[CCTECH]74123 woes

From: Jim Battle <frustum_at_pacbell.net>
Date: Mon Jul 15 04:56:00 2002

I've been working on trying to get a Sol Helios disk system up and running
so I can recover and archive a bunch of disk images. If I can't get it
working (I'd really like to get it running for its own sake), I'll get a
catweasel card, a conventional 8" disk, and attempt to write my own
bit-level decoding software to recover the disks.

Anyway, I found and fixed the first problem: one of the satellite circuit
boards on the persci had a tantalum cap that had a dead short, taking out
the whole +24V power supply (it used a 3-terminal rgulator that detected
the problem and cut off power, thus there was no pop nor any telltale burnt
traces or wires). I fixed that and things are working somewhat better, but
I still can't seek to any track except 0 (via the restore line).

After discovering that the schematics I have are for a somewhat different
revision of the logic board than I have (doh!), I tracked down the problem
to what appears to be a busted 74123 (not LS123, which is slightly
different logically). The positive trigger is always high, and the Q-bar
output is always low -- that is, the thing looks like it is always
triggered despite the fact that there is no edge on the trigger input.

I could remove the 74123, replace the resistor and cap and see if that
fixes it, but doing it would require a lot more disassembly than I'd like
to do without being confident that what I'm seeing really is a problem.

Here is where I need advice. I can't find my TTL databook, but it would
appear that the trigger inputs are edge triggered, not level triggered, right?

Secondly, I haven't done board-level design in more than 10 years, but from
what I recall, 99 times out of 100 when I thought I had tracked down a bug
to a back chip, it was something else (then again, that was during design,
not just working on ostensibly correctly designed boards). I have little
experience with the '123 since using one for any timing critical function
simply hasn't been kosher for the past 20 years at least. Does anybody
know in such a situation like this, is it more likely to be a bad cap or a
bad '123, or a short somewhere else on the trace? I can measure the
resistance of the R and it is in spec. I've buzzed the circuit to see that
the R and C are connected to the pins of the '123. Any other
suggestions? Oh, the C is a mylar (I think) with a 5% precision rating
stamped on it (100pf).

Jim Battle == frustum_at_pacbell.net

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Received on Mon Jul 15 2002 - 04:56:00 BST

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