[Collins] KWM-2 Popping Noise

[Dr.Gerald Johnson]

Could be the VOX tripping. Too much VOX gain or too little antivox 
gain.

A fast AVC can pop.

RF Gain comes from a negative bias using the negative bias supply 
on the main power supply. They used an encased 20 ma Sarkes 
Tarzian selenium rectifier that should have been replaced with a 
silicon diode by now. As it ages it develops lots of back leakage 
and a high forward resistance. That bias supply has its own filter 
capacitor. It won't last much longer than the selenium, and the 
back leakage from the selenium will hasten the demise of the filter 
capacitor.

Loss of RF gain control points at the bias supply. You can make a 
quick check of the bias supply by checking the PA idling current by 
going to SSB transmit mode with no voice or carrier. PA plate current 
should be 40 ma. If its significantly higher, the bias supply needs 
attention before you transmit again. That same bias supply is used 
for receiver RF gain control and switching some stages off when not 
needed (e.g. transmit stages when receiving and receiving stages 
with transmitting).

Popping could be motorboating of the receiver from old 
electrolytics not being effective low frequency bypasses anymore. 
applies both to the power supply and the capacitors in the radio, 
though such motorboating tends to not be affected much by signal. 
More audio gain when the motorboating condition is possible makes 
it motorboat faster or louder.

73, Jerry, K0CQ, Technical Advisor to the CRA

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.