Pioneer A-40 (SA-940) Resistor keeps burning

[agamenon]

This amplifier has been a long time problem for me, main power transistors are changed, some other stuff has been fixed and it was working fine until I decided to apply some deoxit to pots and switches, turned it on and smoke came out of it. On closer inspection, I could see a resistor burning brightly! 😱

I use this schematics (PIONEER SA-940 SERVICE MANUAL Pdf Download.) for it, I removed the resistor and it measures 60 ohm, while according to service manual it should be 47, changed it and the same happened, this happens even when power transistors are removed.

The resistor is marked as R210 in schematics,
So I am lost here, checked q205 with a transistor tester and seems fine...

Any help is appreciated

 

[rayma]

The resistor is marked as R210 in schematics,
So I am lost here, checked q205 with a transistor tester and seems fine.

There must be a short on the output of the Q205 regulator.
Check the shunt capacitor at its output emitter first.
If capacitor is good, disconnect the regulator output wire
to the other pcbs, to verify the short is off the board.

 

[llwhtt]

Q205 is the voltage regulator that supplies voltage to the meter display and that's all it does. Check the voltage at the emitter of Q205, should be +11.8VDC. If that's real low the display circuit could be the problem. I was thinking that D7 could be shorted but with R211 in series with it I don't think that would fry R210, maybe check D7 anyway, should have 13VDC across it. If the display doesn't matter to you just pull R210 and be done with it.

Craig

 

[joaoveludo]

Not sure i'm helping but for what its worth i'll tell you that the pcb quality on this pioneer series is utter crap. I got an sa-750 (almost the same but 55 watts) and i was was afraid to sneeze at it when i did the recap. Lots of lifted pads and tracks with the job being done carefully with a 30 watt quality iron. I did plenty of recaps over the years.
Even accidental light touch on some resistors/capacitors was enough to lift the pads and leave them hanging. You have to make sure you didn't touch a component with enough pressure to lift a pad or a trace during the cleaning. So unscrew and inspect the underside of that pcb carefully while lightly touching the components to test for looseness.
You may find that you have to re-solder and repair a trace or pad. Do it by removing the green coating exposing the copper and using thin wire. I would re-flow the whole board too, just like i did in mine...
Those output transistors sockets really suck too. I used those straps just like you did to make them firmer.

My Yamaha ax-400 i been using to mod and experiment with is built like a tank compared this one. The pcb can take all kinds of abuse without a flinch.

My Denon PMA is so-so...

 

[agamenon]

Thanks for all the replies.

It seems that I am out of luck with this amp.

Narrowed down the problem to the meter display. Since I had some difficulty putting it back I may have placed pins in the wrong slots.

Replaced it carefully and turned on the unit, no smoke, lights on, and looks fine!

Put everything back together, connected to speakers, played something...but absolutely no sound!

I could even see the meter display playing the music, after like 30 seconds a click and a little smoke (on the right side of top grid)

Now when I turn it on there is no click (basic self-diag).

Could not find anything burned on the PCB...