Got me a nice IBM 5150 PC. This is the first PC of its class and I will make a separate post about it when I will make it work. When turned it on first, the PSU fan was spinning but no other life symptoms, no beeps, no video. Made a minimal setup with only motherboard, psu and speaker. Investigated motherboard, checked voltages and no voltages were present between ground and +5 or +12 volt pins ( when the connector was still connected to the motherboard ). Disconnected it and checked resistances between ground and voltage pins and found that +12 v rail was most probably shorted as its resistance was way too low, just a couple of ohms. Some googling revealed that tantalum capacitor can be faulty and can be removed. I have removed this capacitor just by cutting it off and the short was gone. But PSU was still kaput.
PSU in question was Schrack Elektronik EG 07068-B, Fab nr 2756, IBM p/n 1501438, 130W, 220VAC.
After fiddling with it for one evening, I found those faulty components:
Main electrolytic capacitor 220uF 385V. Replaced it with two 100uF 400v capacitors.
Three BUV46 transistors. All three were dead. Replaced with same type transistors.
Power resistor towers. Those were 2 2×4.7ohm resistors connected in series. They were not showing any continuity. Replaced those with whatever I found in my component box and it was two 5w 1.2ohm resistors.
Also replaced fuse from 2.5A to 1.5A. I don’t know why the fuse was not blown but by replacing it with a lower one I hope that next time when it fails, the lower wattage fuse will fail first.
After replacing those components PSU started working again. It will not work without load. When debugging I was using two lamps connected to +5 and +12v rails. On the final test, I tested it with just one old hard drive.
The first test of PSU in IBM 5150. One long beep and two shorts following error codes 0400 201 301.
0400 201 indicated a problem with RAM and that will be the next thing to fix.
301 is a keyboard error, and it is because I am using the wrong keyboard that works on the wrong protocol. I need some XT to PS/2 adapter. That will be a project on its own.