Commodore SX64 probs...

I've got a pair of non-working Commodore SX64's here. One machine powers up, but the floppy drive light remains lit and I can hear the drive spinning continuously, and there's a 50% grey pattern filling the screen (i.e. pixels look to be alternating white/black). Actually, there's also short run of pixels middle-bottom of the screen which aren't illuminated at all. Pressing reset has no effect. Pressing caps-lock does result in the caps-lock light illuminating, but I don't know if that's a simple circuit hardwired to the key and doesn't go via the CPU... The second machine is totally dead - no activity whatsoever. There's a *very faint* humming noise from the monitor area, typical of a display that's at least getting power though - but on the CPU side of things no chips seem to be getting remotely warm, suggesting that there's no +5V. Before I start digging deeper, does the fault with the first machine sound familiar, and are there any common PSU faults which affect these systems? Plus, anyone have schematics they could scan? Be nice to get one of them going. One keyboard is damaged (plus I only have the one original keyboard cable), so it's only really viable to have one running, but I'd like to get to the bottom of what's up with both of them... cheers, Jules