SX-828 FM issues

[burnoil]

Hi folks,

I have an issue with an SX-828. Started to rebuild it and all has gone well except for the tuner. BEFORE any work was done, the tuner is working on AM, but FM just issued static on any channel No tuning at all. Neither meter will move or show any signs of life. Stereo lamp is new, but does not light. Also tried mono and the muting button just for kicks... nothing. AM still working.

Rebuilt the power supply only for now. Recapped the tuner board, but FM still not showing any life except static. Thinking someone may have messed with the board in the past, I even swapped the tuner board. Same issue. Voltages look ok from what I can tell. Thanks for any help.

 

[EastPoint]

You have an antenna hooked to the back terminals, right?

 

[burnoil]

You have an antenna hooked to the back terminals, right?

Yes, sure do. As I scan the FM range with the dial, the static does not change. It stays uniform. Signal strength and center don't move even a little.

 

[AE Bob]

My bet is that the local oscillator in the FM front end is not running.

 

[burnoil]

Sooo...possibly a failed Q17? I think T7 runs to that transistor. Hopefully, IF that's the source of the issue, it is only the 2SC870 at Q17. Here's to hoping T7 isn't bad. Any other possibilities?

 

[AE Bob]

You can verify that the local oscillator is not running with an oscilloscope, or you can substitute for it with a signal generator. If you don't have a scope or RF generator you can sometimes use another receiver connected to the same antenna as the malfunctioning one tuned to a station (as a sort of signal generator), and tune the same station on the malfunctioning unit, and see if it gets a little signal (you may need to rock the tuning a little on the good unit). You are using the small bleed of the local oscillator from the good receiver back through the antenna into the malfunctioning one.

 

[burnoil]

Well, definitely something else. I took a known working board and swapped it in. Same result. So, I have something mis-routed or not generating power correctly for the board. I see melt marks on soime wires so someone's been in here. Verified wiring is correct, but still stumped.

 

[burnoil]

Ah, Got it. Probably not worth the effort as that would require a lot of disassembly to get to the underside of that board buried in there. It'll now be a parts unit.