Uploaded May 2024 | Updated September 2026, 2 weeks ago
Self trained hobbyist here, working on a Pioneer SX-980 that a friend gave me to work on, apparently it’s been sitting for 20 years and has been dead for quite some time.
Bad Zener diode in the power supply, bad 13 V rail transistor, one capacitor completely open, 4 others were quite charred and barely in-spec.
Purchased a power supply rebuild kit with all the essential components from Affinity for Artifacts out of Wisconsin, I’ve purchased a number of rebuild kits from him.
Chased down an open resistor after about two hours of investigation, and also replaced another. Also found a couple bad circuitboard traces, virtually hidden to the naked eye. 
Output transistors are good, it’s running solid via a separate external preamp. The internal preamp is very very weak, it’s extremely low volume, and the tone circuit is working, but there’s something wrong.
Also, the amp biasing and idle current is strangely off on both channels from the service manual specs. I’ve got them equal and what I feel is reasonable but there’s just something strange. I may have to look for a different service manual. Perhaps this is a different revision. for example I just can’t get either of the settings to be at the proper voltage or amperage, but I can make them equal and within a safe range.
I’m going to need to do some tracing of the power rails to the tone board, but I’m guessing that there are bad transistors in the circuit. Something catastrophic happened in the power supply and maybe it went downstream.
I’m pretty good at fixing almost anything and it takes a lot of extra time, it’s definitely worth it in the end.
I really wish I had more formal electronics training because I probably could diagnose things a hell of a lot quicker.
Self trained hobbyist here, working on a Pioneer SX-980 that a friend gave me to work on, apparently it’s been sitting for 20 years and has been dead for quite some time.
Bad Zener diode in the power supply, bad 13 V rail transistor, one capacitor completely open, 4 others were quite charred and barely in-spec.
Purchased a power supply rebuild kit with all the essential components from Affinity for Artifacts out of Wisconsin, I’ve purchased a number of rebuild kits from him.
Chased down an open resistor after about two hours of investigation, and also replaced another. Also found a couple bad circuitboard traces, virtually hidden to the naked eye. 
Output transistors are good, it’s running solid via a separate external preamp. The internal preamp is very very weak, it’s extremely low volume, and the tone circuit is working, but there’s something wrong.
Also, the amp biasing and idle current is strangely off on both channels from the service manual specs. I’ve got them equal and what I feel is reasonable but there’s just something strange. I may have to look for a different service manual. Perhaps this is a different revision. for example I just can’t get either of the settings to be at the proper voltage or amperage, but I can make them equal and within a safe range.
I’m going to need to do some tracing of the power rails to the tone board, but I’m guessing that there are bad transistors in the circuit. Something catastrophic happened in the power supply and maybe it went downstream.
I’m pretty good at fixing almost anything and it takes a lot of extra time, it’s definitely worth it in the end.
I really wish I had more formal electronics training because I probably could diagnose things a hell of a lot quicker.
