Here's a fun and easy one to fix.
One commodore plus/4. Here's what it displayed.
Looks like the TED chip is outputting something, but it isn't setup right.
So I pop it open. There's the right clock on pin 1 of the CPU - switching between 1 and 2MHz as it does. Ok, so let's check the ICs.
Hm, there's something wrong with the CPU pins.