Overhaul of an R1155
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A few days ago this example
of an R1155 arrived for some work. It seems removal of cobwebs
after a very long period of dormancy had disturbed something
vital. From the outside this old receiver looks typical
of a surplus wartime set that has been worked on by a radio ham
and in fact silent key G3PNV was responsible. |
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The first thing an R1155
devotee will notice is the strange tuning knob as well as more
common additions such as an S-Meter, mains socket etc, but a
view of the inside reveals more changes much less common. I can
see eleven valves including an EF50 and a couple of B7G based
valves as well as what looks like a full wave rectifier. Clearly
a basic R1155 circuit diagram will be of limited use as proper
examples use only octal valves. Also visible (and wired up) are
the original can-based condensers. That socket adjacent to the
aerial socket looks a bit odd and after some later investigation
I think its for a converter, either for say the 10/15m bands
or for 2m. My R206 was pressed into service with a nuvistor converter
for the 2m band about the same time G3PNV was using this receiver. |
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The first thing to tackle
is the dial. The perspex cover is completely obscuring the dial
markings and I decided that this needs to be removed and cleaned.
Normally this is a straightforward job but in this example the
task is very tricky. I've shown below the mechanism revealed
after prising off the replacement slow motion drive. The drive
is a version of a Muirhead type common in lots of WW2 equipments
but in this instance it's difficult to see exactly how it was
fitted to the R1155. Below is shown the pointer which is covered
by the steel ring marked 10A/12684. The brass fitting is home-made
and is clamped to the end of the tuning condenser shaft which
has been cut short by an inch or so. Below the brass part is
a piece of fibre material used to clamp in place the Muirhead
mechanism. The clamp is secured to the chassis by a long 8BA
screw. The only way to remove the Muirhead mechanism was by prising
it off as its fixing screw is inaccessible. Of course refitting
it after cleaning everything will be a puzzle. |
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Above you can see the
dial markings have been modified by erasing two sets of numbers
corresponding to two wavebands. The range 75-200KHz is painted
over as are markings for the Trawler band. Without getting the
set to work I can't say whether these wavebands have been fitted
with alternative coils or just disabled in some way. |
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With the cleaned-up perspex
in place the dial is now legible but I need to re-fix the slow
motion drive (below). The pointer is secured to a brass bush
clamped to the truncated tuning condenser shaft by a concealed
screw. This is almost completely inaccessible but has to be tight
and with the brass bush positioned to allow free movement of
the pointer from its exact start and end positions. It seems
the pointer has to be fixed securely before the panel is fitted
or a hole drilled in the chassis....
Below is a description of the
Muirhead Drive with tips on dismantling and reassembly.
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Above, upper right, you
can see the fibre material part that's screwed to the chassis
for clamping the mechanism. The reverse is shown on the right.
The problem is how to fix the assembly to the brass drive shaft
protruding from the pointer. The fixing screw for the pointer
bush and the fixing screw for the drive are sunken below the
front panel. How this was achieved by the late G3PNV must remain
a mystery. A specially angled screwdriver might be OK for the
pointer bush but other than trial and error to set the screw
that secures the drive to the brass pointer bush then jamming
it into place or perhaps bending the whole mechanism away from
the chassis and using a very thin screwdriver the only way I
can see to do it is by drilling holes through the lower edge
of the front panel and then the chassis. Maybe I could drill
the black plastic part of the dial as I'm fairly sure some of
these Muirhead drives have a screw access hole? This one doesn't...
Those two pointer knobs are
from two extra pots labelled RF Osc and RF gain. Both of these
pots needed to be detched to allow the panel to be removed. |
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I think I managed to fit
the drive back together OK?One reason it proved difficult was
the square brass part to which the knob attaches was stuck in
place with dried lubricant and I hadn't realised it just pulled
off. The centre disks were also gummed together unlike the top
couple which have their disks riveted together.
