crosley: Ch= 1217; Prestotune 12 - Molded cap color code

OK, I have another question now.  I'm not at all familiar with these old style molded capacitors.  The capacitor on the left in mounted upside down, so it reads black-brown-red (you can see the arrows indicating this).  The other one is black-green red.

Now, what are the values?  According to a table I found on the web, the black-brown-red should be 0-1-00, or 100, and the other one is 500.  But what are the units?  Is this 100 and 500 uF, nF, or pF?

Also, does anyone have an opinion on the reliability of these components?  If they read OK, should I just leave them alone?  Are they more reliable than paper capacitors?  Any help would be most appreciated.  Thanks!

 

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Hello Michele,

Yes you are correct! They read like resistors with the values in pF.  First two colours the significant numbers and the third the multiplier.

Hence the first is 0-1 x 100 = 100pF, the second 500pF.   They are usually moulded mica and I have found them to be extremely reliable and they are the last component that I normally suspect!

Best regards,

Roy

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Thank you so much, Roy, for the confirmation.  Now I know they're in pF.  I think I'll just leave these little guys in place for the time being.  I still have a few more bad paper capacitors to replace, so I can't power up the radio for a test just yet.

Cheers!

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Dear Michele and Roy,

For many years my experience with molded mica capacitors was the same as what Roy reports above.  However, in the last few years, I have begun finding bad molded mica capacitors, particularly in televisions (I probably notice them more there simply because there are far more of them in some TVs).

I don't think it is necessary or wise to make a practice of always replacing them.  On the other hand, we should no longer count on them to be always good -- which was a pretty safe bet up until the recent past.

No need to worry about these for now, Michele.  I simply mention this so that we can begin to share with each other in the community what kind of reliability we are seeing in vintage molded mica capacitors.  I know from discussions on other forums that I am not at all alone in seeing mica capacitor failures more frequently now.

Best regards,

Tom

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I came across this article while looking for other capacitor information. 

These can be paper capacitor if they have the "Micamold" manufacturer label  marked on the other side.  As with other paper caps, replace them.

Paul Pinyot.

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Paul,

You're right that some Micamold molded capacitors have paper dielectric.  The ones with higher capacitance values (500 pF or greater) seem to be the ones most likely to have paper dielectric.  Low value caps in the 100 pF range or lower always seem to be mica.

Best regards,

Tom

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Tom and Paul,

The possibility for a "mica" cap to have a paper dielectric now makes sense for a supply bypass failure I saw a few years ago in an FM tuner chassis I no longer own.

The capacitor value must have been between 1000pF and 0.01uF, and the package was only a little larger than what Michele showed. So chances are the dielectric was paper.

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I am one of those that is loathed to "modernize" a set any more than is necessary. I am more likely to measure all capacitors, in particular those coupling from plate to grid, instead of recapping.

Other caps I pay special attention to, are the power line filter caps that are often wired between the power line an the chassis.

Measuring the leakage of the capacitors with a high voltage capacitance tester like a Heathkit IT-28 takes less time than recapping, and eliminates nearly all recapping mistakes. Only one wire of the cap needs to be cut or unsoldered to test it.

A simple High voltage supply, or the Bplus of a radio, can provide a handy test voltage for capacitor leakage. The microamp-meter can be an inexpensive $5 DVM set to the DC voltage range.

Start with the meter, in series with the cap and the supply, at a scale that could measure the supply voltage directly, should the cap be shorted, or very leaky. Then keep lowering the voltmeter scale until you see a steady reading after the capacitor has charged. Divide this voltage, if any, by the input resistance of the meter, which could be 1Meg, or 10Meg and you get the leakage current.

You can measure the input resistance of the DVM in the DC voltage range with a separate meter in the Resistance range.

One example would be that you read 1V on the meter, which has a 1Meg internal resistance in the DC voltage scales. This means 1uA of leakage at the test voltage you applied. 1uA of leakage is too much for a cap between plate and grid, but is OK for a cap across the supply.

A $5 dvm with a 1Meg input resistance in the DC scales can resolve down to 100pA in the 200mV scale.

The tolerable current leakage for an electrolytic cap is much higher. Up to 1mA is reasonable for a supply bypass cap. If memory serves me, that is how the IT-28 sets the test limits.

Adding a low current fuse in series with Bplus is a good safety measure for radios that have the original caps.

It is generally a good practice to not leave antique radios playing unattended. Capacitors are not the only things that can fail.

Regards,

-joe

 

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