OK here is the situation so far.
Apple /// 1, well call it 1 for less typing. 1 was in very bad shape. Lots of mouse pee, dropping, dust, and cobwebs all over the MB. The PUS looked good.
I did a very careful job of cleaning it up. I washed the MB. let it dry for many days, after blowing out water with my air compressor. I reseated every IC, inspecting each for bad pins. Some had a bit of corrosion. I scraped those pins and reseated a couple times to loosen anything in the sockets. I air blow it all off careful again. I let it dry for another day just in case. Hooked it up to it's PSU, and nothing. Not even a blink on the screen. The red LED near the rear of the MB did not light. I tested the PSU with my multimeter, not under load, and got 5v on some line. I do not know which should have a voltage, as this 1's psu cable is all black wires.
Ok now for Apple /// 2. It was just very dusty. I dusted it off, hit it with the air compressor. I also hit the psu with some air. reconnected it all up and booted. I lives. I got a beep at first power. I get a memory test, then a screen full of '5v+'with several varying shades of green squares between them. This repeats until I press Reset. Then the floppy drive (FD) spins, there is a hiss from the speaker. and 'RETRY' shows on the screen. I have not cleaned the case, keyboard, or the FD of 2 yet.
I then connected 1's Mb to 2's psu,keyboard, and FD, and I get a beep, and brief flicker on the screen. I press reset, and the speaker has a his, but no FD spin, or anything on the screen.
From this I assume 1's psu is actually bad in some way. And that 1's MB is partly alive. I have cleaned the case, keyboard, and floppy of 1
I watch a youtube video of a A3 booting and after a reset press, 'Apple III' shows on screen. But I can't tell if a disk was inserted or not.
These are my first A3's. Both are A3 Plus, 256k, 5v memory board systems. I have several NOS Apple Utility disks. I can use a multimeter, and am fairly confidant in my soldering skills. No scope, no EE knowledge. I know the difference between a resistor, and a capacitor, but do not understand fully the marking on either. I know just enough electronics repair to be my own worst enemy.
I think I should put 2's Mb, and PSU in 1's case, with it's clean but untested keyboard, and untested FD. Y/N?
How to fix 1's MB? Would swapping 1's IC's, one at a time with 2's IC's work, with a boot of each after each swap? I have 2 working A3 screens, but only known working psu.
Suggestions on how to proceed.