silver mica Capacitors -newbie alert

[John Sherman]

Hello everyone,
I am very new to Canadian Vintage Radio society. I was going through a schematic of an 1941 Dominion Electrohome Z40A82-X-E on the Schematic it says that there is one 5750 mmf silver mica capacitor–all the other silver mica capacitors are much lower. Do you think this is a misprint. I am new to the restoration of antique radios and hope this is not a stupid question. some of the mica caps looked damaged so I was just going to go ahead a replace.

Thanks so much everyone and i am excited to get working on this project.

Attachments:

1. 

   Schematic-Phonala-Z40A82XE.pdf
   

[Alistair Thomson]

It’s certainly large. They do exists, but not with that exact value. I found one at 5100pF for $4.25, and if the value is correct you could parallel that with a 650pF or similar size.

But I believe that it’s a typo. Looking at C21, it has a similar job to do as C14, and it’s 260pF. I’d be tempted to try a 570pF in C14 first.

[John Sherman]

Thanks Alistair, I very much appreciate the reply.

[Jean Marcotte]

Sorry Alistair I’m no specialist but I do not agree. Although it looks similar on the schematic, C21 is in the IF section and C14 in the SW oscillator section at a much higher frequency. Values can be totally different. C8 has the same function as C14 but in the AM oscillator section and has a value from 500pf to 800pf, quite higher than C21. Also, C14 has tolerance of +/- 5% wich makes the precise value of 5750pf quite possible. Another model of the same era shows a 3750pf +/- 5% mica in parallel with a 200-1000pf padder. Anyway, if it is a mica it should normally still be good

[Alistair Thomson]

Actually, I read it that C8 serial tunes with one of the T4 windings, and is therefore variable for tuning purposes whereas C14 is providing an AC ground at high frequency. Of course, tuning is also involved with L3… It looks like aligning this receiver might be a can of worms!

But, as you say, whatever its value, if it is a mica, it will probably not need to be replaced.

[John Sherman]

You guys sound pretty competent on your schematics, I am just learning how to read them and it seem all backwards.LOL
I am just going to recap it and not worry abo ut Mica caps, clean it up and test the tubes and hopefully it will work. It is missing dial glass and needs re stringing if you have any idea where I can find info on stringing it?

Thanks

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