Now I've started to work on my
military radio collection (and any others sets delivered to my
workshop), starting with the R1132, it may be useful to record
the odd note on the various sets as I tackle them.
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.
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.
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.
I bet a pound to a penny the B40 Design Project Chief Mechanical Engineer had a Meccano Set No10.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.