Thanks, I appreciate the help. I am going to fire up the test bench and give
it a go. I will let you and the folks know what I fine.
Ok on that RF engineering experience...........that is also what I am doing
right now. I worked at Collins Radio in the 2nd half of the 90s, switching
to Motorola when we moved back to chicago. For the last couple of years, I
have been working as a contractor/consultant at Motorola. I think that's
what happens when you get to be a semi-old guy like me. They make you a
consultant!
Thanks again to you guys for the help!

Pete

"John Doe" wrote in message
...
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.