Deunan wrote on 2023-03-28, 11:33:

If you are not in a hurry you can wash that PCB in running water. Some solid soap and a brush with long hair will help clean the […]
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anetanel wrote on 2023-03-28, 10:57:

I will do as you said and start examining the components to the best of my ability.
I started with overlaying the pcb solder side over the component side so it will be easier to identify each component from the back.

If you are not in a hurry you can wash that PCB in running water. Some solid soap and a brush with long hair will help clean the difficult to reach spots. Leave it be for 1-2 days to dry completly in room temperature (you can inspect it during that time but not connect to mains). Cleaning helps spot burn marks and also removes possible accumulation of dirt that could get conductive with enough ambient humidity.

At this point you are most interested in the parts between the smaller transformer and the transistors. The power resistor and a ceramic disc capacitor next to it are most likely the snubber circuit (R4 + C8 per the schematic I've linked). The big blue cap is C7 on the schematic, make sure it's not shorted. This particular cap should in theory prevent a direct short from input rectifier through transistors, if it's shorted it would not do it's job anymore. Although the PSU should regulate the transistor pulses before this capacitor starts limiting, but it might just be the brief power-on phase that it needs to limit.

BTW when the fuses blew, did the load lightbulbs (on 12V and 5V) even blink? If not, if the fuse blew right away, then I'd start checking the blue cap for short as the first thing.

When the fuse blew, the 12v light briefly light up.
The blue cap seems to be ok. not shorted.
The "R4" resistor seems ok. It is giving me 48 ohm, and it is indeed a 5W, 47 ohm resistor.
The disc cap however, was shorted in circuit. I took it out and it is not shorted, but the pads it was on are still shorted. I measured a voltage drop of 0.048 (err... volts?), and resistance of 48 ohm.
I also noticed that those pads are shorted to the transformer leg with 0.03 ohm resistance. Not sure if that's ok..., but I guess it explains why there is a short (or at least 48 ohm resistance) across the cap.

The snubber is in parallel with main transfomer primary winding so you can't measure that capacitor in-circuit. Not even with 100kHz AC (with something like ESR/cap meter) because the primary side is relatively low inductance and next to no resistance. So all you are going to see is the resistance of the R4 plus of the winding (which is very small). But if the cap is not shorted outside circuit it should be OK, high voltage disk capacitors rarely die in quiet fashion, there's burn marks and missing chunks on blown ones.

So the bulbs did light up, it could be output regulation but I'm still suspecting the transistors. Run another test or even two. Run the PSU with 60W limiter bulb (after you re-check the diodes for shorts and replace fuse) with no load at all. The 60W bulb should be dark but will probably light up bright again but try to measure all output rails (you can connect the meter first and then do quick runs for 10 seconds to read the values). Write down values. You'll most likely see 5V a bit high, 12V a bit low but both within 10% of nominal. -12V can be a bit more off, but -5V should be almost perfect. Close to zero volts on either line or way higher output voltage would tell us where to look next.

If that looks OK then try 100W bulb and just 5V line loaded with 21W 12V bulb. Again measure all voltages and write down. You can skip this if the 60W no load test already gives weird voltages.