Mostek MK8022 QBUS memory repair tips

[Noel Chiappa]

So I have one of these Mostek memory cards, and it was non-functional, and a
Google didn't reveal _anything_ online about them.

The fault was a couple of picked bits, so I started off tracing the signal
paths for those bits from the bus fingers, to the transceivers, to an octal
latch, etc - and then it dawned on me that this card has two banks (i.e. it
has a 9x4 array of xx64 64Kx1 chips; i.e. to provide 16 bits wide plus byte
parity, there are two groups of 18 chips), and one bank was picking, and the
other was not. So that meant that data paths were all OK, it was a simple
matter of finding some bad memory chips.

(It turns out that diagnostic heuristic is quite useful, since many PDP-11
semiconductor memory cards have two banks of xxKx1 chips; so if you have
dropped/picked bits, look to see if both banks have the same fault. If 'no',
it's pretty much guaranteed to be a memory chip, and it should be easy to
find. I just fixed for someone an M8044 with this failure mode, without doing
any hardware debugging at all; the symptoms, and the prints, were all I
needed.)

There was nothing to indicate which banks/bits were where on the MK8022, but
by pulling memory chips (luckily, they were socketed, so this was pretty
painless), I managed to work it out (unlike many memory cards, it's not
semi-random). For reference for others, here it is:

Low bank:

01 - H1
...
0200 - H8
0400 - E1
...
0100000 - E8

High bank:

01 - F1
...
0200 - F8
0400 - D1
...
0100000 - D8

D-H9 seem to be parity.

If anyone has one of these cards, and it's busted and they're not up to
dealing with it, let me know. Depending on the failure mode, I _may_ be
able to help (no documentation of any kind, after all...)

	Noel