Overhaul of an R1155

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.

 

 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.

 

 

 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.

 

 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.

 

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.

 

 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.

 

 

 

 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).

 

 

 

 

 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.

 

 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.

 

 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.

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.

 

 DIAL MARKING

DIAL MARKING

ACTUAL TUNING

ACTUAL TUNING

CORRECT OSC

CORRECT OSC 

IMAGE 

IMAGE 

  RANGE

 LOW

 HIGH

 LOW

 HIGH

 LOW

 HIGH

 LOW

 HIGH

 5

 75KHz

 200KHz

 80KHz

 210KHz

 635KHz

760KHz

 1195KHz

 1320KHz

 4

 200KHz

500KHz

200KHz

530KHz

 760KHz

 1060KHz

 1320KHz

 1620KHz

 3

 600KHz

1500KHz

900KHz

1600KHz

 1160KHz

 2060KHz

 1720KHz

 2620KHz

 2

 3MHz

7.5MHz

3.5MHz

7.7MHz

3.56MHz

 8.06MHz

 4.12MHz

 8.62MHz

 1

 7.5MHz

18MHz

7.7MHz

19MHz

 8.62MHz

 18.56MHz

 9.18MHz

 19.12MHz

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.

 

 

 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

 

 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

 

 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.

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.

 

 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.

 
 

 pending

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