
I had the same problem with my 512 a long time ago.  Resoldering
the joints on the motherboard (all of them) fixed it.  Turns out
that continuous heating and cooling caused annular (ring-shaped)
cracks to develop in the solder, effectively cutting the video
circuitry off.  If you're not a solder-jockey you might want to
have someone else do it -- I took mine to an electrical engineer
buddy -- but it was a 20-minute job, tops.

-Kelley-
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Thomas Kelley Boylan, PowerPC, IBM Austin, kelleyb@austin.ibm.com