----- Original Message -----
Sent: Thursday, November 13, 2008 10:10
AM
Subject: {Collins} KWM-2 loss of audio
and transmitter output
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
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