Sears Roadtalker 40 SSB base 934.38310700 weak AM receive

[Michigan Muskrat]

I've a Sear Roadtalker 40 SSB base model 934.38310700 that has an issue with AM receive only, SSB LSB and USB receive works great. When the switch is turned to AM, the volume drops down to next to nothing. It's like to volume slide switch isn't working. The receive voices are barely heard. AM transmits fine, dead keys at about 4 watts and swings to maybe 6 watts. Using an antenna with SWR of 1: 1.1, needle barely moves.

Here's what I've done:
1. Used contact spray on the AM/LSB/USB switch, PA switch, volume slide switch, and took off the small plastic cover of the relay and used contact spray on it too. Remember both LSB and USB work well, volume works well on both SSB. However on AM receive, I can barely hear the voices, and there's no loud static sound like normal.

2. Replace the capacitors, there were about 52 of them. Still, no volume on AM receive. LSB and USB work great. Checked the wires, and this radio has a bunch of them, and did not see any disconnected.

3. Bought a SAMS manual on the radio, and checked alignment the best I could. It seems to be aligned well. Checked voltages on major test points, from the SAMS manual and there were a few that were off slightly. Less than 1 volt however.

4. The AM squelch level pot does not have an affect on receive sound, if turned in any direction. Looked for cold solder joints, pressed board gently, but no Ah-Ha moment.

Here's the question: If LSB and USB work as normal, is there a resistor, diode or transformer that sticks out in anyone's mind that would silence the AM receive, but have the SSB work as normal?

Thanks, I'm not one to ask for help much, but this one has me stumped.

 

[guitar_199]

A couple of things come to mind (and, please, others with more experience on this one jump in!!!)

SSB and AM use totally different circuits to achieve modulation. I don't THINK that they tie that closely together.

As many typically suggest, have you tried the PA through an external speaker? The PA and AM Modulation are VERY closely tied together. Depending on where an audio failure occurs...it will hit both of these....

In addition, can you plug an external speaker in to the EXT SPKR jack? Speakers do fail......

That is a couple of pieces of low hanging fruit.....

 

[sp5it]

2. Replace the capacitors, there were about 52 of them. Still, no volume on AM receive.

LMAO.
First I would check AM detector.
Check marked elements.

Mike

 

[Michigan Muskrat]

A couple of things come to mind (and, please, others with more experience on this one jump in!!!)

SSB and AM use totally different circuits to achieve modulation. I don't THINK that they tie that closely together.

As many typically suggest, have you tried the PA through an external speaker? The PA and AM Modulation are VERY closely tied together. Depending on where an audio failure occurs...it will hit both of these....

In addition, can you plug an external speaker in to the EXT SPKR jack? Speakers do fail......

That is a couple of pieces of low hanging fruit.....

Thanks, I will give the PA /external speaker a try for modulation/receive.

 

[Michigan Muskrat]

LMAO.
First I would check AM detector.
Check marked elements.
View attachment 34610
Mike

OK, the AM detector will be checked, this is what I'm looking for, some direction of where to start on the board for AM receive. Thanks!

 

[sp5it]

Use schematic from PC122. AM/SSB is common than it splits on L11 tuning can.
Mike

 

[Michigan Muskrat]

Use schematic from PC122. AM/SSB is common than it splits on L11 tuning can.
Mike

Thank you Mike. I appreciate your knowledge.

 

[guitar_199]

I hadn't had my coffee yet.

I see now it was weak receive.... duhhhhhh.... nevermind.

Thanks, I will give the PA /external speaker a try for modulation/receive.

 

[Rwb]

Any results?

 

[Michigan Muskrat]

Any results?

Yes!!!!

It was a bad transistor (Q102 on schematic), it was receiving the 8.2v but was not giving the proper voltage at the base and emitter, changed the transistor and it it works like crazy! I also just recapped the Roadtalker with 52 Nichicons, and it barks like never before! I talked ssb last night 45 miles barefoot, with a T2LT antenna in a tree! Couldn't be happier with the results.

Thank you for your time and help, much appreciated.

 

[Rwb]

Yes!!!!

It was a bad transistor (Q102 on schematic), it was receiving the 8.2v but was not giving the proper voltage at the base and emitter, changed the transistor and it it works like crazy! I also just recapped the Roadtalker with 52 Nichicons, and it barks like never before! I talked ssb last night 45 miles barefoot, with a T2LT antenna in a tree! Couldn't be happier with the results.

Thank you for your time and help, much appreciated.

Kool i got 1 also i keep track of trouble people have

 

[guitar_199]

Yes, thank you for that result. I , too, like to catalog these things in my head.

Was it much of a job getting to all of the caps to replace them? Does any place stand out as particularly tough to get to?

 

[Alex7590]

Hello everyone, I have worked on several of these radio's and found lots of cold solder joints. Just resolder everything and it will come alive. Does anyone know how to unlock the clarifier? I have the schematic but not sure how to do it the correct way. Any help is much appreciated.

Alex