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

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.

Sounds like a RESISTOR is changing value causing the bias to change on the
input preamp.

"Pete KE9OA" wrote in message
. ..
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

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.

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.

I wondered about that myself. Looking at the schematic, I do see a couple of
electrolytic coupling caps that can be causing the problem. Since Tx audio
is there for the first few seconds, this tells me that it is a Tx keyed
stage that has the problem. I am going to have to trace the Tx enable line
to see which stages are being keyed in Tx mode only.
I am looking at the block diagram, and it appears that either TR27, the PA
switch or TR21, the AF switch might be causing the problem. There is also a
feedback network that starts back from TR34, the ALC amp. This feeds a
couple of other stages caled AMC amps, that appear to go through a summing
network into TR23, the Mike amp.
Since the Tx audio problem affects both AM and SSB modes, it has to be in a
keyed stage that is common to both modes.
The nice thing about this radio is that no mods appear tohave been
done.............even the clarifier control has not been "married" to the Tx
mode. This one is worth fixing........the only problem is getting at all of
the wiring. Much of it is hidden behind a shield plate at the front panel.
It could be something as simple as a cold solder joint, but that might just
be wishfull thinking!

Pete

"John Doe" wrote in message
...
Sounds like a RESISTOR is changing value causing the bias to change on the
input preamp.

"Pete KE9OA" wrote in message
. ..
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

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.

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.

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

I am not sure what you mean.


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.

I might inject the signal right at the audio amplifier and work backwards.


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.

I'll give that at ry.


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 will do that. I only can scan up to 11 X 14 size, so the schematics will
be broken pages,but that should be fine.


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


On occasion.

On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA" wrote:

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



I had this problem on my 2510. Ended up finding out that my power supply wasn't
cutting it. I went to a larger power supply. Since yours in internal, I am not
sure that is the problem. But the damn symptom iwas the exact same.

Vinnie S.