Dwight d (858) distorted tx audio.

[KK4MSA]

Good evening everyone. I have replaced all electrolytic caps in this radio, as well as the 4 to22 style transistors in the mic circuit. The only one I have not replaced is the 2sk. The receive audio sounds beautiful, the pa audio sounds great, but the TX audio sounds distorted, it's understandable just distorted. I have also tried a couple of different microphones as well.
What else would cause distorted transmit audio? It also seems as though it's not forward modulating as much as it should. A whistle will get about 17 Watts pep as opposed to a good strong audio only getting about 10. Again I have replaced all electrolytics from the microphone input to the audio chip itself. Could it possibly be the mic preamp ic? Any help would be greatly appreciated.

 

[Dmans]

Could it possibly be the mic preamp ic?

The mic preamp ic is also used during the PA function I would think. Have you completed a full transmit section alignment?

73
David

 

[KK4MSA]

Hey David,
Yes, fully aligned. I should add that I got this issue squared away. I took out the audio chip,mic pre amp, all 3 xtals, and resoldered the xformer connections. I scraped every leg Individually, reinstalled and it's all good now. Sadly to say what component was the issue I do not know as I took them all out at once. Either way a win..lol. Thanks for replying.

73

 

[nomadradio]

took them all out at once

I believe you have discovered why the common advice is to remove caps one at a time, replacing each one as you go. Makes it harder to get a capacitor backwards when you're comparing the new one to the one that just came out.

I see a coin flip between a backwards electrolytic cap and a bad uPD1156 audio power amp chip.

Or a backwards cap may have damaged the 1156?

Occam's razor is a principle that all other things being equal the simplest explanation tends to be the right one. One cap, or maybe more than one with the polarity backwards is where I would look first. That's all it takes. Polarity marked on a circuit board's screen-print legend may or may not be correct. The old part that came out is nearly always turned the right way.

Take a bright light and magnifier to the pc board's solder side. A tiny solder bridge between adjacent foil traces is a common hazard when replacing that many parts. The problem could be that simple, too.

73