Realistic STA78 Resistor/Transistor fun

[Impact]

Looks like il be adding to the fun as I am as well a STA78 owner that has crackling/popping out of only the left channel. After searching the forums, I put together that this is likely a bad transistor and decided to start doing a cold test of the transistors using super cold spray.

During testing I was just narrowing down the exact transistor that was giving me the left channel noise issue and suddenly the crackling noise was gone. Issue was I had about 3 transistors cold so im not sure which one is the exact one that is blown. (Noob mistake)

Then suddenly before I had a chance to do anymore testing my speakers start squealing like a baby in a doctors office. Extremely high pitched noise that did not go away with volume/input controls. I turned the unit off and unplugged it to find my R623 resistor smoking away and black.

I'm relatively new to this so I may be guessing wrong, but it seems that R623 resistor likely was overloaded and blew due to the transistor that I am actively trying to troubleshoot. My suspected transistors are TR602, TR604, TR606, TR608 but they are all a long way away on the board. Not sure if this theory makes sense or if I have other issues. Just looking for any advice/input before I try to just order a bunch of new transistors.

Photo attached to provide a visual. Bad resistor is marked in red.

 

[petehall347]

those transistors shouldn't affect that resistor as they are in the other channel

 

[dr*audio]

Looks like il be adding to the fun as I am as well a STA78 owner that has crackling/popping out of only the left channel. After searching the forums, I put together that this is likely a bad transistor and decided to start doing a cold test of the transistors using super cold spray.

During testing I was just narrowing down the exact transistor that was giving me the left channel noise issue and suddenly the crackling noise was gone. Issue was I had about 3 transistors cold so im not sure which one is the exact one that is blown. (Noob mistake)

Then suddenly before I had a chance to do anymore testing my speakers start squealing like a baby in a doctors office. Extremely high pitched noise that did not go away with volume/input controls. I turned the unit off and unplugged it to find my R623 resistor smoking away and black.

I'm relatively new to this so I may be guessing wrong, but it seems that R623 resistor likely was overloaded and blew due to the transistor that I am actively trying to troubleshoot. My suspected transistors are TR602, TR604, TR606, TR608 but they are all a long way away on the board. Not sure if this theory makes sense or if I have other issues. Just looking for any advice/input before I try to just order a bunch of new transistors.

Photo attached to provide a visual. Bad resistor is marked in red.

A schematic would be helpful. That is a component layout. Partial schematic below. Looks like it may have started oscillating and blew the outputs. Start testing the output transistors and work back toward the input until you stop finding shorted transistors.