SWTPC 6800 weirdness

[william degnan]

On Tue, Sep 6, 2016 at 2:25 PM, Brad H <vintagecomputer at bettercomputing.net>
wrote:

> My 6800 has been mostly working, but it seems to be occasionally flaking
> out.  I don't know why.  Sometimes you go to power it up, and there's no
> response on terminal side.  The 'fix' is sometimes to wiggle the memory/CPU
> boards and then for some reason it's fine(ish).  There are five cards
> installed right now - the MP-A, MP-S, a heavily modified MP-M board (with
> rams piggybacked on all the original RAM chips) and then two Digital
> Research 16k boards.  The system was modified for Flex 2.0
>
>
>
> Today it flaked again and would not come back up, so I pulled the MP-M
> board
> and the MP-A board and swapped slots.  It came up, but memory at $0100 was
> missing.    I tried powering up, swapping slots, etc.. same deal.  Then I
> left the machine for an hour, powered up again.. boom.. now $0100 is back.
> I don't fully understand the addressing system but if that MP-M is
> configured as $A000 would that cover $0100 as well?
>
>
>
> I've tried changing the jumpers on each of the DR 16k board to cover
> $A000..
> but the machine will not boot.  I will only work with either the MP-M alone
> or those 16k cards with it configured to other spaces.  In other words, the
> machine does not accept any other card configured for $A000.
>
>
>
> I'm wondering if anyone has any suggestions on how I might nail this down.
> I'm thinking some of the RAMs on that MP-M are flaky, but it would be a
> *nightmare* to try and diagnose it, with all the RAMs piggybacked, all the
> little jumper wires, and everything soldered.  I'd prefer to bypass it and
> either use my other, less modified MP-M or just the DR boards, which are
> socketed.  If any of you have suggestions on how I might take this MP-M out
> of the equation that would be awesome!
>
>
> Brad
>
>
Brad,
You'll need to make electrical measurements, from the system checkout in
the manual.   You very possibly will have marginal components that need to
be replaced, but it's best to try to locate which is bad rather that to
replace at random.

A000 is not the same place as 0100.    In the 64K space, they're quite
distant.

Eliminate all but the one RAM board, setting it to 0000.  Test that
thoroughly, then add the next at the next RAM space beyond the first card.
Continue until you have enough RAM for a minimal Flex boot.  It should tell
you in the version of Flex you're using how much that is (24K?)  It's hard
to do everything at the same time, break it down into chunks..
Bill