Sony AV1010, basically works but heatsink is cold, resistors overheating

[foxer]

Weird stuff.
I press power on this receiver, I hear the relay click, I can play music and get radio, everything apparently works but not really. The big heatsink stays cold to the touch, the small heatsinks to the two voltage regulators on the mainboard get super hot, and there are two overheated brown areas on the mainboard with an identical pattern (circled in red), centered on the components on each side of the big capacitors (removed in these pictures). Those last components, of course, also get burning hot.

So...I spent the week with my novice skill trying to locate the problem.

I removed all the transistors and checked them with an analog meter's ohm setting and they appear to be good.
All the diodes appear to be good.
The rectifiers, too.
Q815 on the power supply board shows a short between emitter and collector but I don't see how this could be cooking components so far downstream. People say this symptom usually means a short.

There are no bridged joints anywhere in this receiver, no leads/wires touching other metal on top, no solder splatter underneath or on top, no metal debris, etc.

Studying the circuit diagram, I can't possibly see what special connection those cooked parts on the pictured board areas share, something that may be the bad component here.

I'm hoping someone here has seen this sort of thing before and can give me a clue.

[R_J]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

The transistors are acting as regulators, If they are shorted, then the full voltage is being supplied down stream.
The board is discolored from heat over time, it is not a concern.
You have the schematic, so check the voltages around q815 etc. after it is replaced.
The transistors Q707 and Q708 that are getting warm are likely ok, if they were bad, or if there was a problem in that circuit, the protection circuit would engage and the relay would not turn on, and you would not have any audio.

[foxer]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

The board is discolored from heat over time, it is not a concern.

I'm not sure about that. The heatsink and its big transistors stay cold to the touch no matter how long the unit is on, this can't be normal operation. At least, in my experience it's not. Why does something have a heatsink if its not going to be sinking any heat.

[petehall347]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

measure the millivolts across those big white square resistors after checking that they are reading the correct resistances .. be something low ohms and should be written on them .
also check all the resistors that stand up off the board .they look to be fuse resistors to me

[foxer]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

Those standing ones aren't fusible resistors. The resistances are correct on those ceramics. Alright, I need to reconstruct this dinosaur then and check its voltages....tomorrow.

[petehall347]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

volts across them white ones will tell you the idle current

[CapLeaker]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

If it’s not broken, don’t fix it. The regulators do get warm and cause the PCB to be discoloured, but as RJ already said, it’s nothing to be concerned about. About the heat sink of the finals running cold, they will get warm once you get them enough “exercise”. �� Put that thing back together and enjoy it as long the relay clicks and music is coming out of the speakers.

[foxer]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

Found time to check the emitter resistors today. I set my meter to 10 dcv. There's nothing. Maybe the voltage could be too low, but the needle did not move even an infinitesimal degree on any of them. Hmm..

[CapLeaker]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

Something doesn’t add up. You are saying the emitter resistors are overheating but yet there is 0V. That can’t be.

[stj]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

how are you metering them?

[foxer]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

Something doesn't add up. You are saying the emitter resistors are overheating but yet there is 0V. That can't be.

I'm not referring to the resistors circled in the pics but to the large white 0.1ohm ones, sorry if I bollixed the name. Many of these things are new to me.

how are you metering them?

Powered on and measured the resistors themselves. Out of curiosity I let the unit stay on a few minutes, then added speakers and played music but the load had no effect on my readings.

[CapLeaker]

Re: Sony AV1010, basically works but heatsink is cold, resistors overheating

Here is the service manual for it. The white square 0.1Ohm 5W resistors next to the output transistors are the emitter resistors.