Also televisions, and computer video monitors; anything with a CRT and focus circuit.
It's partly the high ohms value that does it. For a carbon film resistor, the high ohms means a thin film (so a few microns of surface oxidation does major damage). The situation for composition resistors is similar, you cannot get high value in composition resistors without small particle sizes.
** Only last week I had a 50 year old valve amplifier on the bench ( Roger s Cadet II) that used all CC resistors. Most had drifted high by 45% to 300 %. A couple were open cct. All eight UK made Mullard valves were original a nd OK, so the amp had not seen excessive use.
.... Phil
** I have a quantity of one like that - banded 100ohms, 5%.All of them show 109 ohms +/- 1 ohm on my Fluke DMM.
.... Phil
Our observed drifting CCs were "new" in drawers. It appears that higher values suffer higher drift.
But the tolerance itself doesn't change anything in the context of aging, so it must be another factor. The 1% parts are mostly metal (oxide) layer, so is it that? Does metal (oxide) layer not age noticeably? What else does not age? Mica caps?
Another thing to mention: I have a 2k/0,02% resistor older than 30 years and it still gives 2,000 readings on all my meters. It is big and sealed - what can be inside?
c.
Either conductive bulk, or conductive layer, or fleas transporting charge packages.
Probably a card sloppily (low tension) wound with manganin or constantan wire, possibly with a series or shunt trim select-according-to-test resistor. I'd wager (if manganin) it's good for +/- 10 C temperature range for that accuracy. To exclude oxygen (so the wire doesn't corrode) there may also be an oil bath or potting material in addition to the sealant. Lacquer on any uninsulated surfaces is likely.
It's possible to make metal films quite well nowadays, but annealed bulk wire has superior aging (a metal film on ceramic substrate can undergo mechanical stress as the dissimilar materials temperature-cycle).
** The tolerance spec of a resistor includes effects from soldering and aging for a reasonable lifespan of the part. This is why the value when new is well within the marked percentage. 1% resistors normally have much less aging than 5% or 10% types - they would be useless as precision parts if they did not.
An exception is WW types, where aging is very low but tolerances are rarely better than 5% cos of the very different manufacturing process involved.
... Phil
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