Repair of a Murphy 62B receiver

 (a customer repair from 2011)

 This particular B40 dates from late 1951 or early 1952, and belongs to a customer. It looks like it was rescued from somewhere, perhaps a very damp garage, where it looks as if it had lain for centuries, as the chassis is heavily encrusted with oxide.
I downloaded an enormous manual. I already had a B40 manual, but certainly nothing like the size of the version I found on the Net which has approaching 500 pages, and for which one almost needs the advice of the original technical author to find one's way around.

Basically most of the B40 and 62B sets use valves with a fairly rare type of base. These are the B8G type which had a relatively short period of manufacture and are cousins of the B9G EF50 range, preceding all-glass miniature valves. The last version of the 62B and the B40D used all-glass valves.

The first thing of which to be aware is the 2-pin mains input. The set is definitely NOT double insulated so I'm afraid the 2-pin connector must go, otherwise (without an isolation transformer) sooner or later someone will get a nasty shock. This may be a user or even a passer-by touching the aerial, if this is draped around one's garden.
My thoughts go back to my 80m long wire running down a back alley behind a block of terraced houses in Liverpool. This was "invisible" using 28SWG copper wire but certainly gave one tingling sensations if touched (and a nasty burn if touched when transmissions were in progress!)
A modern kettle socket fitted perfectly after some judicious filing. I suppose a comment such as, "a live user is better than a dead purist", is appropriate?

The tuning arrangements used in the B40 have to be seen to be believed! My experience in the Defence Industry leads me to believe the project manager in charge of its development must have been an engineer of the mechanical persuasion, otherwise an electronics engineer easily led astray by the mechanical engineering team. I say "team" because there is just too much mechanical design associated with the B40 for one or even two engineers.
Then again, maybe it's just a reflection of a specification released by a vast MoD team with such exotic titles such as PE1A/2b-3c2, PEA/3d-5f1(test) etc etc (you get the drift if you've ever seen a document distribution list for circulation in MoD?
No expense spared… especially if it's the British taxpayer footing the bill!
Don't start me on this….

Looking at the mechanical design of the wavechange system; the origin of the receiver has to be in the R206 dating from at least the early years of WW2, and I'm reliably informed the late 30s.
Both receivers used a turret carrying the tuning coils, and both employed serious mechanical components.

I bet a pound to a penny the B40 Design Project Chief Mechanical Engineer had a Meccano Set No10.

Possibly that same engineer in charge moved to Murphy from another Company, perhaps Marconi, but certainly the same brains that came up with the R206 came up with the even more complex arrangements in the B40.
Murphy Radio was always extremely innovative so I don't think it was a case of plagiarism. Maybe someone knows the answer to this riddle?

Back to the example on the bench; marked on its front panel "62B", is an earlyish version. It was reported to be deaf and has plenty of surface oxidation on its die cast chassis.

How to tackle it was the first consideration?

I chose to find out just how deaf it was and my Wavetek signal generator would be useful as it can put out a volt of RF at any frequency covered by the 62B.
First check the IF frequency. This is, unusually, 500KHz and, after removing a side panel to expose the components in the RF section, I connected the generator to the mixer stage and, after setting the various 62B controls eventually found positions where the set responded to a 500KHz signal.
Next, I twiddled countless IF tuning slugs and declared the IF strip was working well enough to proceed to the front end.
I started with Band 1 which covers 150 to 300KHz. After setting the tuning dial to 150KHz I looked for the Wavetek output which was initially set to 150KHz. Nothing, and after 20 minutes of hunting around decided to switch to my Marconi signal generator. The advantage of this analogue equipment is that it has continuously variable tuning rather than digital. Using the latter is like trying to find the light switch in a coal cellar.

I swished across the band and found a good strong signal at something like 279KHz, miles away from the indicated 150KHz; in fact, tuning the 62B had absolutely no effect on the strong 279KHz signal. Very odd, so I switched to Band 2, then Band 3 etc. In each case I found a good strong signal, fixed and invariable, at a frequency quite remote from the dial settings.
My first reaction was that the tuning condenser wasn't connected.

