[CCTECH]74123 woes

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Mon Jul 15 15:09:00 2002

>From: "John Honniball" <coredump_at_gifford.co.uk>
>
>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
Received on Mon Jul 15 2002 - 15:09:00 BST