Amiga 1200 rev 2BScreen stuck on one colour

Red: the ROM (Kickstart 2.04 and later)

The ROM failed its checksum. Only Kickstart 2.04 and later run this check: on 1.x, red does not point to the ROM. Clean the socket contacts and reseat it; if it stays red, the ROM is damaged.

Green: Chip RAM

Chip RAM error. The usual suspects are the RAM chips and the traces between them and Agnus or Alice, often eaten by a leaking capacitor. CIA-A can also cause it, if it fails to move the ROM out of the way at start-up.

Blue: the custom chips

Custom chip error: Agnus, Denise and Paula (Alice, Lisa and Paula on AGA), according to community lists; we have not found this test in the Kickstarts we could check (1.2 doesn't have it, it is switched off in the 3.1 source and 1.3 is unconfirmed). Start by pressing down the socketed ones; the A500’s Agnus tends to work loose.

Yellow: a processor exception

The processor took an exception (a bus error, for example) before Kickstart had the handlers that show the Guru. It does not mean the processor is faulty: it can be a bad ROM read, the bus or an expansion. Remove any expansion first; if it persists, check the ROM, the processor and the sockets.