I'm working on a very early Apple II power supply, serial number in the 3000's. I want to restore it to proper working order, with a minimum of fuss to keep it reasonably original and historically accurate. Although I did have to drill out the 2 rivets that Apple decided to use to keep us safe from ourselves. I suppose I could replace them after I'm done.
Anyways, the problem is that it has a 1A fuse (soldered) on the PCB of the PSU, and the fuse instantly blows when you power it on with no load. I have found that R22, which is a 10 ohm resistor is exploded. Well, the color bands say it's supposed to be 10 ohms, but it tests as about a 1k ohm resistor on my DMM. See photo below.
Looking at the schematic for this PSU (see photo below from my copy of Winston Gayler's "The Apple II Circuit Description"), R22 is surrounded by C22 (a 470uF @ 6.3V electrolytic capacitor), Q3 (the large TO-3 switching transistor), among a couple other small signal diodes and resistors. I removed C22 and tested it on my LCR meter, and it tests flaky right after the fuse blows but then if you wait a couple hours C22 tests normal again on the LCR. So obviously I'm going to replace C22, especially given it's low 6.3 voltage rating.
I don't have an easy way to test Q3, but I could rig something up to measure the gain if I had to. The other surrounding components (diodes and resistors) look good and test to reasonable values (in-cicuit FWIW). I'm wondering if I should just order C22 and replace it (and replace the exploded R22) and give it a try. Seems the worst that could happen is I blow another fuse, and I have plenty of them.
Any suggestions from any experts here in switching power supplies? I'm far more skilled in the digital circuit realm, but I'm pretty sure I can get this PSU running again with enough perserverance. Unfortunately I could not find any documentation that identifies the physical components with the components in the schematic. Worse, the schematic doesn't identify the values of the components, so the only way to identify component IDs was to reverse-engineer the PCB layout to match the physical components with the schematic. The schematic and my physical PCB matched perfectly, but it was a tedious task. I've shared my results showing this component annotation in the 3rd photo below.