[R-390] Module swapping for troubleshooting

[Tisha Hayes]

The modular design of the R-390A and the standardization across all of the
manufacturers should make it possible to mix any manufacturers modules with
any other manufacturers. That was the intent of a standardized design, so
maintenance folks could swap modules to make working radios and to ease in
troubleshooting down to the module level.

Ideally, higher tiers of depot maintenance would chase problems down to the
component level, bring the module back up to spec and then the module would
be returned as a spare or put into stock in case another radio went down.

As with any sort of electronics assembly there will be some slight
differences between modules due to component tolerances and how some of the
alignment works across modules. This would be more apparent with the RF,
IF, PTO and crystal decks where those sorts of interactions become more
apparent.

Your problem with an excessively high audio level should follow the bad
module. If you have an assortment of known-good modules you should be able
to swap your way into isolating the problem to a bad module.

It sounds like you are finding all sorts of little, nagging problems with
potential shorts between components, bad solder joints, possibly bad
capacitors and who knows what you will find with out of spec resistors.
This would not inspire any confidence that by going through a module that
you will have something that is 100% known good. For module level
troubleshooting you really do need pieces that you are absolutely certain
that are good, otherwise you are doing nothing more than component level
troubleshooting that requires a bit more skill and understanding of how of
the elements of the radio circuitry work together.

My concern is that by undergoing a wholesale swap-out of capacitors really
does not put you any closer to solving the problem. While you may
eventually luck-out in replacing the bad part it may get pretty expensive
and frustrating until you reach that point.

--------------
A number of years ago I was keeping my eye on a tech who was involved in a
particularly puzzling problem with a power supply. The stock manager had
called me to say "Todd seems to be having a problem with something, he has
been in here four times today to get parts to repair a unit". For me that
was a clue that either he had a unit that was beyond economical repair or
he was "easter-egging" a problem. I stopped by his bench and he showed me
where he had this supply that just had a terrible problem with AC ripple on
the supply. On such a seemingly simple circuit he was pulling his hair out.
After a day of him struggling with it I suggested that he turn it over to
another tech to just check out. When the unit was moved to a different test
bench they could not find the problem. The power supply worked perfectly,
ripple was down in the low millivolts range.

The problem was that Todd's oscilloscope had a bad ground connection on the
probe. He was so focused on solving the problem that he never considered
that maybe his test gear was at fault.

-- 
Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama