"Mark D. Zacharias" wrote in
. net:



I will often sub in an external power supply and just see what gets hot.
No need to cut any foil runs at this point.

You need to be a bit careful to isolate the original supply because back
feeding some supplies can give misleading indications or damage the supply.
For example, I have been told that some 3 terminal regulators are damaged by
back feeding.

Also, you need to make sure you provide ALL + and - voltages in the right
order as some ICs do NOT like it when one or more supply voltages is missing.

Cutting out the 5 volts is unlikely to do MORE damage than has already
occurred but feeding +5 without the -5, +12, -12 [or whatever]
could take out good ICs.

My FAVORITE tool is the 'Curve Tracer' that I build from a 1970's article in
Radio/Television magazine.
It can spot 'sick' gates on an IC, and a lot of other problems 'in circuit'.

It is similar to this one:
http://www.techlib.com/electronics/curvetrace.html

Mine is like the single range model. It is all built into a wall wart except
for the test leads.
I clip my scope to resistor leads that come out through holes in the case.

Anyway, I find it very useful for locating bad transistors and bad gates on
IC because the 'knee' gets rounded or is absent on bad junctions.

Often, you can test 'in circuit', but sometimes you will have to cut some
traces to isolate the junction from caps or resistors that are in parallel
with it.

A little practice with one of these, and a few 'reference junctions' to
compare with the curves you see, and you may love your curve tracer too.

Here is a modern version using your computer's sound card:
http://www.arrl.org/qst/2006/07/steber.pdf





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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