Red Screen on Startup

[ferix]

As far as I know, a red screen at boot means there's something wrong with the kickstart rom...
Try pushing down those chips (two chips on A4000, I think), to prevent bad contacts.
Or may be that printer drains too much power from the parallel port data lines... and It becomes on a malfunction of the rom chips, they are power munchers in the way they are connected on the amiga, with the CS signal permanently tied to ground.

[ferix]

But it only happens with the printer on. If it's off, no red screen!

Yes, but, as I sugested, I could be a power problem. The printer could be drainig too much current from the data (or control) pins. It could be a bad cap, a bad isolation, or watherver on the printer side.
If the printer drains too much power, but not enought to get the A4000 down, the first things to fail are the most "power hungry" chips, as roms, and even some datapath drivers (74LS/F245, 74LS/F373, etc...).
I saw such simptoms on other computers.
On the A4000 side, It could be a bad cap, but It would fail with or without printer...