As the 62B is extremely well shielded the only way to proceed was to remove the front panel, which is surprisingly easy after removing all the knobs. Next the large drum dial has to be removed. This would enable me to see the business end of the tuning condenser. This done I immediately saw the reason for the tuning problem. Lying loose on the inside casting was a tuning chain. The chain connects the tuning knob to the gearwheel on the tuning condenser.
The manual is helpful but not entirely clear. It certainly shows the path of the chain, but only in diagrammatic form, and there is some vagueness in the actual layout.

After puzzling over the options I attempted to thread the chain around the gears and the two idlers. There was only one position where the chain fitted and the tensioner did its job so, quietly confident, I continued.
Initially, I had to reset the position of the driving gear to match operation. The tuning gear incorporates a mechanical counter with end stops so needs to synchronise precisely with the tuning condenser to enable its full tuning range. This is tricky as the tuning condenser has an integral slow motion drive and the various gear securing screws are not that accessible, but finally the tuning knob rotated from 150KHz to 300KHz and the tuning condenser followed.

A quick check with the signal generator appeared to show tuning points of 150KHz and 300KHz at the scale ends. I say "appeared" but in fact appearances aren't everything as I'll explain later.

Next, I refitted the drum dial and continued checking. At this point I had the first suspicion that all was not well. True I could now tune Radio 4 Long Wave at something like 200KHz, but something didn't seem right.

I'll attempt to explain further. As the tuning knob is rotated clockwise the drum dial rotates and, if correctly fitted and adjusted, not only turns, but rises upwards so that the scale which winds around the drum always appears in the fixed window: a really clever mechanical design feature.
The penny ever so slowly began to drop.

Tuning upwards from 200Khz to 210KHz revealed the set responded not to 210KHz but to 190KHz.

Tuning so that the dial read 190KHz showed it actually responded to a signal at 210KHz.

The tuning condenser spindle turns clockwise to reduce the frequency and anti-clockwise to increase the frequency, whereas the tuning knob rotates clockwise to increase the frequency and vice versa. Of course the tuning condenser plates are not visible due to metal screening plates otherwise I'd have spotted the problem earlier.
As I'd been checking the long wave band with its harmonically related end frequencies, this had clouded the issue. The 300KHz signal I thought I'd heard must have actually been the 2nd harmonic of 150KHz.

After removing the drum dial again, I worked out a possible alternative route for the chain, and after re-threading it correct contra-rotation of the gears was achieved.

Now I could see why the chain had fallen off.

The tensioning idler had moved to the extreme end of its range and the slightest snag in the chain pushed it beyond its now horizontal position and it just flopped away.

The chain now relieved of its tension just fell off the gears.

Whether it was chain wear, gear wear or perhaps even the wrong chain who knows? It certainly needed to be sorted out as, without correct chain drive, the set is useless except in its secondary role (in keeping with many sets of the period from WW2 and just after) as a boatanchor.

The solution turned out to be relatively simple. The position of the fixed idler can be shifted so that this takes up most of the slack in the chain, then the tensioning idler can do its job afresh.

The fixed idler is secured directly to the casting with a special 4BA screw. There's no indentation in the casting, just a flat surface through which is a 4BA tapped hole, so I experimentally moved the fixed idler to a position where the slack was taken up, marked this then drilled it and tapped it 4BA.

Re-fixing the fixed idler with its special screw and setting back the tensioning idler thankfully did the trick.



  Now, when the tuning knob was turned clockwise the tuning condenser turned anti-clockwise and everything worked as the designers intended.

Next I have to turn my attention to an annoying hum from the headphones. The hum is constant and masks the audio. Grounding the audio amplifier valve grid has no effect on the hum, just removing the audio. I also noticed that the HT measured at the anode of the audio output valve was 295 volts. A bit on the high side because the mains transformer is rated for 230-volts.That's fine you might say… UK mains is rated at 230 volts too. However, although the government in their infinite wisdom said that UK mains shall be 230 volts, just like many other things the government says does not make it true. Saying something does definitely not make it so, whether it's inflation or today's exams being just as difficult as 50 years ago, the truth is often radically different

UK mains has for some time been 240 volts not 230 volts, and until all the generating plant and infrastructure has been changed, UK mains will be nominally 240 volts. In fact during most of the day and night you can expect the mains voltage to be between 240 and 250 volts or more. It depends where you are and how many electric kettles etc are being used.
In the case of the 62B therefore, according to the manual, the HT voltage can be up to 275 volts. As my set's HT measures 295 volts, my mains supply is about 246 volts.

