[snvzz]

Black screen both. Removed SIDs as they are unneeded for boot.

Some RAM chips were getting hot, so I socketed and replaced these. No progress otherwise, so I got myself a dead test cartridge.

One of the C64s had video capture card blink the screen on power on, the other not even that.

From this I correctly guessed dead VIC-II. Moving the working one to the other machine made that machine show the memory error blinks from the test cartridge. Swapping the other PLA finally allowed the memory test to end, and the display to show up.

I also had to replace a 74-series logic chip in one of the RAM rows, not sure which computer, do not remember how I figured that out. A color SRAM was also bad.

The remaining machine, I found that 6510 clock output was missing, from which I deduced dead 6510, which a swap confirmed.

Oh, and one of the two sids caused errors, so I had to replace that too.

[wkjagt]

Oh wow, seems like you encountered all the possible problems! Good thing you had two C64 so you could swap components. Do you have an idea why so many things had failed? Bad power supply blowing up chips maybe?

[300bps]

There is a voltage regulator in the standard Commodore 64 power supply that fails catastrophically. Specifically, it sends > 5 volts down the 5 volt line.

When this happens, depending on how long it's on it can fry every chip on that line.

That's why everyone advises not to use the original power supplies even for a quick test. Even if it tests 5 volts before you plug it into the computer it can fail at any moment.

[snvzz]

It was the power supply, no doubt.

Back at the time, we (little me and my family) already knew it was bad. But we weren't aware of its failure modes. All we observed when measured was low voltage, and that it "sometimes worked".

That's why so many C64s were damaged, and still are, when people connect their old supply without the knowledge (or against warnings) of such a nasty design flaw.