In article 2HI3c.3458$re1.1290@newsfe1-win,
"Roger Hamlett" wrote:

"Al" wrote in message
...
In article om,
"Henry Kolesnik" wrote:

I was a GCA radar tech in the RCAF in the 1960s and in one of the
excercises
(war games) we had to find and fix a fault so the incoming aircraft
wouldn't crash, it was zero visibility . Sometimes it was as easy as a
bad
or loose tube, but some seargents had subchassis with cold solder
joints,
shorted black beauty capacitors or fried resistors. Time was critcal as
the
weather was closing fast and the aircraft was low on fuel. Sometimes
the
excercise left us without many parts, partially functional test equpt,
and
only partial manuals. To better simulate battle conditions, one end of
the
hut could be on fire and CO2 smoke to hinder visibilty! A shorted .01
uF
400VDC black beauty was easliy replaced with a .01 uF 600VDC or .02 a
fried
22K 1/2 watt with a 22K 2 watt or something close. Color codes were
quite
useful in many cases. The objective was to save the aircraft using
limited
resources. Today I don't think we see component level repair in the
field
but in battle anything may be necessary for survival. I'd much rather
have
something with component values rather than a bunch of codes that
required
decifering. I still contend this is a result of "military
intelligence."
And the codes make it tough on us hobbyists but we not under a critical
time
crunch and with the Internet it's usually a piece of cake.

In the sixties that was possible. But now you can't really fly by the
seat of your pants. Repair is by replacing LRUs (Least Replaceable
Units). Even if the LRU makes it back to the depot for failure
confirmation, it may not be repairable. 6, 8 or 12 layer PCBs cannot be
readily repaired. And would you trust one that was repaired if it did
not go through a burn-in cycle afterward? Would you depend on a
fail-safe circuit to prevent a nuclear launch if it had a component
replaced in it that was "close enough?" Maybe in your cars brake system,
but not on my missile!
True, in the 'close enough' stakes, but it is well worth reflecting that 90%
of simple systems use a relatively small 'subset' of parts from the avilable
world pool. An engineer, with one each of the IC's for a range of boards,
and a few dozen resistors and capacitors, can potentially repair most faults
on such boards 'on site', especially if the board is designed with this in
mind (possibly with some form of self diagnostics for many parts). However
if the same units are built with SM parts in quantity, using custom IC's,
the solution becomes to carry a complete replacement board. Doing this for a
few dozen products is often not practical. The problem here is that the
custom IC/SM solution is cheaper once production reaches a reasonable level,
but is not the best solution where the units are going to be a long way from
'spares'.
I designed a range of units used on sites across many third-world countries,
and it became necessary to deliberately design the units with both
redundancy, and repairability in mind. Given that the service engineer may
have to travel 1000miles to get to a unit, having it so that repair is
likely to be possible, was a vital design criterion. The parts list was
deliberately 'shrunk', retaining as far as was practical a limited range of
parts used in all. Unfortunately the relative costs of truly 'mass'
production, combined with robot construction/assembly, make such designs a
'dying art'...

Best Wishes

If your design criterion is that the equipment be field repairable with
readily available parts, then so be it. I have no argument with that!
But in the high-reliability military electronics world in which I worked
in the late 60's, that was not possible. Would you believe that one
printed circuit card, 4 in by 6 in, was needed just to implement 4
flip-flops using descrete components. Each component, yes even a carbon
composition resistor, had a serial number on it. Why? So it could be
traced back to the lot from it which it had been selected if it failed.
And boy, did those components have to be reliable! So that's why the
military specifications with their "strange" component markings were
invented. Expensive? Lordy, lordy! I was very shocked one day when I
requisitioned a capacitor from stock to compare to a rejected one. The
price for that unit, a precision paper mylar cap. was $100 - in 1970's
dollars! I almost fell out of my chair! And now you can buy stuff like
that, surplus, for just pennies on the dollar...and with their strange
markings.

I have a bag full of CSR13G825KR's! Anybody need one?

Al

Al

--
There's never enough time to do it right the first time.......