FT-840 and 2SC2879 HG

[victorlad]

Hello. Working on a 840 that had blown finals. Replaced drivers and Finals. Bias the matched HG 2sc2879 with 200 ma. 100 ma each. Has good output but the is RF getting into the radio from the 13 volt rail and bleeding into the AF. I am assuming they are not perfectly matched causing the issue. Each 2879 has a matching output cap of 360p. I'm guessing I could replace the caps with adjustable ones to try to match them better? Or maybe just clean up the 13 volts going to the rest of the radio? Any ideas? Thanks!

Victor

 

[DoctorAudio]

C5023, a 10uF - 50V electrolytic on the PA-unit board could have opened up(lost value).

 

[victorlad]

I didn't test that one specifically but I try an additional one on Pin 5 of JP5001. It was a 1000uF electro. It did reduce the feedback a bit. Maybe 25%.

 

[DoctorAudio]

Yeah, if C5023 was the only culprit, adding the 1000uF where you did would have eliminated it. There must be more going on.

 

[victorlad]

These HG 2879s probably have slightly different input/output impedances the the original Japan 2879s. I was thinking I could play with the value of C5019 and C5020. But I'm not a RF engineer. I could install variable Caps and peak the outputs then replace with fixed ones. Of course the input match could be off too. Obviously I don't want to blow 35$ Transistors.