RA81's hanging??

[Rob Warnock]

+---------------
| DEC feels that the problems have to do with heat. Apparently
| something goes wrong when the boards get hot, even if they
| cool off. We had an air conditioner problem about the time
| the RA-81's started failing.
| 			Marty Sasaki
+---------------

A general warning note (on ALL electronics, not just RA81's):

Some failure modes are STRONGLY accelerated by operation at elevated
temperatures, and the accelerated failure rate can continue on return
to normal temperatures. This is especially true of the big electrolytic
capacitors in power supplies. They handle a lot of ripple current (A.C.
component), and are protected only by their low A.C. impedance. If you
overheat those babies, they can dry out a little bit. This raises their
impedance, and they start running hot internally from then on (impedance
times current-squared equals power dissipated). This in turn causes them
to die at a much earlier age than spec'd, say like a week to a month
after the original temperature problem was fixed. (This can be checked
by looking at the ripple voltage ( = current x impedance).)

Other similar effects: power transistors (e.g. in disk drive servos)
can get "hot spots" which continue to make the transistor run hot later;
power resistors can change value when overheated (sometime +/- 30% or more)
causing servo loops to be less accurate or stable (leads to seek errors or
just plain data errors); motors/transformers/speakers can short "one turn"
of a winding (or worse, just get "leaky" between two turns). Generally,
such failures are in high-power sections of the equipment (but not always),
produce no immediately observable effect (negative feedback designs can hide
horrible sins!), and the only way you know there was any damage is that the
equipment dies sooner than (or is flakier than) similar equipment that was
never overheated.

I once worked on an old DEC KA-10 which had such big power supply caps.
At least one of them would blow up (LITERALLY!), spraying goop all over
the inside of the cabinet, almost exactly two weeks after any major air
conditioning failure. (The system was installed in 1970 and is still running,
and is in the South, so there were a few A/C failures in its life... ;-} )

Remember, temperature-induced failure rates double every 10 degrees C.
Failures that include volatile materials are accelerated worse than that
around the melting/boiling point of the particular substance. The insides
of some of those parts are NORMALLY near boiling, already.

Rob Warnock

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