Cobra 142gtl no audio on AM transmit but yes on SSB transmit

[mattsowders1989]

I have a Cobra 142gtl with no audio on AM transmit but it does have audio on SSB transmit. My guess is I overdrove the Darlington pair direct injecting it years ago maybe? Would that take away my audio on SSB, too, though? just trying to think what I fried. Any help would be appreciated.

 

[nomadradio]

If you can adjust the carrier with VR6, this suggests that TR35 and TR34 are okay. C118 can block the AM transmit audio if it fails. The radio is old enough for electrolytic caps to start going bad from age alone.

73

 

[TM86]

If you've got an oscilloscope or have a buddy who does you can use a cellphone app to create a tone that you feed into the microphone, then key the radio and use the scope to find where the audio signal disappears. Someone here recommended "f Generator", it works. Helps to have a schematic too. OK, it helps me when I have one, your mileage may vary.

Also, the reason SSB transmit audio still works is that it takes a different path after C83, and doesn't go through the AM modulation circuit.

If it still has the original caps in it that were installed at the factory, take Nomad's advice to heart. Bad caps can do a variety of nasty things.

 

[mattsowders1989]

Yeah I should have check electrolytics beforehand. I'll post my results. Thanks, guys!