No modulation from Connex CX-4300HP

Unit 75 · Jun 23, 2020

Weird problem with this one. I used my audio probe and traced back to where I lost my modulation. I have modulation coming up to and out of TR44 (2sc2166) collector going into the junction of C209 (820pf) and C338 (390pf) but that is where it stops. It is not getting to the input (base) of the 2sc2290. The caps check out ok on my capacitance meter. All components on the 2sc2290 board have been checked as well. Any insight?

Handy Andy · Jun 23, 2020

Have you done a board foil from that pill board to the foil of the main PCB?

All traces?

When it pulls current - that is it's weakest spot - pops like a fuse

When it "dies" it can also affect that 820pF but you said it checked ok.

If that cap is silver mica - just swap another in there to make sure that the meter is not fooling you.

They can check ok (marginal) but when pressed with RF they quit (out of tolerance open-lead action) - the 390pF takes all the drivers' RF current. the 390 can show ok, and act like a dead short in RF if its' a simple ceramic.

Unit 75 · Jun 24, 2020

Thanks for your reply, Handy Andy. Both input and output foils including ground foils are good going to and from the 2290 board. I know exactly what your referring to as I have seen those foils lift from the board from previous radios I have worked on. Looked like the tech had over heated them with the iron when removing the 2290 board. The 820pf and 390pf caps in this radio are both ceramic. It actually makes sense to me now why both those caps checked ok when removed but will not pass the modulation under RF. That is what had me scratching my head at this whole ordeal. They both look like 100v rating only. Cheers.

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No modulation from Connex CX-4300HP

Unit 75

"TRAINMAN"

Handy Andy

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Unit 75

"TRAINMAN"

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