In order to fit the parts you
need to simultaneously engage the three disk pairs with the brass
centre part (E,F,G,H) and locate the bearing holes top and bottom
(A,B,C,D) for the outer disk pairs. To do this you need to keep
the upper ring slackly in place (nuts I plus three others)to
allow some jiggling. Screws J,K,L need to be fairly slack to
allow the disk pairs to be slightly separated so that the two
upper disks can be engaged. You'll notice that one upper disk
is fairly easy to locate but the other isn't. The way around
this is to open the lower disk pair using a plastic rod with
a flattened end. If this is jammed in place the second upper
disk can be engaged with the centre spindle followed by the lower
disk pair. The final part of the assembly is to very carefully
locate the lower bearing of the second upper disk. Once all the
parts are located correctly tighten the four nuts holding the
upper ring then tighten screws J,K,L (note there are three screws
located 120 degrees apart L is hidden behind the centre spindle). |
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Above you can see the
Muirhead drive is now put back and seems to be nice and smooth
in operation.The most difficult part of the job was refitting
the pointer because the securing screw was inaccessible. I drilled
several holes in the lower edge of the panel (out of sight) and
the chassis and was able to engage the securing screw after grinding
a screwdriver to perfectly fit the screw used to tighten the
bush.
I found by trial and error I
was able to fit the home-brew drive securing block so it matched
the screwhole in the panel and gave enough clearance to get to
the grubscrew in the Muirhead drive. Because the clearance between
the rear of the drive and the R1155 panel was only a couple of
mm I had to grind a second screwdriver to fit.
Left; compare with the picture
above... after cleaning the plate you can see the original ranges.
It seems at first sight 1500/600KHz has been replaced as has
200/75KHz. |
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I guess nobody will ever
undertake this conversion but it's interesting to record it.
The accuracy of the drawing isn't perfect but it gives you the
general idea.
One drawback with this is what
would happen if excessive force is used once an endstop is reached?
If the pointer shifts inadvertently
there's a pointer access problem, hence the trouble taken to
drill the screwdriver access holes so the quarter inch adaptor
could be really secure. I did wonder whether a new method could
be used by drilling a hole from the front of the adaptor into
the tuning condenser shaft, tapping the latter and putting a
star washer into the gap before tightening a screw to hold them
together.
Commercial tuning arrangements
usually design a safety feature to overcome misalignment or damage
from excess force which can result in a very complicated design. |
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While I was fitting the
tuning bits together I noticed not one but several components
and wires that had come adrift so getting this R1155 working
might be more tricky than I first imagined. Producing a rough
circuit diagram is next on the agenda...
Looking at the valves used in
the receiver it looks like the original design must have been
changed with now a 6AK5 RF stage, an X66 mixer using a 6SN7 local
oscillator. The extra valves lower right of the
picture above may even suggest it's now a double superhet
as the extra 6AG5 and EF50 don't seem to go too well with the
KTW62 and EF39 unless they're employed as a second local oscillator
and mixer? I looked on the Net for likely modifications and there
are many but I reckon none describe exactly what's been done.
I looked again at dangling wiring and its possible that some
instances may be design changes dating back decades so maybe
the best option is to power the receiver from an external HT
PSU which I can slowly increase to a working level and if there's
no smoke or anything untoward try reconnecting the various floating
components...
After a week or so of repairing
circuit boards in the day job I looked again at the R1155 and
decided it would be too time-consuming to trace circuit details
so instead decided on the option of attaching an external power
supply and attempt to diagnose any faults that are present. Lots
of the hanging wires looked like they may have been deliberately
disconnected so powering the receiver may be a worthwhile proposition.
I attached an HT supply between the rectifier cathode and chassis
(I had intended to look for an HT negative feed but gave up,
at least temporarily, in the rat's nest of wires).
I wound up the HT whilst monitoring
the current and found the latter dropped slowly from around 25mA
at a low initial voltage and as the voltage increased to 200.
Once the value had finished dropping off I wound it up to about
250 volts with no ill effects. Under the chassis is the audio
output transformer which has a 47 ohm resistor across its output
tags and to these I connected a loudspeaker. I then connected
a 6.3 volt supply to the valve heater circuit. The HT current
slowly rose to 45mA with an HT reading of 257 volts with the
wavechange switch producing nice cracklings noises in the speaker
as it was turned.
