Hello all, this is my first post as I'm new to the list.
I found in the trash a fat Mac that gives a checkerboard pattern when
the power is applied. There is no startup tone audible. The pattern is
about 15 squares wide and 40 squares high. Any ideas on repairing the
problem (besides buying a Macintosh 128 logic board)? Here is what I have
tried so far, after Googling:
Pull the RAM and ROM chips and upgrade card (see below), and reseat
them (in correct orientation, of course). Same checkerboard.
Replace the 2 banks of 1256-15 chips with known good 1256-12 chips.
Same checkerboard.
Replace the 2 banks of 1256-15 chips with known good 1256-10 chips.
Vertical stripes.
Replace the 2 banks of 1256-15 chips with known good 164-12 chips.
Blank screen.
Quick power off, power on cycle. Same checkerboard, but a few pixels
reversed.
On opening the Mac, I discovered that this was a Macintosh 128 that
had been upgraded to 512K. The upgrade appears to entail: replacing the
old RAM chips with sockets, plugging 1256 chips in the sockets, replacing
one TTL chip with a socket, and plugging a small circuit board in the
socket. The identity of the TTL chips on the small circuit board has been
unfortunately wiped off. Logic board is clean, and voltages to the logic
board seem OK. I found ca. 2.5V, 5V, +-12V and ground on the wires.
Any help would be most appreciated,
Edwin
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