geoff wrote:

In our electronics lab where we maintain *very* expensive electronic and
electro/hydraulic equipment, whacking things with a rubber hammer is a
standard fault-finding technique.


The other good one is a can of freeze spray.

Any area of the board that creates or clear a fault with application of
freezer, is almost certainly suffering from a dry joint..

I resoldered ALL the eyelets - over a hundred - that were the substitute
for plated through holes - on one telly.

Fixed all the problems that banging it cured. That was after resoldering
half a dozen ID'd by freeze spray fixed SOME of the problems.




The vast majority of faults are not fancy electronic failures.
90% are mechanical, connector, chaffed wire, etc.
Of the remaining 10% electronic problems, 90% of those are PSU-related.
Only a very small percentage is actual gubbins-failure."


Totally my experience as well.

However the ones that I used to get handed to me were a skewed sample:
they were the ones that could NOT be fixed by such techniques.


And were far and away the most interesting.

I recall one, which was a shorted polystyrene capacitor. Solder bath had
actually melted it internally.

Showed up by prodding it.

Another power transistor failure was due to getting hot and the actual
lead out wire on the emitter expanding enough to short to the base..I
sawed it open and could JUST see where a microspark had welded the base
to the emitter. Completely off target spot weld on the wire, that JUST
passed testing, but failed the first time it was put in a unit.

Other nasties are digital timings and propagation delays..Yuk. stuff
that statistically mostly works until you get a hot (or cold) day, and a
bunch of all out of spec one way.

Conservative design and soak testing is the way you get round this sort
of stuff.

You HOPE that you have put the unit through worse than it will ever get
in service.