Not mica and not the grey plastic-like stuff, anyone familiar.?
They are on a Kustom 6160A mixer amp, USA Kustom , not Custom, under TOP66
trannies.
They have shrunk on the upper section (hottest ?) to the point of very
nearly shrinking under the tranny bodies and the uncovered parts are
ruffled, showing the heated area has shrunk significantly, even split
possibly, not taken apart yet, although check out cold ok.
No other over-temperature indications.
I will replace with mica but is this usual for these, I've not come across
white , relatively thick rubbery ones before.
Almost look like thin slabs of slicone bath sealant.

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Diverse Devices, Southampton, England
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