[Collins] KWM2 feedback

[Dr. Gerald N. Johnson]

On Fri, 2007-08-17 at 16:47 -0400, john wrote:
> Thanks to help gotten here, and some hours under the hood, I've gotten an 
> early KWM2 playing pretty nicely. One interesting problem remains.
> 
> If the rig is in SSB transmit (either USB or LSB) and the audio gain is 
> advanced more than half way, audio feedback occurs....now the weird part of 
> this is that the feedback is not coming from the attached speaker...I'm 
> wondering if there is some piezoelectric effect that accounts for it....its 
> coming from the rig itself.
> 
> If I turn the AF gain up, (again, during SSB transmit) the amplitude of the 
> feedback increases, but it's not being coupled into the transmitted signal 
> (there's no power out, no change in grid or plate current).
> 
> It's quite loud!
> 
> Any ideas where to look? All other functions appear normal. 6146s have been 
> changed (for another reason) and there was no change in this phenomenon.
> 
> Ideas?
> 
> John K5MO 
> 
The audio output transformer will "talk." Its not an efficient speaker
though. There is likely a disk ceramic capacitor from audio output plate
to ground to roll off high frequencies. Actually its C104 across the
transformer primary and C235 for AF feedback that both get lots of audio
voltage.

First thing I'd check is the on chassis can electrolytic by shunting it
with known good electrolytics. 

But I recall there was a service bulletin about such feedback also.
Check the usual KWM-2 website for such service bulletins. Their KWM-2
manual also has a list of updates and their effects with suggestions on
which one are needed and which ones are nice but not necessary.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer