Pilot "Little Maestro"
Repair No.F065
This type of set was made over a long period in many guises.
The model whose chassis is shown below was brought in for
repair recently and is the "1939" version manufactured
in 1940.
The valves are 6A8 frequency changer, 6K7
IF amplifier, 6Q7 detector/audio amplifier, 25A6 audio output
and 35Z4 rectifier.
The set used a line cord which has failed and I am modifying
it to use a simple ballast resistor and diode arrangement. Although
this is not going to be a repair for the purist, it will end up
a lot safer with a modern mains lead and moulded 13amp plug. I
have written some notes on this which will be published in the
"Repair Tips" section when they're complete.
The set has a live chassis so all tests are carried out using
an isolation transformer otherwise nasty things would happen to
my oscilloscope and signal generator. Some "breadboarding"
is apparent in the picture where a chain of ceramic resistors
can be seen. These air cooled types run so hot that the solder
melts and they fall apart but the final ballast resistor will
be a 50 watt metal clad device bolted to the chassis.
I tested the valves and found they were all
in excellent shape but a quick check under the chassis revealed
an open circuit 10uF electrolytic at the cathode of the output
valve and a 0.025uF audio coupling capacitor at the grid of the
6Q7 with a leak of around 3 megohm. Other components were deemed
to be serviceable but I replaced the two bad components with new
devices. The electrolytic is now a miniscule Japanese component
and the 25nF capacitor a polycarbonate type. Incidentally an excellent
source of capacitors is virtually any old TV chassis. Most use
lots of high voltage types eminently suitable for old radios.
Given three or four different makes of TV chassis virtually any
value capacitor may be salvaged. These will range from disk types,
high voltage pulse types and decoupling varieties used in mains
input circuitry. Because radios have been repaired over the years
with whatever components were around at the time I have no compunction
against using the latest types.
I found an interesting design fault on the chassis. The dial
lamp was set to one side of the illuminating hole in the metal
at the back of the scale. Thinking it had come loose and shifted,
I repositioned it to line up properly and later wondered why the
tuning capacitor jammed half-way closed. A grub screw holding
a pulley was fouling the dial lamp. After setting the lamp back
to its angle of 45 degrees I restored it to the way the factory
had probably been told to do it back in 1939 when the fault had
no doubt been corrected on the production line by modifications
to their drawings!
When powered up the audio was distorted. This was primarily
due to a loose speaker cone. The glue at the back of the paper
re-inforcing ring had failed and the cone was moving in an uncontrolled
manner. One of the tuning capacitor vanes was bent and a loud
crackle manifested itself at one third mesh. Straightening the
vane sorted this out.
I checked the remainder of the capacitors and resistors during
the realignment because I found that although medium waves were
tolerable there was some instability in the form of motor-boating
at the low end of the band and besides long waves being pretty
deaf the RF stage seemed to be oscillating. All the components
tested OK except the odd leaky capacitor in places were leakyness
didn't matter. I input 451 KHz which is the slightly odd IF and
peaked up its four trimmers for maximum output. Now there was
not only instability across the long wave band but the whole of
the medium waveband was similarly affected. Not to be put off
I continued with the realignment and the number of stations increased
tenfold... but so did the instability. Something was decidely
wrong! I tuned the 451KHz rejector in the aerial circuit then
started to decouple various parts of the circuit looking for the
point where the circuitry was misbehaving. Nothing helped and
though signal strengths were now really good the complete medium
and long wavebands were full of heterodyned signals. I checked
the HT line and found that without the line cord that was a little
high. Was this the answer? I inserted an additional 470 ohms in
series with the speaker energising coil. Now the voltages were
right but absolutely no change to the oscillating RF stage. I
wrapped foil round the valves one at a time... no help. Back to
first principles... just look at the circuit and think hard. I
noticed that there was a moulded capacitor decoupling the audio
stage to ground. I also noticed a second moulded capacitor decoupling
the RF stage to ground. But wait... it wasn't ground, it was the
metal case of the volume control. This looked like a Radiospares
component and must have been fitted to replace the original many
years ago. But unlike the original, the new control had a metal
can that was electrically floating so that the two decoupling
capacitors were effectively in series, connecting the audio stage
to the RF stage... a feedback loop! I shorted the potentiometer
case to ground; the oscillations ceased and Radio Solent appeared
like magic. Presumably the set had never worked properly since
its last repair, maybe 40 odd years ago!
When I tested the receiver, with its new 50 watt ballast resistor
bolted to the chassis adjacent to the output valve and the underside
of the speaker, things got uncomfortably hot. The resistor has
a design temperature of up to 250 degrees centigrade and needs
to be better cooled than the small chassis can manage. As room
inside the set is very limited I had to reposition the main HT
smoothing capacitor and fit an aluminium finned heatsink in its
original place. To this is bolted the ballast resistor. Not original
maybe but not too disimilar to the model that followed this one.
That's designers didn't have such a difficult job however as they
used valves with 150mA heaters, not the 300mA types fitted in
the early model. Dissipation is proportional to the square of
the current hence I was dealing with four times their heat problem.
I suppose I cheated a little as the silicon diode I fitted wasn't
available to the old designers and this effectively reduced the
problem by half.
While chatting about this receiver, which is fairly typical
of AC/DC valve sets, a word on safety aspects. 
click on meter
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