On Sat, 17 Feb 2007 13:15:27 -0600, "Pete KE9OA"
wrote in
:

Thanks for the tip. The problem is definitely in the modulation stage. It
appears that the operating point of one of the preamplifier transistors is
changing into a nonlinear region. I didn't see any new soldering that had
been done, and all of the coils still have the wax intact. The receiver
sensitivity measures at , 0.1uV across the whole band, so that section is
working fine. This unit has one of the hottest receivers I have seen in a
long time; this is the reason I want to restore it.


Sounds like it might be a little too hot for the band.....


I do have the service
manual, but I have never seen this kind of problem.


Never assume the manual is correct. Establish a baseline by injecting
a single tone post-AGC, run it up to 100% and measure. Then re-align
the AGC to your mic according to that baseline.


For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like somebody
tried to align the exciter/output stages.


Sounds like it may have an RF limiter. If you have a pulse generator,
hit the audio with 0.1ms spikes. That will also tell you if the audio
limiter has been disabled.


It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice!


No problem. If you ever get around to scanning the manual I wouldn't
mind having a copy.


I think I remember you as also
working in the RF industry.


On occasion.