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With a safe way of powering the
set I connected a loudspeaker and applied power. Something was
wrong. Although valves lit up and HT started to establish some
wisps of smoke appeared so I turned it off. I decided to check
the two electrolytic condensers C34/C35 by disconnecting their
red and brown leads and applying HT to each condenser in turn
via a 1.8K resistor. I'd already checked the condensers with
an ESR meter and found one to be OK but the second showed a short.
However, powering with an HT supply showed both to be OK with
only a small and acceptable leakage. Presumably the short indicated
by the ESR meter had been removed when HT was applied.
I removed the rectifier valve and turned
on the set, but using an external variable HT supply. Connecting
the HT to the red lead (from C34) and turning up the voltage
showed a drain of 50mA at 200V but as the voltage was increased
the current rose to over 70mA and wisps of smoke appeared again
(from R19). Unplugging the output valve (an EL37 which was in
place of the original KT61) reduced the current drain to a low
value of circa 15 to 20mA. Checking the grid connection at the
IO valve socket revealed about 1.5 volts. This fault is a stock
problem with nearly all the valve radios I've looked at.
The problem was a leaky condenser C24.
This couples the audio stage anode to the output stage and any
leak in this (because of the high circuit impedances involved)
places a small positive voltage on its grid which causes the
valve to draw too much current. As with many leaky condensers
the leak increases as leakage current raises the internal temperature
of the condenser hence after a minute or so the HT current would
rise to the point where R19 smoked. I located C24 and replaced
it with a new, modern capacitor. If you look at the circuit diagram
you'll see that C24 is buffered from V4 grid by C25 (47pF) and
therefore the effect of leaky C24 will not be apparent unless
C28 (part of the tone selection) is closed. As the EL37 cathode
decoupling condenser (C31) was adjacent to C24 I and tested it
with my ESR meter. This indicated that C31 was open circuit so
I removed it and checked it out of circuit. It was indeed open
circuit so I fitted a new 47uF capacitor. Retesting showed that
the HT current was now stable although a bit on the high side
for comfort, but by now the receiver was working and pulling
in stations at good strength. I swapped the EL37 for a 6V6GT
and the HT current dropped by 20mA or so. Removing the external
HT supply and refitting the U10 rectifier proved the set was
now working normally.
The next job is to align the IF strip
and the four wavebands because over the past 65 years components
will have drifted and no doubt at least one twiddler will have
messed up the tuning. To assist with alignment the designers
have incorporated a scale (in inches) printed on the inside of
the large tuning pulley fitted on the tuning condenser spindle.
There's a pointer which is used to make readings and this needs
to be aligned to exactly 9 inches when the tuning condenser is
fully meshed. You don't need to move the pulley, just bend the
pointer.
It's important to take care of the dial
cord and to check all is well because you don't want the cord
to detach itself once the set has been reassembled and returned
to its owner. Visually check that the various cords aim exactly
at the centre of the grooves in the pulleys. In this set the
cord detached itself during initial testing and was due to the
position of the large pulley being a tiny amount out of alignment.
Repositioning the pully by slackening its securing screws, moving
it a millimetre along the tuning condenser shaft fixed the problem.
There's nothing worse than a customer on the phone reporting
the tuning knob has no effect. |
