I FIXED IT!!!
It was U2 (6526 CIA). I replaced it with a 6526 CIA from a non functional C64C board I had available. I used U1 from the spare board rather than U2 since the two are identical IC’s and U1 isn’t routed directly to any ports, unlike U2, which manages the user and serial ports. Therefore, the likelihood of U1 being damaged is less than that of U2.
I was a bit concerned with heat damage since this IC has already proven to be sensitive. So rather than desoldering the IC, I ended up clipping it from the board and soldering forty little legs consisting of leads saved from trimmed resistors, diodes, etc. which were scrapped from past projects subsequent to installing those components on PC boards. It wasn’t pretty, but this method minimized the amount of time necessary to heat each pin of the IC. Actually, it appeared quite humorous before I trimmed it up. I’ll share an image. It should be good for a laugh.
Thanks for your help, carlsson. If I hadn’t found those C64C schematics while rummaging through your extensive C64 archive, I may not have been successful. Thank you!
The reason I didn’t want to, simply, give up on this particular board is due to the fact that I had performed several modifications to it which I would rather not repeat. I will add images of those modifications to the
hacks/mods section of
bigmech.com. In addition, while resolving issues on my own, I always end up learning something.
I hope this thread may be of some help to anyone who has a C64 with serial port issues, or anyone who may have made the same mistake of inserting a cartridge into a C64 while the power is on.