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 Post subject: Philco 39-30 Capacitors
PostPosted: Nov Fri 08, 2019 3:00 am 
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Are these capacitors in the tuning section of a Philco 39-30 (probably Canadian) trustworthy? Note the flat wire leads.


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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Fri 08, 2019 5:49 am 
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Yes, and they are silver mica and should only be replaced with the same if necessary, but they seldom go bad.

DM


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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Fri 08, 2019 4:53 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Short of obvious physical damage, there is no way anybody can look at any electronic component and say it is good or bad. The only way to know for sure that a component is good (or bad for that matter) is by testing it, either by observing how it functions in the circuit or by measuring it with suitable test gear.

In this particular case we note that these are very early ceramic capacitors. Ceramic capacitors are generally extremely reliable and seldom need to be changed. It is often the case that ceramic caps used in the RF, mixer, IF, and oscillator circuits of vintage radios were specially made to OEM spec for characteristics like tolerance, temperature coefficient, and voltage coefficient. Often there are no exact commercial replacements or direct equivalents, and if you replace them with generic parts you could be letting yourself in for drift as the radio warms up, and maybe other problems. For these reasons the advice is usually to leave ceramic capacitors alone unless you discover they have failed through careful troubleshooting.

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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Sat 09, 2019 7:26 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
We have one vote for silver mica and one for ceramic.

Has anyone opened one up to see what's actually inside?

I would have to say that they resemble the common mica caps of that era, but are likely not silver mica. Probably made by stacking metal plates and mica sheets, then squeezing the lot together mechanically. Note the rivets or eyelets.

In any case, the are very possibly still perfectly functional.

Ted


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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Sat 09, 2019 10:12 pm 
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DM


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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Sun 10, 2019 2:20 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
No doubt they contain silvered mica plates, but the construction is atypical with outer covers that appear to be ceramic and rivets. They still could have been designed to change dimensionally with temperature, and thereby compensate for temperature differences. I still wouldn't change them unless it was absolutely necessary.

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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Mon 11, 2019 2:00 am 
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Thanks all, I will leave them as is until proven bad.
Lee

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 Post subject: Re: Philco 39-30 Capacitors
PostPosted: Nov Mon 11, 2019 4:58 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Charleston, W.Va.
Hi Lee,
+1 to previous advice to leave these capacitors alone until/unless troubleshooting indicates otherwise.

As for testing them, they would first need to be isolated in the circuit. This can be problematic in and of itself. If they can be isolated by removing leads of other components or wires, then all is well. But if you have to lift one of these capacitor's leads to isolate it, then you risk damaging it either from excessive heat or from manipulation of its lead(s). Above all, do not rotate these capacitors with their leads attached. If you do so, you are almost certain to damage them internally.

If the capacitor can be safely isolated in the circuit, the only reliable test is one done via a high-quality metered bridge-type capacitor tester such as a Sprague TO-series Tel-Ohmike. With one of these, you can test the cap for insulation resistance (i.e., leakage) at its full rated voltage. And if it passes that test, you can then test it for capacitance value. But the above being said, the best test will always be how it performs in the radio.

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