Hello everybody!
A bit of background for me. I've been in the electronics hobby for about two years and I started with tube audio amplifiers. Nothing fancy, just grasping the basics and doing some experiments with simple circuits. Also I've repaired some broken computer supplies that had obvious faults like blown caps. I only have 2 DMMs and my newly aquired oscilloscope is on it's way. I should have it in a week or so. Also I've just bought an ESR meter that I've used for the first time in this project that I'm about to tell you about.
I came across this power supply that's rated for 12V/84A output (about 1000W). The problem is that it's faulty. I would like to get it working again. It had one of the two fuses blown. Trying to get the fuse out I broke it and saw that the internal wire burnt at one end. I resoldered it on both ends so I can do some experiments. I did that as I didn't have any at hand (16A/250V). Since I learnt something from the tube world I decided to power the unit with a lightbulb in series (poor man's current limiter). I used a 75W bulb.
When I power the psu the bulb lights up and then slowly fades for about two seconds. Then the relay comes on and the bulb starts flashing brighly. The power supply has two status LEDs (DC Good and AC Good). At startup for the two seconds that the relay is still off the AC Good is lit up. After the lightbulb starts flashing the AC Good turns off and then I have no clue whats happening
I used the ESR meter on all the large electrolitics (there are a few anyway). I've also tested a few smd ones but it's not that easy since the whole board+components are covered in a clear silicone like substance. I have to scratch that off so I can get a good electrical connection with probes. Also I've tested the large silicon for shorts and there aren't any. I would have removed these components to test individually but it's a real pain to do that. The board is double sided and somehow there's solder on both sides for all of these parts. I guess I'd have to cut all the legs and replace with new parts if I ever have to take them out.
I figured that everything is ok until the large caps fill up and then the bulb flickering occurs. I measured 12V at the relay supply so that's ok. I traced that back to a board that is attached to the large pcb that's next to the two large electrolytic capacitors. Also there are some large diodes that tested ok for voltage drop and shorts and I also managed to remove the rectifier from the board and test it and that's ok as well.
Other than that I don't know what else to do. There isn't a single component that looks damaged, burnt or bad solder. I've also probed for temperature but there was nothing above 30 degrees C.
What would you recommend to do next? I'd really like to repair this PSU and is also a nice learning project. I attached some pictures with some notations for the large silicon and connectors etc. I can take more detailed pictures and more clear. I made these in a hurry with my cell phone.
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