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. |
