[Drake] T4XC with poor audio on SSB

[fkamp@comcast.net]

I just finished repairing a T4XC that had poor transmit audio.  I think
my experiences might be of interest to others.

The symptom was tinney sounding audio and the signal was difficult to
tune in at the receiving end.

At first I suspected a bad sideband filter but the transmitter had
similar syptoms on both upper and lower sideband.  It seemed unlikely
that both filters were bad.

I checked the carrier oscillator frequency and found it a little over
500hz low.  The C6 frequency adjustment would move the frequency even
lower but would not trim it up to 5645.0

I then suspected a bad crystal.  I had that happen before on other Drake
equipment.

Turns out that the problem was due to a collector to emitter short in Q4
on the carrier oscillator board.  Q4 is a PNP transistor used to add
capacitance and shift the carrier frequency by 500hz.  This is intended
to move the signal higher in the passband of the filter for use in
transmitting AFSK generated RTTY.  It allows cleaner transmission of the
2125/2295 tones.  This feature is energized by placing a short across
the RTTY input connector and should only function when using LSB on a
properly working transmitter.

The original Q4 is a 2N4125.  I replaced it with a 2N3906.  Seems to
work fine now.  Still wondering why the original 2N4125 failed.
Everything else seems to be okay.  Maybe it just got tired.

Regards,
Frank Kamp
K5DKZ