[Collins] KWM-380 Problem

[Dr. Gerald N. Johnson, electrical engineer]

I presume the synthesizer is a VCO with digital controlled divider then
a phase detector, low pass filter, e.g. Phase locked loop.

First thing to check is that each of the VCO's is oscillating within its
required frequency range. Then trace that signal through the shaping and
programmed dividers. Then to the phase detector (4046 probably), then
the low pass filter then the VCO control input (with essentially pure
DC. Parallel to that testing, one needs to check the reference frequency
and its divider. Having multiple loops unlocked, my first suspicion is
the reference oscillator and its dividers. That's what I found wrong
(broken cable) in a Yaesu 220 module I bought last summer. When the test
voltages which are probably the locked VCO control voltages are out of
range (and they will vary according to the programmed frequency. You can
try to adjust the resting frequency of the VCOs to see if they lock. But
you can also wear out the VCO frequency adjustment without solving the
problem.

If there are aluminum electrolytics on that faulty board, I'd replace
them. Get the highest temperature, lowest ESR variety for better life.

Then if signal tracing of the divider and PLL doesn't work, it may be
time to check that the control signals to the programmable divider are
correct. An open in the board, a solder splash, or a bad chip, all can
kill the loop.

73, Jerry, K0CQ, Technical Advisor to the CRA.
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.