[Ron Ramirez]

Knowing that micas can - and do - go bad just like paper and electrolytic capacitors do, here is the approach I take:

If the mica is in the RF section of the set, I generally leave them alone unless there is an obvious defect.

If it is connected between the plate of a tube and ground as in the driver tube of a Model 18/118 set, it gets replaced.

If they are encased in bakelite blocks, they get replaced. (Most older 110 pF caps in the bakelite block containers were paper, but replacement blocks often contained mica caps. The only way you will know is to melt/dig them out, and by the time you do that, you will have to replace anyway.)

And the 370 pF caps in the pushbutton mechanisms of 1939-42 Philcos always get replaced automatically.

In most of the instances given above (2nd detector and audio bypass), 100 pF ceramic disc caps work well as replacements. However, on the 1939-42 pushbutton mechanisms and any replacements in RF sections of radios, new micas should be used for stability. On the pushbutton mechanisms, 370 pF is no longer a standard value. I don't remember the closest standard value right now (360 or 380?), but something very close to 370 will work fine. These needs to be mica, else your pushbuttons will drift in frequency and be useless.