This pertains to the Tasma M1331T mentioned in another post with output transformer woes. After having put in a temporary o/p transformer was letting the radio run for a while.

Again it stopped, signal disappeared although tuneing about seemed to be working like a TRF radio. Suspected an oscillator problem and on a hunch replaced a brown silver mica and one of those small waxed silver micas which were attached to the oscillator coil.

Happily this restored the radio to normal operation. Remember there has been discussions on silver micas which in this era have aged due to silver migration and are now unreliable.

Shortly will replace the rest of the silver micas.

In only one Australian radio struck by lightning have I had cause to replace all of the capacitors. The ones across coils are often there to bring it into spec & are rarely marked with a value. The Australian Micas and Silver Micas after the mid thirties unlike American ones, were quite reliable.

The ones on IF's in USA give trouble with Silver migration. In 60 years I have only replaced about three in IF's and they were mechanical failures. The most prone to failure are those with high DC voltages across them, not to be confused with many in IF's whoes caps have high voltages, on them.

The principal test for Mica's & Ceramic caps in valve radio is "Leakage Testing" 500V DC. with an insulation tester. 99.99 % of Wax paper & a lot of oil filled caps you test (if you must) at operating or working volts (WV /SV) and they will leak like seives.

The only trouble I've had with mica caps is those with a large voltage difference across them, they produce a crackling noise. It's generally the one around the first audio amp.

However, given that these caps on average would be 60-80 years old, I expect anything could happen.

Yes, I seem to be changing out more now than ever before. Mainly ones with voltage potential across them. The post war “simplex” are becoming quite troublesome nowadays with varying leakage issues.

Yes my experience with old TVs is similar.

But if the mica cap is connected across a coil, don't bother changing it. It will almost certainly be OK.

It's the applied DC voltage that invokes the silver migration.

Modern gear has a similar issue looming with the tin in lead-free solder. The Tin Whisker syndrome.