Am recapping the above radio.
C24 , 0.01uF is burieed under layers of components and wires and tough to get to.
I assume that the mode of failure of old paper caps is to leak and open, never to short, am I right?
If the cap is open and I simply tack a new cap things are fine.
If the cap is good I double the capacitance, will this affect the performance?

Hi

Caps usually become leaky. They seldom open. No advantage in adding a second cap.

Does the radio operate? Can you get to leads on this cap with meter probes? With very little voltage on a cap leakage won't be much of a problem.

Norm

:Am recapping the above radio.
:C24 , 0.01uF is burieed under layers of components and wires and tough to get to.
:I assume that the mode of failure of old paper caps is to leak and open, never to short, am I right?
:If the cap is open and I simply tack a new cap things are fine.
:If the cap is good I double the capacitance, will this affect the performance?

You are right in assuming that caps leak, but I believe you may need some help regarding your conception of “open” and “short”. What Norm told you is correct, but if you do not understand what is happening, it may not make much sense to you. Therefore, I will attempt an explanation in plain language without getting technical about it.

Roughly speaking, you can think of all good caps as being open. In other words, a good paper cap will appear open if an ohm meter is continuously connected to both leads. All caps do about the same thing one way or the other, even the tuning capacitor of your radio. I mention a tuning condenser here because you can easily see what is going on. As you can see, the rotating plates never touch the stationary plates, and the only thing that separates them is air. Therefore, it could be called “open” if an ohmmeter is continuously connected across the plates. However, if you bend one plate on the rotor, it will come in contact with a stator plate and cause a short. So yes, caps do short.

By comparison, the paper caps in your radio do almost the same thing. They are just not adjustable, and instead of air between the plates, they have very thin paper. Several different things can cause this paper to break down. If moisture happens to be the culprit, even a slight amount in one small spot in the paper can cause a leakage between the foil plates. This could be called a leaky condenser, and in this case, the amount of leakage would depend on how much moisture is present throughout the paper. If a high enough voltage is applied across this cap, then it can easily arc through the damp paper and allow the foil plates to momentarily short. This could also burn a hole in the paper. When paper deteriorates far enough for whatever reason, the foil plates can come in physical contact with each other and become permanently shorted.

Technically speaking, no capacitor is truly open. About the only way a paper cap could be considered “open” would be if it exploded, but then, it would no longer be a capacitor, would it? So I doubt very seriously if C24 is really open.

Apparently your reasoning is that it may be OK to parallel a new cap with C24 because it is too difficult to get to. If my reasoning was the same as yours, then I would say to go ahead and do it. However. my reasoning would go even further. If it is possible to get to C24 to parallel it, then it should also be possible to cut the old one away from it afterwards. So why not do that an be safe about the correct capacity?

While we are on the subject, is a cap called "leaky" because it allows DC to pass or is it leaky because it can no longer hold a charge and anything injected into it bleeds off faster than it should?

Randy

:While we are on the subject, is a cap called "leaky" because it allows DC to pass or is it leaky because it can no longer hold a charge and anything injected into it bleeds off faster than it should?
:
:Randy
:

DC current is "leaking" from one plate to the other across the dielectric -- like it shouldn't.

Chuck

Randy, a cap could be called “leaky” if electrons cross the dielectric, be they from AC or DC voltage. Therefore, you could say that a cap does not hold a charge when electrons “bleed” from one plate to the other. However, no dielectric is perfect, so a capacitor will eventually loose its charge.