Metallisation creep ?

A slightly different kind of creep happens in the silver-mica capacitors in old radios. Many old radios from the 1950's had a small silver-mica sheet in the base of each IF transformer. Each sheet had two small capacitors deposited on it. One is in the plate circuit of one tube, with 100 to 150 volts on both its terminals. The other capacitor, about 1 cm away, is in the grid circuit of the next tube, at near zero volts. Over the years many of these have shorted out from one capacitor to the other. You can see the shorting is due to metal creeping across the gap.

The shorting in this case is often intermittent-- as the 150 volts is quite capable of zapping the thin metal film. If you ever hear a radio that emits a constant crash of static, as if in a monsoon, this is likely the reason.

I've never seen this happen in transistor radios, and I've played hundreds of old ones. Is this problem limited to certain transistor designs? Most US transistors went from alloy-junction type directly to silicon planar, circa 1962.

I could understand where there is a heavy polarisation DC but there would be minimal DC

disimilar mertal potential at most ) for stored trannies. Usually for ceramic resonators they are in ac only/(dc only by association) in filter circuits. I can only assume it is some weird chemical process going on with no DC around.

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

I have heard that such phenomenon happens in old Philips AF116/AF117 transistors, commonly used in the sixties in the FM tuner.

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Interesting, but OC71's are surely just a collectible now?