[CCTECH]74123 woes - test-drb@ccmp.vtda.org

15 Jul 2002

...
 From: "John Honniball"
&lt;coredump(a)gifford.co.uk&gt;

Jim Battle wrote:
  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... 
Yes, that's a typical failure mode for tantalums!

  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). 
Until recently, I'd have blamed something other than a 74 logic chip
just going bad.  However, I've seen a few chips do just that in RL01
disk drives, which must be about the same age as your chips (late
1970s.)

So, I'd say it's possible that the 74123 has just ceased to function
"all by itself".  Due to age, that is, not due to external factors.
In the end though, it's up to you to decide whether you suspect chip
failure enough to warrant replacing it (with all the associated taking
apart, soldering and putting together again). 

Hi
 The capacitors used with 74123's tend to fail over time
because they are slightly back biased at the of the discharge
part of the cycle. This does in tantalums and electrolytics,
over time. Non-polar parts are usually OK.
Dwight