On Sun, 17 Jul 2016 11:27:18 -0000 (UTC), Cursitor Doom
wrote:
False assumptions have always plagued me. In this instance I falsely
assumpted that because the reg was functioning fine under a heavy passive
load, there was nothing whatsoever wrong with it. I didn't admit to the
possibility of partial damage which could manifest under other load
conditions and that made me completely blind to fixing the problem.
Lesson learned - I hope!
Assumption is the mother of all screwups. If it helps, I think
everyone has the same problems with bad assumptions. It's our way of
simplifying our lives. Our brains are pre-programmed to look for
patterns and repetitions, so we generalize based on the available but
usually insufficient evidence. Don't fight it. Assumption and
generalization work well if properly used.
In my life, I've noticed that first impressions are usually correct.
That applies mostly to people, but also to things. I just look at the
circuit, the components, the PCB, or the schematic, and ask myself
"what's the most likely culprit". Never mind logic, deduction,
measurement, and flow charts. Just intuition based on experience. For
example, I always look for bulging capacitors, previous repairs,
crappy soldering, corrosion, bad wire connections, loose parts, signs
of overheating, loose screws, and an assortment of common problems
that can be found by inspection. Do it often enough, and it becomes
automatic[1].
We have one big advantage with repair. We can assume that the circuit
was designed correctly and that it did work at one time. All we need
to do is find the one part that died. However, this was not really a
repair task, but rather a construction or engineering task, which
cannot make the assumption that the circuit was previously working.
So, I ask myself, what is the most likely screwup someone could do and
what were the likely causes of the circuit behavior? The behavior
made no sense, so went through my history of all the things I've
destroyed on the bench. I started with improper construction and
ended with the effects of overvoltage and overload while looked for a
pattern. Overvoltage and temporary wiring errors are my big problems.
Often, the combination blows things up.
If intuition doesn't pay off, I make measurements. I suggested that
you use a scope to look for oscillation and current spikes. It would
have helped identify the exact symptoms, but would not have pointed
directly to the LM317T.
I could go on forever on how I do troubleshooting. However much of it
is more intuitive than logical. That's guaranteed to produce more
confusion than enlightenment, so go with your first guess. It's
usually right.
Good luck.
[1] I've been looking at resistor color codes for much of my life. I
don't look at the colors individually and decode them into numbers and
multipliers. I look at the entire resistor, automatically recognize
the pattern, and (for example) 10K pops into my head. The same can be
done with defective PCB's and products. Something that looks "odd" is
often the culprit.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558