Ack! Dead 3b1.

[Paul S McClay]

Ack! My 3b1 has died.

I believe the video RAM is bad. On power up the self test sequence
hangs with the LEDs indicating a fault in the video RAM and there are
glitches in the <16bits on/16bits off> pattern on the screen.

The act of taking the it apart and worrying over it for a few minutes fixes
it, for a while. Last time I tried this it stayed up for ~1 week. After such
a functioning spell, the fault returns and it panics, then may or may not
pass the video RAM test after a reset. If it passes, it lasts for no more
than a few seconds further, then goes down hard.

I don't understand what would cause this behavior. That mechanical
manipulation fixes it for a while suggests a mechanical (eg electrical
rather than electronic) failure on the board, but the bit rot on the
screen suggests bad RAM.

The bit rot always occurs in the upper third or less of the screen. It
is roughly, but not perfectly, confined to one or two bands a few
pixels high and horizontally periodic. I suppose the video RAMs are
64kbit chips, in which case one would cover a bit more than 1/4 of the
screen. I can't say for sure weather the bit rot is confined to the
first chip or not.

Is there any possibility that this might *not* be fixed by replacing
the #1 video RAM? (except that possibility that it might be the first
two chips)

Where can I get replacements?

Procedural problem: I *can not* disconnect the floppy drive power
cable.  I've pulled *very* hard, with no success. I can not see any
sort of locking mechanism on the connector. Is there a lock?
(This is a 3b1. Someone said some 3b1s were built with 7300 boards, which
power the floppy from the board. It was implied, then, that in "real" 3b1s,
the floppy power comes direct from the PS and the top plate can be trivially
removed from the rest of the frame. Is this the case? Do I then have a
wimpy PS for sure, or might it be a 3b1 PS?)

Your advice will be greatly appreciated.

  -Paul
--
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Paul McClay                            "Where are we going?"   "Planet 10!"
pmc at engin.umich.edu                    "When?"                 "Real soon!"
CompE & Physics @ U of MI            -                -        -    -  - --
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