KHz worked a treat
and showed me the IF alignment was hopeless with the various
bandwidth settings giving widely varying results. After some
twiddling I realised alignment is not straightforward so I'll
leave that to the spectrum analyser after moving the receiver
back into the workshop. The reason for the complication is that
some bandwidth settings use a crystal to help shape narrow IF
filtering whilst wider settings do not so that it's important
to base overall IF alignment on the crystal characteristics rather
than a precise figure (in this case)of 735KHz. If this isn't
done you'll find that changing bandwidth settings results in
signals being off-tune.
In the meantime I'll sort out
the bathtub condensers. A quick check with an ohmeter told me
that in most cases the outer condensers are around 1Mohm and
the centre several Mohms so I'll need to swap the lot. I'm a
little unsure whether the 0.01uF examples can be changed to 0.1uF
without degrading or altering performance. For example, changing
the AVC response? A resistance of 2Mohm and a decoupler of 0.01uF
gives 20mS and 0.1uF will give 200mS so in practice, with a swing
of around 10 volts it shouldn't really matter. |
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The picture above shows
three pairs of 100nF capacitors and on the left three single
capacitors. These are rated at 500v working. Initial tests showed
resistance reading around 1.5Mohm each and with the new capacitors
these resistances were basically not measurable or equal to the
resistance of surrounding circuitry. Common earth points are
made by adding a solder tag under a bathtub screw. |
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Here's a couple of Canadian
bathtub condensers all of whose sections are used for decoupling
. Top left a 3 x 0.25uF x 400v working (C99=audio amp bias, C112=audio
amp screen, C113= audio amp anode supply) and below this a 3
x 0.1uF x 400v working (C56=mixer screen, C76=1st IF amp AVC,
C93=2nd IF amp AVC). See the simplified sketch above.
I was going to open these and
replace the innards but they may be filled with a dodgy type
of oil and the larger started to leak when I applied a large
soldering iron to the back where a plate is soldered in position.so
I decided to instead use surface mount capacitors. I used two
different methods as you can see. It wasn't easy because the
wiring which is cotton over rubber is perished. Luckily not to
the extent it needs replacing, but its very stiff because the
rubber has hardened. Removing the end of a wire reveals corroded
strands which fortunately respond to wetting with a hot iron
and tin/lead solder. During this process the rubber melts and
the results looks messy but is perfectly sound. I used a junkbox
bathtub whose three section all read about 250pF in place of
the badly leaking one (to keep some semblance of authenticity!)
A quick test then showed Radio
4 LW now at +84dB on the S-Meter, a 6dB improvement. |
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I treated this 3 x 0.1uF 400v
leaky bathtub like the first, using its tags to mount the grounded
ends of the capacitors (C79=1st IF amp anode supply, C84=BFO
anode supply, C92=2nd IFT bandpass).
Manipulating wiring is difficult because
it's very stiff and brittle with hardened and cracked rubber
insulation. Fortunately the cotton covering keeps the wiring
from developing shorts.
Radio 4 is now at nearly 90dB on the
S-Meter and the shortwave broadcast bands are full of very strong
stations. The BFO centre position has moved slightly from 735KHz
due to the new C84 capacitor. |
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I completed the swapping
of new surface mount chips for the bathtub condensers. As I bought
a vast number of 100nF x 500v of these it seemed logical to change
everything up to 0.1uF with single chips and use a couple in
parallel for the few 0.25uF condensers. The first three bathtub
swaps were fine then I swapped out the third and everything was
still fine, but after changing the final couple the IF strip
burst into oscillation. One set was 3 x 0.1uF so I left these
but the other was 3 x 0.05uF so I added a second capacitor in
series with each and the oscillation stopped. Presumably the
overall gain has oustripped shielding with the extra decoupling?
The set was working after swapping
the block containing C68=? , C109 and C110= in parallel, decoupling
noise limiter circuitry.
Oscillation started after the
final pair had been swapped. One block carries 3 x 0.1uF but
the other 3 x 0.05uF includes two decouplers that explain the
oscillation, being a cathode decoupler for V7, the third IF amplifier
and its screen grid. Both would contribute to extra IF gain and
to make matters worse this stage is self-biased. C71= g2 for
V5 & V6, C95= anode supply V6, and C102 decouples the band-pass
filter between V6 and V7. The other has C103=g2 for V7, C106=
cathode of V7 and C107=anode supply for V7.
