The CBM 3xxx and 4xxx series were revisions of the original PET 2001.
The range was rebranded CBM (Commodore Business Machine), as PET was dropped.
The 3xxx series introduced the 3000 series ROM sets, BASIC v3, and a completely redesigned keyboard layout, which used a full typewriter keyboard instead of the "mickey mouse" keyboard of the PET, and dropped the on-board cassette unit.
Well, at least you know the video circuits are working. Looks like the CPU isn't resetting right and isn't executing its code in ROM and fails to clear its video RAM. I had same problem with old Apple 2. It's probably just some corrosion build up on chip leads or sockets. Cleaning up the contacts with a soft pencil eraser and/or reseating the chips will be enough to restart your vintage computer. Hope that helps.
Monday 8th May 2006
Yves McDonald (the North, Quebec, Canada)
When I switch my PET on the screen is full of rubbish. It used to work fine sometimes but not now. Can it be fixed?
Friday 6th February 2004
A Hodson (England)
BUILT IN LANGUAGE
QWERTY full-stroke keyboard, and sometimes calculator type