{Collins} KWM-2 loss of audio and transmitter output

["Keith" <kman9@xxxxxxx>]

I have an older KWM-2 WE transceiver that loses audio and the ability to transmit about 20-25 minutes after turn-on.  At that time, the receive audio cuts off when in USB or CW modes; the receive audio continues to work fine in LSB mode.  Along with the audio outage, the transmitter no longer puts out any power nor will any plate current flow in any mode including TUNE and LOCK.  However, before the 20-25 minutes are up, the transmitter will put out around 100-110 watts in LOCK into a 50 ohm dummy load.  The problem does not affect bias current which can be set to 40 ma. The problem occurs with both PM-2 and 516F-2 power supplies. 

 

In switching from LSB to USB, receiver output will be audible for a second or so before cutting out; sometimes a little chirp or burp is heard before the audio cuts out.  In USB or CW modes, S-meter movement is visible even though no audio is heard, indicating that the receiver is actually detecting signals but the audio output is muted.

 

I have checked the usual problem suspects to no avail.  I checked all tubes and swapped out numerous 6AZ8s, 6U8s, and 6BN8s with new tubes.  I have used Deoxit on wafer switches, pots, and relay contacts.  I have replaced C102, the 100uF cap in the AVC rectifier circuit.  A previous owner had replaced K3 with a new relay but I cleaned the (relatively new) contacts on this component anyway.

 

It would seem that some component in the VOX or T/R circuitry is perhaps heating up and changing value just enough to cause the problem but I cannot seem to find the cause.

 

Any suggestions for what might be causing this problem would be greatly appreciated.

 

73,

Keith WB2ZAP