During testing after bathtub
condenser replacement the S-Meter stopped working. The reason
was that Radio 4 was now so strong that the meter reached over
90dB on its scale and the needle jammed because the extra paper
thickness had cancelled the minute clearance between needle and
scale. I had to detach the meter, open it up and bend the pointer
away from the scale. Once done and refitted I changed the 68
ohm shunt and fitted an 82 ohm shunt which allowed me to set
the pointer at zero reading for noise-free reception (I selected
a frequency near 30MHz for this.
Now, tuning Radio 4 gives me
something over 80dB (In terms of the AR88 S-Meter 80dB over 1uV,
or if S9=100uV +80 represents S9+60dB) which represents almost
zero current through V5, the 1st IF amplifier.
The next stage will be to accurately
align the IF amplifier which means carrying the AR88 back to
the workshop, although very slightly lighter less its outer case. |
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Results of IF alignment
are shown here; BW1 to BW5. I'd suspected the rough method using
an audio power meter had resulted in a dodgy response and sure
enough the wider bandwidths were skewed about 5KHz HF of the
crystal frequency. It took a lot of adjusting to move the response
downwards to match the crystal, but now the bandwidth can be
narrowed without having to retune the signal.
These scans of BW1 to BW5 have
a 200KHz span (20KHz per vertical division) so that you can see
the skirts of the responses close to -70dB.
Below I've shown the curves
for BW1 and BW2 (which I use for comfortable listening) with
a narrower span where each vertical division represents 5KHz.
The user handbook has curves
shown to -30dB and for the widest setting (BW1) has +/-21KHz
with the narrowest (BW5) +/-6KHz. The curves which I measured
show +/-12KHz and +/-5KHz respectively so are nicely in spec. |
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Click to see a larger picture.
I don't know why this official picture
is upside down from convention. Presumably the graph needs to
have a zero reference although the explanation is simple: the
axis title "Times Normal Input" means the same thing
as attenuation in terms of the resultant curves as long as AVC
is disabled.
The scales shown on the vertical axis
are logarithmic with wider spacing for low numbers and smaller
for high numbers. Usually these sort of scales are rationalised
as in the spectrum analyser pictures above to show even spacing.
10, 100, 1,000 and 10,000 are the same as -10, -20, -30 and -40dB
and early test equipment would not have been able to work at
levels that are common nowadays. The DSA815TG shows noise at
-70dB indicating the huge improvements made since the 1940s.
It's interesting to note that the official
spec here is a drawing and as such will be a somewhat theoretical
indication, hence the perfect symmetry compared with the real
curves shown above.
The notes about the crystal indicate
that this is not in circuit for the two wider bandwidth ranges
and does not mean the crystal should be unplugged. |
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I've noticed that the
audio sounds very clean no doubt helped by the 6V6 operating
in Class A. Listening to Radio 4 on the AR88 and on a digital
radio there's no comparison. The AR88 sounds a lot nicer than
the rather muffled digital receiver. The circuitry around the
audio amplifier is quite interesting. The designers used oil-paper
decoupling condensers throughout, but using lozenge-shaped mica
condesers elsewhere. There are two 4700pF of these mica condensers
in parallel connecting the audio amplifier to the 6V6GT output
valve and appear to be leak-free as the 6V6 control grid is solidly
negative at minus 16 volts. If you look at the impedance of this
pair of condensers and compare this with the 6V6 330Kohm grid
leak there's a considerable loss of fidelity of some 30% at 50Hz.
This however is dealt with by shunting the 6V6 grid with a 560pF
mica condenser and, a tone control using a 4700pF mica. These
additional parts tend to restore some audio fidelity. Another
component was also added, this is C119, a 0.003uF-1000v oil-filled,
or 2700pF-mica (depending on which issue of the documentation
you have) which again acts to top cut the audio and help with
fidelity, but because this is prone to failure it is often cut,
as I determined when I first began this exercise. Without this
extra top cut you will not experience the designer's intended
results, however.. as one gets older your ears will compensate
and give you the desired top cut automatically.. so you don't
really need C119 after all once you turn a certain age... |
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Before replacing the RF
screens and putting the chassis back into its case I decided
to remove the new decoupling capacitors that seemed to be the
source of IF instability and put back the connections to the
bathtub. That done the IF instability went away. |
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IN PROGRESS |
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