Page 174 - Troubleshooting Analog Circuits

Letters to Bob 161

Figure 13.3. This scheme will make your tantalum capaci-
tors immune to damage from reversal.

forward-bias the transistor’s intrinsic base-emitter zener, you could cause damage. If,
instead, the circuit prevents zener breakdown and its clamps prevent excessive re-
verse VB~, then everything is fine. I probably wasn’t clear enough about those points.
Note that many discrete transistors are not as fragile-are not going to be damaged or
degraded by zener breakdown-as monolithic transistors are. (I’m not sure why.)
Also, you recommend against plugging CMOS components into live (powered up)
circuits and working without a ground strap. You say you have seen such procedures
directly cause unreliability and failures. OK, you have and other people have. but I
haven’t. Perhaps RAM chips are more fragile than 74Cxx chips. So, I must retract my
cavalier and flippant remarks: In general, you should use ground straps and not plug
ICs into live circuits unless you are sure you know what you’re doing and are pre-
pared to accept IC failures.
However, when troubleshooting, sometimes you may have to resort to these mea-
sures. You should then be aware that they may not necessarily cause harm. But I, and
the readers, should be aware that sometimes they my cause harm, so don’t hack
around if you don’t have to.
As for tantalum capacitors, I’ve seen very few fail with no provocation. I’ve used a
lot of cheap tantalum capacitors, and they must have been more reliable than I de-
served. To ensure that they survive reversal, I suggest the arrangement in Figure
13.3.
Using a current probe to find short circuits is a viable technique, but I don’t under-
stand how you can clip a current probe around a board trace. I find that my DC mi-
crovolt detector, which appeared in Chapter 2 of the series, will let you track down
such DC shorts. If I had to troubleshoot a lot of boards, I’m sure one of your audio-
output, milliwatt detectors would be tembly useful.
Although I’ve never seen a problem with floating ’ITL gates, you are correct to
caution against circuits like the one in Figure 13.2.

RAP

Dear Bob:

As an old hand with 30 years in the business, I have run into some anomalies you
did not mention.
1. LS logic is totally unforgiving of negative undershoots at its inputs. The worst chips I
have seen are 74LS86s, which hang up for microseconds, totally confusing other
circuitry. The second worst is the 74LS75, which can go into either logic state after a
negative undershoot but will recover upon the next clock pulse.

2. I have seen circuits with a 7470, -73, -76, -107, -109, -1 10, or -1 1 1 that “remember”