Tagboard insulation failure in Quad II amplifier

[Murphyv310]

What's the mechanism for WWs going high if overrun? All I can think of is oxidation of the wire making the metal thinner and hence higher R.

I can understand WW resistors reducing in value due to heat induced shorts between turns and/or deposition of conductive crud.

What happens Jeffrey is when the resistor is over-run, it could be due to a fault elsewhere or the wrong rating of the resistor. The resistor overheats and at this point in effect the value rises, if this continues and the resistor is not changed overtime this pulsing of overheat and cooling of temperature can cause the wire to expand and contract then physically wear on the ceramic or whatever the actual wire is encased in. This then gives the resistor wire less cross sectional area and the value rises, the old Radio-spares wirewounds were guilty of this no doubt due to engineers replacing 10 watt resistors with 5 watt types.

I normally start with the cold value of the resistor and bring that back to nominal. Its value was originally specified cold of course, so any resistance changes due to the original temperature coefficient should have been built into the equipment's design. There may be an additional 'new' tempco arising from ageing effects, but if there is then, as the above posts show, people's experiences of that do vary. If the new tempco is negative then padding 'too low' will at least provide some future-proofing against continued upward drift. I did some measurements on a single resistor set over a five year period and, broadly speaking, the drift which had got them to their initial value seemed to be continuing at about the same rate over the five years. So it's not that the resistors drift and then settle.

As folks have said, the long term reliability of a Quad II might well be better if its innards were stripped out and modern components were substituted. Fortunately Quad have met that need for people who want it. The Quad II Classic is exactly that http://www.quad-hifi.co.uk/product-detail.php?pid=20, made very nicely in China. I own a pair and they are lovely, and I never worry about them. My customers don't want this though. They want original. Many of them buy classic vinyl and old stock valves and a few even drive classic cars. If you're going to suggest that they should drop the original engine out of the latter and fit a modern one then you might want to do that from a safe distance   .

I do draw the line at leaving Hunts Micamoulds in critical positions though !

Sorry, I forgot to say, I don't use hidden micro types Gary, I just tack ordinary half-wattish metal films underneath the original Eries. That way people can see what I have done and remove it if they don't like it. They can also swap the resistors out for new ones in a decade or two if  the drift requires it.

VB

Interesting reply VB.
I must admit that I have rejected a couple of amp repairs as the customer would not let me replace faulty resistors. I made the point that I don't patch in other resistors to restore values of old and dubious resistors.
I could never understand why some of these people would cut out rusty sections of classic cars and then fit new pattern parts fit 5 speed Ford Sierra gearboxes etc and not change old duff and poorly specced resistors in quality amps, one thing that really baffles me.