How about swapping around U26 and U27 for a test to
see if the pattern changes in the self test? Might
need to replace these if we can get it to change by
doing this. At this point it could still be CPU, MMU
and a host of others too, but hey what fun!
There must be a bug in the self test code as usually
three rows is all I've seen. And this could bad ROM
itself or the bug acting only with the particular
defect. At any rate BASIC occupies the last half of
the last row of three.
So your first screenshot if it were all green squares
is what we want to see if it was booted with BASIC ON.
Last screenshot is not possible, but there it is anyway.
I'm leaning towards a failing ROM chip U5 at the moment.
- kirby76 likes this