Next I'll test the valves including a check for heater-cathode shorts. 6.3V AC would nicely modulate the anode current. In days of old it wasn't unusual to find an audio output valve with a directly heated filament without a separate cathode element. Removal of hum which would modulate the audio was achieved by connecting a ground connection via a rheostat across the filament. One set the position of the wiper carrying ground to the middle of the track at which point any hum disappeared like magic. In some sets both sides of the heaters are floating above ground and an earth connection made via a centre tap in the heater winding on the transformer. This would help minimise any potential hum in high gain circuits.

The answer to the source of the hum was eventually found although via a rather roundabout route.
I'd noticed that the sound quality, although masked by hum, was pretty awful. I'd vaguely put this down to the fact it was an old set and not a hi-fi receiver, however as the performance slowly improved due to other adjustments and repairs, it became clear something was wrong.
For example, sound quality was just about acceptable if the various knobs were set just so… like fine tuning sound quality. A fraction either side of optimum settings and the sound became so distorted it was clear that something was clearly not right. The penny dropped and I decided it was probably a faulty component in the audio stages.

At some point, although I'd been using headphones and then an external amplifier and speaker, I switched to checking the internal speaker and found it was pretty useless. I disconnected it and connected it to my audio signal generator. It had a scratchy intermittent sound. I looked in my Farnell catalogue and found a perfect replacement, a tiny 2.5" speaker rated at 15W RMS and a 4-ohm impedance, close enough to the spec of the original.

At this point I discovered a huge advantage that the B40 has over other military sets of that era. The mechanical design allows for simple removal of major sections of the set. This is a very clever design feature and permits the repair of an operational set, given suitable spares, in minutes.

I hadn't realised the facility was quite so useful until I read a particular page in the manual. It seems that the entire power supply and audio section can be removed, once the outer case is detached, by slackening a couple of screws and removal of a pair of connectors.
Because of the corrosion on the chassis I hadn't realised this particular chassis could be removed, but it was just as the manual stated. The whole chassis slid back and after only a few minutes I'd found the cause of the distortion. The anode load resistor had risen from 47Kohm to several megohms. The audio stage had become a limiter. The merest whiff of signal on its grid clamped the anode to ground.

I found three bad resistors. The other two were in the input circuit of the audio output stage. The 47Kohm "stopper" was 62Kohm and the bias resistor was 670Kohm instead of 470Kohm.
I replaced all three and then tested the various coupling and decoupling capacitors. Surprisingly all were in excellent shape with no discernable leakage and pretty accurate values.

Next, I found the reason for the hum. There are two miniature "Jones" style connectors joining the audio/PSU chassis to the rest of the set. The smaller connector has 8 pins, and pin 8 carries the audio to the grid of the audio amplifier.
Although the audio is carried by a coax connection from its source, only the inner wire goes to the 8-pin Jones plug. The coax ground connection isn't made, and relies substantially on a path through the various chassis. As the chassis are all badly corroded there isn't a clean ground return from the coax shield to the audio amplifier.

I'd actually discovered this because the coax connection had broken, when detaching the plug, leaving the cable sticking out with no obvious way of getting to the audio section. Connected to pin 8 of the Jones plug was a short length of blue wire. Maybe the original rubber sleeve had perished and fallen off as there was no sign of insulation. The solution was simple. I connected a short length of black wire to the ground pin on the Jones plug and reconnected the coax, exposing its shield and connecting this to the new black wire.

Maybe the original omission had some design sense behind it? Perhaps there was a problem with a ground loop? Well ground loop there now is. In fact a 50Hz modulation loop using the resistance between the various chassis semi-insulated in their grey corrosion.

Refitting the power supply and audio chassis proved the repairs had transformed results. Gone was the raucous hum, the audio quality was perfect, and the new speaker performed brilliantly.

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