On Wed, 09 Mar 2016 16:16:19 -0500, legg wrote:

From previous traffic on this issue:

"If these are the maroon-colored parts, they are Philips flame-proof
parts designed to run with body surface temperatures in excess of 175C.

So you're saying these ARE the original factory parts? Wow!

"The long preformed leads are thin dia steel, with poor thermal
conductivity, in order to reduce thermal conduction to the printed
wiring.

"Your real concern should be the temperature of film caps and insulators
in the immediate viscinity, which have a lower tolerance to
overtemperatures. They should not touch.

I think I can now see how that first diode met its end. Having its own
radiant 1 bar fire juxtaposed to it would have lowered its junction
barrier height - we all remember how temperature affects p-n junctions -
enabling increased current flow and the eventual destruction of the diode
through thermal runaway. It was asking for trouble having that resistor
running at such a high temperature in this particular position.

BTW, there's an identically-sized power resistor at right angles to the
one I've now binned and after a half hour's running, it tops out at only
33'C. I know it's got different working conditions, but in this case it
was correctly rated for its job - unlike the other one. Looks like a
design error after all, then.

Larger bodied resistors with the same power ratings may run at a lower
body surface temperature, but this will be of little value if their bulk
results in reduced air flow or physical contact with nearby components.

Not a problem with the new component.

This position should not be filled with a wire-wound component.
Metal glaze, at certain values and body sizes, offers a reduced self
inductance, allowing them to function in higher frequency snubber
circuitry.

Yes, I'm aware of the perils of parasitic inductance in w/w resistors;
the one I've selected doesn't suffer from it.

Be nice to see some traces tomorrow (one can but hope!)