D858 SSB Chassis with no AM modulation and no PA

[Unit 75]

Maybe someone who knows these boards really well can shed some light. It's an early run Madison with the D858 PLL. Pulled this one out of the mothballs. Because it is in mint shape, I decided to put in new electrolytic capacitors (yes the problem was there before I recapped it). It receives really well with good audio and SSB puts out 18-20W of clean audio. . I have no AM modulation at all and no PA output. Carrier is 5W on AM. I just checked TR-20 (2SC945) and it is ok. Going to check out TR-17 shortly as soon as it stops storming outside. Maybe someone can shed some light. I don't know if it is a common problem or not. Thanks.

 

[Unit 75]

I did some more troubleshooting this morning around the relay and found a little bit of conductive glue on top of R46. I removed R46 and used acetone to remove the glue on the board and replaced the resistor with a new one. I now have fully working PA audio. So that part is fixed. I still have a problem.... If I turn the radio on (AM mode) I have full receive. When I key the mic I have very weak modulation using my Turner+2 mic. Once the mic is UNKEYED, the AM receive goes out as well as the AM modulation (if I re-key the mic to talk). If I turn the radio off and back on again I get full receive back and TX audio comes back but still weak (AM mode only). SSB works very well both receive and transmit. Oh, I also replaced the relay with a new one to rule out any relay issues.

 

[Mk121]

Sounds kind of similar to an issue I'm having with my 138xlr. I just haven't had a chance to probe around in it yet.

 

[DARKDUDE]

You might look at the audio output transformer. The output is center tapped half is your speaker/PA half modulates the radio.

 

[Unit 75]

Thanks, DARKDUDE. As stated in my above post, R46 cured the PA issue, T1 is fine, I did a DC resistance check on both primary and secondary sides before I found the glue problem on R46. TR20 is good and IC4 is also good as I have my 7.2v on Pin 10. This is a difficult intermittent issue that I will get back to as I have more pressing work on other radios, but thanks for your input.