Pioneer 980 with loud Thumb

[Leokarma]

My pioneer 980 had developed a problem which I'll try my best to explain.

When I turn it ON, it starts normal i.e. the relay will engage within 4~5 seconds and the receiver will work with perfect sound. But once in a while (within few minutes to any hour) the speaker will make a loud "Thumb" (for fraction of a second) and the relay will disengage and then engage again within 4~5 seconds. Sometimes this will happen within few minutes and sometime the receiver might work OK for over 30 minutes. When there is this "Thumb" noise both watt meter needle goes to extreme right (only for fraction of a second). I'm unable to zero in on the problem and need someone to help. Just fyi I do have another 980 in my attic with bad FM board and if required I can use power supply, protection or other boards from that. The problem is that I'm not sure on where to start.

 

[zebulon1]

Power supply. Probably.
But; Rebuild the PS with new components. It's better than swapping old boards.
The other one with the tuner issue. It might be missing the 13 volt supply for the tuner board. Who knows what's wrong with that? You would have to evaluate it, making sure it's a good donor.

 

[Leokarma]

OK I'll start with the Power supply board. For now I'll order all electrolytic capacitor, transistors and diodes.

Power supply. Probably.
But; Rebuild the PS with new components. It's better than swapping old boards.
The other one with the tuner issue. It might be missing the 13 volt supply for the tuner board. Who knows what's wrong with that? You would have to evaluate it, making sure it's a good donor.

Thanks. I'll start with the Power supply board. For now I've order all electrolytic capacitor, transistors and diodes. Should I also replace resistors ?

 

[zebulon1]

Just check them for value. If one checks out of specs, desolder one leg and lift it from the board and re check it.
It wouldn't be a bad idea to do a good visual check of the board traces and solder connections. It might be something you can see.

 

[Leokarma]

Just check them for value. If one checks out of specs, desolder one leg and lift it from the board and re check it.
It wouldn't be a bad idea to do a good visual check of the board traces and solder connections. It might be something you can see.

Yes I have already given it a good visual check. Everything else looks OK except Q5 (2SD712) of power supply board has been replaced by previous owner. The replaced transistor is ECG 274 and I'm not sure if it's equivalent of 2SD 712. Please see below.