Before I start, let me tell you that I am well aware that conventional wisdom says to start troubleshooting at the source, since there is no point in troubleshooting a circuit that receives dirty, incorrect or no power. For this story's purposes, though, I thought it might be more interesting to investigate what feedback signals look like during abnormal circuit operation before fixing it, so I decided to start from the output instead.
What are my main points of interest going forward?
Looking at the signal path backwards from the 5VSB output to the primary side, we find:
- the TL431 shunt reference partly hidden by the capacitor, acting as a comparator and also as a driver for the photocoupler
- an 817 photocoupler
- the 22µF capacitor I did not change, which may have been roasted by the zener
- some other issue with the auxiliary output; taking advantage of the new capacitor's exposed leads, I hooked up my multimeter to measure its voltage and read 0V there, which seems obviously wrong
Time to start poking around and see where things break down.