[Collins] KWM2 feedback

[Dr. Gerald N. Johnson]

On Tue, 2007-08-21 at 21:09 -0400, James Riach wrote:
> John:There is a 100 mfd electrolytic cap from the audio amp cathode that
> sometimes fails. The positive voltage on the cathode is used as a delay for
> the agc and sometimes audio can get on the agc and create all sorts of
> problems. The best thing to do is just replace it. It might do the trick???
> Jim  VE3DSR

There also was a circuit mod to change that in a service bulletin. KWM-2
SB-8, part B.

But the original post was hearing audio with the receiver audio gain up
while transmitting. 

73, Jerry, Technical Advisor to the CRA

> ----- Original Message ----- 
> From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
> To: <collins at mailman.qth.net>
> Sent: Monday, August 20, 2007 9:01 PM
> Subject: Re: [Collins] KWM2 feedback
> 
> 
> > 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
> >
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