[CCTECH]74123 woes

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). Thanks. ----- Jim Battle == frustum(a)pacbell.net _______________________________________________ cctech mailing list cctech(a)classiccmp.org http://www.classiccmp.org/mailman/listinfo/cctech