Today I got an IBM 5150 B in the mail. It was absolutely filthy. There was a layer of dirt-turned-crud covering everything. There were playing cards stuffed in the B drive and an old disk in the A drive. The system did not power up.
I almost gave up right then, but ..
I took out all of the cards (CGA, AST SixPak, IBM floppy drive controller) and then cleaned everything as well as I could.
Powered up - fan started, long beep two short beeps (missing/bad display). Ah, it must be one of the cards shorting out the power supply.
Correct - It was the display card. I replaced it. Put everything back in to the slots.
Powered up - fully functional.
I cleaned it to "excellent" condition with a magic eraser, paper towels with soap and water, q-tips for the keyboard and drives.
Another computer saved from the dump.