I had a convenient long wire
to hand and looked for somewhere to connect it under the chassis.
At the RF input end of the wavechange switch I noticed a detached
rubber covered wire that appeared to have come off a nearby tag
and connected this to my long wire. I then turned the RF gain
and volume control fully clockwise which brought up some hissing
in the speaker and swung the tuning knob from end to end in the
different settings of the wavechange switch. I was rewarded with
a strong broadcast station (possibly Radio 4?) at a setting corresponding
to a pointer reading of 18MHz which is around what would have
been about 190KHz on the painted-out innner scale. No other signals
were present so I connected a signal generator to the same connection
as the long wire and found all the wavebands appeared to be working...
meaning the local oscillator is probably OK on all wavebands.
At some point I'd found the signals were heterodyning and by
trial and error found the upper switch at the left of the front
panel was responsible. This is an old RAF plastic switch with
its toggle broken off and is used to turn on the BFO.
All told the results are promising.
I found the aerial socket wasn't connected so that needs sorting
out but most of the various controls seem to work including a
pair of potentiometers adjacent to the S-Meter which affect its
sensitivity and zero-setting. Next I'll see if the internal PSU
works, fix the aerial circuit and see what each waveband covers.
The old mains lead terminated
in a circular Belling Lee plug was a tatty cloth covered lead
with decaying rubber insulation and an ancient 13A plug so I
removed and refitted the Belling Lee plug to a new 13A lead,
plugged it into the R1155 and switched on. After 40 seconds the
speaker came to life, much louder than before no doubt because
the heater voltage was now a nominal 6.3V, and after resoldering
a detached coax lead to the Belling Lee aerial socket (which
had a broken inner so needs replacing) and attaching my the long
wire with a croc clip stations were present on all but the highest
range. It looks like the lowest range is consistent with the
painted-out dial calibrations, the next correct and the third,
medium waves, but with reduced sensitivity. HT at the output
transformer measured 280V and presumably the original ground/bias
circuitry is now correct.
The lower switch at the bottom
left, I think switches in a Q Multiplier as tuning across stations
gives a sharper and enhanced response. Overall sensitivity no
doubt helped by the newer valves is really good and I can hear
an unusual long wave broadcast which I think might be a Polish
broadcast at a very good strength a little lower than the Irish
station on 252KHz. I need to check the various wavebands for
alignment, for example to see why medium waves broadcasts are
weaker than normal and why the highest band is flat (of course
this latter may be due to poor H F conditions). The BFO switch
needs replacing as does the aerial socket. |
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The next day I used the Tiny
SA to gauge the waveband coverages and found all worked but miles
out when it came to RF alignment. One end would be very sensitive
and the other over 30dB different in sensitivity. With a long
wire and decent AGC gave a misleading effect. On the lowest frequency
band marked at the knob as 75-200KHz and tuned 80 to 210KHz.
The second range marked 200 to 500KHz tuned 200 to 530KHz. Next
600 to 1500KHz tuned 900 to 1600KHz. The first shortwave range
tuned 3.5 to 7.7MHz and the last tuned 7.7 to 19MHz.
