C! Radio Capacitors

[Mark P.]

How can I test a few radio noise suppression capacitors to ensure they are working correctly ? Do they fail or go out of spec often ?

The 3 I bought for my 1960 are for the Regulator (1917580) rated at .5 MFD, the Generator (1911095) rated at .3 MFD and Coil (1947452) rated at .3 MFD.

Anyone know if the wax capacitor for the Ammeter (7270568) rated at .5 MFD is hard to find ? Are the repros any good ?

Thank you.

[Mike E.]

Re: C! Radio Capacitors

Ammeter capacitors are very hard to find. Repros I've seen have not been terribly accurate. Maybe Jack Humphrey can weigh in on how to test the capacitors--most of us don't ahve the equipment for that.

[Jack H.]

Re: C! Radio Capacitors

You really need an RLC meter (akin to a Digibridge) to evaluate capacitor health. It takes TWO to tango...

Not only do you need to verify storage capacity approximates the part's rating (say 0.5 uF) but it's also important to assess the capacitor's dissipation factor. What's that?

Well, if I sold you a bucket rated to hold 1 gal and I sent you one with a jillion holes in the bottom/sides, would you be happy? YEP, the bucket WAS as advertized (1 gal capacity), but it shore nuf wouldn't hold it very long!!!

That's what dissipation factor is all about... Charge the cap up, remove the source of charge, wait a fixed time interval and then go re-measure how much stored charge is still present. Dissipation factors in the zero to 0.08 range are pretty typical of these parts (they hold all of the initial charge to 92% of the charge after a fixed waiting inverval).

For RF suppression, high precision storage capacity isn't really needed and the factory original parts were typically +/- 25% of their specified value...

[Gary C.]

Re: C! Radio Capacitors

Mark, here's easy steps to check capacitors to see if they're functioning or shorted or open. Won't tell you how effective they are as Jack said. Gary....
To test a capacitor quickly, using an analog ohmmeter.

• On your ohmmeter select Rx1 or Rx10 for a large value of capacitor (>1uF) and Rx100 or above for small values (<1uF).
• Discharge the capacitor first by shorting the capacitors leads together. Don't touch both leads with your fingers until the capacitor has been discharged or you may get shocked.
• Put the positive probe of your ohmmeter to the positive lead of the capacitor, do the same with the negative probe. Your meter pointer should swing to zero resistance then swing to infinity. This’s called a “capacitor kick”.
• Discharge the capacitor again and repeat the test this time touching the negative probe to the positive lead. The meter's pointer should swing to zero resistance then swing close to infinity.
• If you get these results the capacitor is probably OK. If a capacitor is an open-circuit (bad) you will get infinity resistance on both readings. If it is shorted (bad) you will get zero or near zero reading and it will never swing to infinity on either reading.

A good capacitor that can store a charge with less leakage will show an infinity resistance reading, which means no current flow, after it is fully charged.