YES - for a MSS and as a RF bench engineer years ago.

If u have the schematic it should be easy to trace the input and output
stages of each section of the TX audio sections.. Just run a constant 1K
tone to the mic input line - I would tend to think that its in the TX line
only, as most of these radios used the same transistors for both RX/TX in
the audio output stage and you are not reporting any distortion in the RX
audio.
Best 73's

"Pete KE9OA" wrote in message
...
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. I do have the
service manual, but I have never seen this kind of problem.
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.
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! I think I remember you as also
working in the RF industry.

Pete

"Frank Gilliland" wrote in message
...
On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA"
wrote in
:

Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete



Assuming your ground plane is at least nine square feet.....

Someone screwed with the audio AGC. Whoever did it probably screwed up
other stuff, too. You should take it to a shop to have the whole radio
re-aligned.