Help again! Thanks to your help, I'm actually having some success with the restoration of my old Zenith. I now have it running. My newest problem deals with finding an appropriate replacement for the variable capacitors on the grids of the 6K7 R.F. and 6A8 1st DET.OSC. tubes, and one of the trimmers on the 3 gang variable tuning capacitor. All have the mica missing and are presumably not functioning. As the mica appears to be riveted on one end, it would be very difficult to replace should the mica inserts be available. Can anyone tell me the value of these trimmers, and suggest a suitable replacement?

What you could be able to use, is a TO-3 transistors mounting insulator. Cut to fit as best as you can, punch out a hole for the adjusting screw to go through.

:What you could be able to use, is a TO-3 transistors mounting insulator. Cut to fit as best as you can, punch out a hole for the adjusting screw to go through.
:

::What you could be able to use, is a TO-3 transistors mounting insulator. Cut to fit as best as you can, punch out a hole for the adjusting screw to go through.
::
:

Henry,
The size of the plates and thickness of dielectric determine capacitance value. Thinner dielectric higher capacitance when compressed. Some TO-3 insulators are mica, some mylar, some silicon film elastomer. I would even think that plain electrical tape (600 V/layer) would also work, and at AM radio frequencies believe the losses to be negligible. The tape a bit thicker than mica, might not provide max req'd capacitance, but if cap meter is available, it can be confirmed out of circuit. Compression trimmers can be fashioned with double side copper clad fiberglas printed circuit board material, an insulator, screw, brass nut and thin brass shim stock. The brass shim stock can be shaped to form a hinge of sorts, providing the spring action to open the gap. Brass nut soldered to back side captivates the adjustment screw. A few picofarads (old mmf) can be created by twisting one insulated solid copper wire with one bare solid copper wire. Double sticky tape and alum furnace tape can also be fashioned into a capacitor, although non-adjustable.

marv

:::What you could be able to use, is a TO-3 transistors mounting insulator. Cut to fit as best as you can, punch out a hole for the adjusting screw to go through.
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:Yes, I've thought of that. Would the thickness of the insulator be critical?
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:Help again! Thanks to your help, I'm actually having some success with the restoration of my old Zenith. I now have it running. My newest problem deals with finding an appropriate replacement for the variable capacitors on the grids of the 6K7 R.F. and 6A8 1st DET.OSC. tubes, and one of the trimmers on the 3 gang variable tuning capacitor. All have the mica missing and are presumably not functioning. As the mica appears to be riveted on one end, it would be very difficult to replace should the mica inserts be available. Can anyone tell me the value of these trimmers, and suggest a suitable replacement?
: