WOAH baby, it always pays to double-check everything! There was a subtle broken trace on the keyboard on the PA7 line (it almost looks like physical damage, no idea how that can have happened). The diagnostic cart confused me, because it reports both CIAs as failed if you don't have a harness, which...

I took apart the keyboard and gave it a good cleaning. No visible wear and tear, and I tested the keys with a multimeter, so they're definitely confirmed working now. So I'm pretty sure it's the CIAs. Shame they are so freaking expensive! Apparently there is a modern replacement (JCIA) but that's no...

As you guessed, this latest issue was indeed another bad trace. I had checked this one twice before as well, so maybe I was just putting pressure on it just the right way to make a connection, but on a third check I spotted it, this time on the color ram. Probably thanks to me when I replaced it. No...

Latest development: a sad blow to my morale. I replaced the color ram and all the ram chips (with saruman), thinking this HAD to be it now. But I've made things worse. Maybe 80% of power-ons I get a black screen, 20% a screen with border and just a jumble of characters all over the screen. Dead test...

Thanks for all the info, this whole thing has been very educational.
And I'm determined to get my c64 back to life - this thread WILL have a happy ending one way or another!

Thought I'd post an update to all the masses following this riveting story. Well, I've replaced ICs u25, u26, u27, u28, u13, u14, and u16. And I only broke one trace along the way! I'm quite proud of myself. I also replaced all the 6 bigger capacitors while I was at it. But, basically no real change...

I've checked all the resistors (only a few needed to be disconnected in the end) and all are exactly as they should be.
I guess I'll order up those ICs...
I'm not sure if it's patience or obsession .... but at least I'm learning something along the way!

Definitely nothing I can detect visually or with a wiggle. I'll need to disconnect the resistors to check them, so I guess I'll just have to do that. Summarizing the main clues to help me get my thoughts straight: - Ruled out: CIAs, SID, PLA, U27, kernal - IRQ is low. That's on VIC 8, CPU 3, CIA U1 ...

Well, U27 has been replaced and..... nothing has changed! On the bright side that means I didn't break anything further in the process. And, at least I leveled up my soldering and I can cross one chip off the suspect list, but, well, that was basically the only one actually exhibiting weird readings...

Well I feel a bit dumb, of course I should've ordered a pile of ICs since they're like 10 cents and the shipping is $5 well, next time...
I ordered from retroleum, they've been good before.