The AM Forum

I've got a bunch of these and most of them check out OK. I assume these are just paper and foil caps enclosed in plastic and maybe good for about 200 volts.

Anyone know for sure?

(http://img370.imageshack.us/img370/2530/img6529om5.th.jpg) (http://img370.imageshack.us/my.php?image=img6529om5.jpg)

Those look like mica capacitors to me, for RF use. Maybe 1 kV or at least solid 500 WV.

Could be mica, most of them checked out OK in capacity but I haven't checked for leakage yet.

I though I'd add a picture with some scale though...

(http://img267.imageshack.us/img267/2164/img6531kh7.th.jpg) (http://img267.imageshack.us/my.php?image=img6531kh7.jpg)

What picture?

Carl

Those look like the WWII era mica capacitors I used to get at the surplus place when I was a kid in the 1960's

mica

almost certainly.



               _-_-bear

They did, in fact, make paper capacitors that look the same (like micas) in that plastic (or bakelite) case during the 30s-40s time frame.  Hammarlund used some of them inside their IF cans in some of the early Super Pros and they were a pain.  I found several shorted ones in SP-200 series receivers.  Guess they attracted moisture or something.  If you have plans to use some of them...it might be worth sacrificing one and tearing into it to see if it is mica or paper.

73,  Jack, W9GT

Duh, why didn't I think of that, I have about 10 of them that are more than 20% lower in capacity than marked.

I split one apart and they are, indeed, mica.

I suppose I could put one in series with a large resistor to limit current and see how much voltage they could take before arcing over.

Nice construction though.

(http://img178.imageshack.us/img178/8062/img6539yp0.th.jpg) (http://img178.imageshack.us/my.php?image=img6539yp0.jpg)

Thanks to everyone!

20% is probably not too far off specs.  If they show acceptable leakage, they are probably still ok for non-critical applications like coupling and bypass  caps.  You would need them to be closer to nominal for resonant circuits or something involving a critical time constant.

In a lot of older gear, capacitors and resistors may be off 50% or more from nominal.  But most of the time I have found that replacing them with new ones spot on nominal causes little or no improvement, so I don't change them out unless they show high leakage or measurably impair performance.  Especially with pre-WW2 restorations... I hate it when I open up a cosmetically well restored piece of 1932 electronic equipment and find it full of orange drops and metallic film resistors.

OTOH, I have no problem replacing bumblebees in a 75A4 with orange drops.  But I wouldn't put them in a 1935 original HRO.

But in many cases, it is not a matter of preservation.  I simply don't want to waste my time and resources replacing components that are still performing at or near original specs.

Good point, I'll just keep the off spec ones in a separate baggie marked as such.

I have the opposite reaction. If I see a point for point replacement of weak components with like value, and they took great care in doing this, my heart sings - I have a radio that isn't going to burst into flames and will likely work when I plug it in. But I never did the really old radios with the pretty components.


This is true. In all restorations, you balance the risk and cost in performing the work over the benefits (improvement in performance, looks, reliability). I now check all the resistors and replace as needed, it takes little time, over the risk of burning up a hard-to-replace IF coil or bandswitch when 'debugging'.