1/3W resistor failure mode?

:Was in a valve /tube amp HT line as a dropper carying about 250V , no :obvious heating of the body or surround of this 10K resistor and no obvious :burning of the resistor track, microscope view :

:B is the original blue body colour, C is part of the ceramic coating that :got left after my scraping. My scraping was axial so not the cause of the :tapered loss of metal oxide in the spiral between * and *, actual break just :off pic and a slight trace of this loss on the spiral above *-*. What caused :this tapering loss ? electrochemical? manufacturing fault ? All other 1/3 W :resistors in the amp look the same manufacture so wonder if being in the HT :line is significant or could any/all of the other 1/3W resistors fail in :similar fashion

Could it be the voltage rating of the resistor and the possibility of higher than normal transient peaks?

I had a similar case where I used a Beyschlag 0.25W/70C MF resistor in a 240Vac mains driven circuit and after about 6 months the device stopped working. I discovered this resistor, which looked perfectly normal, was open circuit. I replaced it with an identical resistor and some months later, similar fault. I decided to replace it this time with a hi-stab 0.5W carbon film which was physically larger and it hasn't failed since. Checking the specs for the MF resistor showed it had a max voltage rating of only 250V, and it would have been exposed to peaks of more than 300V from inductive components during operation. The carbon film resistor replacing it was rated at 350V.

Perhaps your MO resistor has suffered a similar failure mode. But unless you know the manf specs for the resistor in question it may be difficult to determine.

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That was part of my thinking. Whatever the rating, they were of a type used throughout the amp so presumably whatever is cheapest so not HV rating. What makes the difference in construction or whatever , between HV and LV use , afterall the value of resistance and wattage sets the volts across the length of the resistor. I've always observed the policy for HV ones as droppers for the supply for zero-crossing ICs etc for triac circuits etc, but have never known the physiscal reason, other than the wrong , ie LV ones, will fail in such circumstances.

Anyone know the chemical/physical reasoin for the tapering failure, I suspect some very sophisticated magnetic field distribution effect, but just a guess.

--
Diverse Devices, Southampton, England 
electronic hints and repair briefs , schematics/manuals list on 
http://home.graffiti.net/diverse:graffiti.net/

I should have said there is a waveyness to the terminator between surviving MO track and the erroded section of the taper on the original full resolution pic for this

shown here

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

On 3/15/2009 11:38 PM N_Cook spake thus:

Oh, come on; there's a very simple explanation: the resistor was *made* that way. Shit happens.

Sheesh.

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