The receiver IF is 560KHz and
I'm assuming this is unchanged. The local oscillator in the R1155
is greater than the signal frequency on all bands "quote
from RAF document AP1186". That means if you set a signal
generator to roughly the middle of Range 1 at 13MHz you will
hear a signal when the pointer is close to 13MHz and by tuning
the receiver to 14.12MHz you'll hear a second signal. If alignment
is good the second signal will be weaker in signal strength than
the first but, because the R1155 is very sensitive and has good
automatic volume control the two signals will probably sound
exactly the same. This can be quite confusing but can be resolved
by attenuating the output from the signal generator. In the original
design a magic eye shows signal strength but in this modified
example a complicated S-Meter has been added. |
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DIAL MARKING |
DIAL MARKING |
ACTUAL TUNING |
ACTUAL TUNING |
CORRECT OSC |
CORRECT OSC |
IMAGE |
IMAGE |
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RANGE |
LOW |
HIGH |
LOW |
HIGH |
LOW |
HIGH |
LOW |
HIGH |
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5 |
75KHz |
200KHz |
80KHz |
210KHz |
635KHz |
760KHz |
1195KHz |
1320KHz |
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4 |
200KHz |
500KHz |
200KHz |
530KHz |
760KHz |
1060KHz |
1320KHz |
1620KHz |
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3 |
600KHz |
1500KHz |
900KHz |
1600KHz |
1160KHz |
2060KHz |
1720KHz |
2620KHz |
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2 |
3MHz |
7.5MHz |
3.5MHz |
7.7MHz |
3.56MHz |
8.06MHz |
4.12MHz |
8.62MHz |
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1 |
7.5MHz |
18MHz |
7.7MHz |
19MHz |
8.62MHz |
18.56MHz |
9.18MHz |
19.12MHz |
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I've marked up the table above
to indicate what's needed during alignment. The DIAL MARKING
columns reflect the figures at the wavechange switch, the ACTUAL
TUNING figures indicate the measured frequencies, CORRECT OSC
are the correct frequencies for the local oscillator based on
the marked frequency and IMAGE figures are false signals. Because
of the way the dial has been painted over only Ranges 1, 2 and
4 are clearly OK but Range 3 has probably been tweaked to cover
the top end of the mediumwave broadcast band. I started to align
the stages and quickly found the trimmers seemed not to be logically
positioned so I checked a previous alignment I'd done a few years
back and (re)discovered the seemingly random trimmer layout (below)
which explained everything. |
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Alignment involves tweaking
coils as well as trimmers and this can be a tricky business with
the R1155 especially if this metal screen is missing as was the
case in my last R1155 commissioning job. If the set has been
previously worked on it's possible the coil dust cores have been
damaged or just stuck. Access is awkward but can be eased by
tackling the other ends of the coils with a suitable tool.
HF end=trimmer adjustments
LF end=coil tweaking |
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The view below is as seen
from the rear of the receiver and is the reverse of the view
of the trimmers above.
The larger coils for RF amplifier
1 are mounted on the chassis |
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During the testing I discovered
slight discrepancies and sure enough I should have checked the
IF response as it turned out to be 568KHz. Using a monitor shortwave
receiver to check the R1155 local oscillator I found out that
Range 1 oscillator was wrongly set to be below the tuned frequency
and correctly above for Range 2.
It's unlikely the IF would have
been modified so my first step will be to correct this and realign
the IF amplifier to 560KHz. Associated with this is the BFO.
The R1155 designers chose to use the second harmonic of a 280KHz
oscillator which could be mighty confusing to a restorer.
Fortunately the IF cores were
all tweakable and I tuned them so the response is now 560KHz
then I looked at the RF alignment. The two HF ranges cannot be
aligned without freeing the coil dust cores. I can tune the local
oscillator so that it roughly matches the dial markings but as
the receiver is tuned LF the response drops off quite rapidly.
In fact to set the second RF amplifier coils on Range 1 for 18MHz
I had to add 50pF across its trimmer. This is quite a lot so
I'd guess the coil dust cores are too far away from optimum.
I noticed some cores have damaged slots so it'll be difficult
to align them. |
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I decided to replace the broken
BFO switch and as I had a spare one matching it I used this as
it fits the larger than average hole in the panel. This is an
Air Ministry switch type 10F-10338 and has a pair of contacts
for both make and break. I fitted the new switch then found
no trace of the BFO. |
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I noticed this very clean example of an R1155B
on Ebay the other day. Clearly that replacement dial wasn't as
unusual as I'd imagined. Also note the S-Meter with that adjacent
adjusting control. This one, mounted upside down, suggests a
simpler drive circuit because that on the one I'm looking at
provides more current the stronger the signal. |
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