Micamold did make real mica capacitors. Pre-WWII Micamold made mica caps in sizes from pF to as large as 0.02 uF. They started making lots of paper caps during WWII because mica was considered a strategic mineral resource. Afterward the war, it was just an accepted & cheaper method. And Micamold wasn't the only manufacturer who put paper-insulated caps into what looked like tabular mica capacitor bodies.
The most likely to be paper seem to me to be the black bodied rectangular ones with beveled corners and/or edges. Small brown square-cornered ones seem most likely to be mica internally. Low picofarad values are also more likely to be real mica. Location in the schematic is also a tip; caps used in RF & IF tuned circuits are more likely mica, audio & bypass caps more likely to be paper.
The attached Micamold advertisement (1954) hints at shapes that can be used as clues, with the exception of one body shape that it shows was used for both mica & paper.
On Monday, August 4, 2025 at 11:38:28 AM MDT,
[email protected] <
[email protected]> wrote:
On 3 Aug 2025 at 22:15, Charlie L. wrote:
>
> The old mica postage stamp caps are known to go bad in all the removable modules,
> and they can contribute to instability in the HFO and BFO stages. If it has those large
> Micamold caps on the terminal boards, I guarantee those are bad and causing issues.
>
> Charlie, W4MEC
Those cursed "MicaMold" flat, usually dark-brown, plastic-cased, capacitors are total and
complete pieces of garbage. Every single one of those I have ever tested were bad. ALL
leaked like a sieve, some were dead short.
And there is absolutely NOTHING "mica" about them: they're simply squashed flat paper
caps.
Total junk.
Get rid of every single one you ever find anywhere in any radio.
Ken W7EKB