[Tom Hamilton]

Hi all,

I've picked up a Philco 39-30 recently and, in doing some research, found that Ron recommends replacing the two 370pF silver mica caps with silver mica caps that have a 390pF value. I believe these caps are used in the push button tuning mechanism. What voltage should I be using? I also have a 41-256 that I believe has silver mica caps as well. Would I use the same value and voltage for this radio as well?

And since I'm on the subject of voltage, is there any way to find out what voltage I should use when replacing the electrolytic caps on my 39-30? I seem to recall reading somewhere that 450 volts is acceptable, generally speaking. Is that correct? Or, am I totally wrong? The service notes don't say what voltage the electrolytic caps are and the cans on top of the chassis with all that helpful information are missing. I know the voltage and mfd is usually printed on the can.

Thanks a lot, Tom

[Mondial]

Modern silver micas are rated at either 300V or 500V and either rating would be acceptable, although the 500V rating would be preferred.

The typical voltage rating of the power supply filter caps is 450V and these are readily available as replacements.

[morzh]

For instance in my 37-116 110pF caps are in the circuits where the voltage will be on the order of units of volts or maybe 10-30 volts max if even that.

So 100V rating would do.

However when going to Mouse and looking at the prices I did not notice significant difference between a 100V cap and 500V cap. What makes the difference is toleranc - 1% cap costs a buck more than 2% cap. So, this considered, might as well buy more 500V caps and use them across the board without much thinking.

[Tom Hamilton]

Thank you for the advice. I knew someone out there would know for sure. I hate guessing, only to find out later that I guessed wrong.

Tom

[Arran]

If it's in the pushbutton bank circuitry the 500 volt mica caps will work fine, I don't know how precise they have to be.
Regards
Arran

[Bob Andersen]

I used 390pf 500v micas in my 39-30 and no problems so far

[codefox1]

Agree, and if you want to get an assortment of often used values to have on hand, the 500 volt types would be the ones to get. Always OK to use higher voltage rated caps; space usually not an issue.

[Tom Hamilton]

Hey, guys,

Thanks a lot for the voltage information. Codefox 1, what values would you recommend? Is there any way to tell if a capacitor is silver mica capacitor? I mean, how can I tell it apart from regular mica/ceramic capacitors?

Thanks, Tom

[Bob Andersen]

I don't think ceramic caps were used much in the 30s (if at all). Also, there is only silver mica - no regular mica. Please someone correct me if I'm wrong ;)

[Ron Ramirez]

You're right, I believe

Tom, whatever you do, be sure you use either silver mica or NP0 ceramic caps to replace those 370 pF micas. Do not use general purpose ceramic caps...they will allow your presets to drift. Don't ask how I know...

390 pF mica caps work fine as replacements; they won't be seeing very high voltage in this application, but I think I would go ahead and get the 500V micas, as Bob suggested.

[Mondial]

Ceramic caps did not see widespread use until WW2 where they were first used in military equipment. After the war they began to show up in consumer electronics and became very common in the 1950s.

Regular stacked foil mica caps were common up into the 1950's when they were replaced by the modern dipped silver micas and ceramics. Except for special power RF versions (Unelco's) regular stacked mica caps are no longer made.

In their early "postage stamp" varieties, regular micas have brown or tan plastic cases while silver micas have red cases, so it is easy to tell the difference. All presently available epoxy dipped types are now silver mica construction, which involves depositing a thin coating of silver conductor directly on the mica dielectric.

[Tom Hamilton]

Thank you all for the advice and answers on silver mica caps. I really appreciate all your answers.

Tom