I have a Heath Apache that I acquired with the audio modifications already made
which included changing coupling caps and removing the clipper circuit.
Recently, it developed a peculiar problem. That is that when the AF gain
control is increased, with no audio being applied to the audio preamp section
without a microphone being connected, the modulator plate current rises as the
audio gain control is turned clockwise. I suspected microphone problems and
maybe hum driving the modulator, but WITH NO MICROPHONE CONNECTED, the
situation still occurs.
When I remove the first audio preamp tube, 12AX7, the modulator current does
not rise when the AF gain pot is increased with no audio input. The modulator
resting current remains the same. Removing the 12AX7 or the 12BY7 audio driver
stops the current rise when advancing the audio gain pot. Something is
introducing audio to drive the modulators with the microphone removed. Sounds
like something is being generated within the audio stages to cause the EL 34s
to draw more current with merely and increase in the audio potentiometer.
I have listened to the signal on a receiver and there is noise, but not hum
such as ac hum. There was added a feedback loop consisting of a .1uf cap and
270 k resistor from the 500 ohm tap on the secondary of the modulation
transformer to the cathode of the 12BY7 audio driver cathode.
I would suspect a shorted .1uf cap in the feedback loop causing feedback or
oscillation except if that was happening, would removal of the 12AX7 audio
pre-amp tube clear up the problem since the feedback circuit is from the
modulation transformer to the cathode of the audio driver tube?
As you can see, I am puzzled. I have changed the 6AQ5 clamper tube in the final
amp section and swapped out the 12AX7 and 12BY7 tubes with no change in
symptoms.
Have any of you AM audio experts encountered such a problem. Your input will be
greatly appreciated, but for those who always ask "is it plugged in" IT IS
PLUGGED IN, but maybe the operator isn't plugged in. Thanks and 73, John, K5PGW
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