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Author Topic: Silver mica cap's  (Read 9125 times)
AC5UP
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Posts: 3927




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« Reply #15 on: November 10, 2011, 05:01:26 PM »

I overheated the ground connection that had a SMC on it also. Do I need to be concerned about over heating and if so, where do you obtain replacements, or what could be used as a substitute.

Overheated what? "SMC" usually refers to a surface mounted component which could be any number of things. And why are we talking surface mount parts in the Boatankers forum? Any component overheated can be expected to be compromised in some way... Reduced life expectancy or a permanent change in value. When in doubt, replace it.

The service manual can tell you what the part is and Google is your friend when looking for a vendor.
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K1ZJH
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« Reply #16 on: November 10, 2011, 06:05:03 PM »

As I remember it, silver migration can happen with the effects of DC and a humid atmosphere, in a somewhat analagous way to the effects of current density and temperature on the aluminium in an IC.

Not very common in my experience, although I've seen it many years ago in ceramic caps being pushed on RF current and operating at high DC simultaneously.

It is a very common in the American "K-Tran" snap-mount style IF transformers. The tuning capacitors are in the base,
and the mica is fully exposed.  I remember seeing radios exhibit the problem when was I a youngster in the early sixties.

Pete
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N4NYY
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« Reply #17 on: November 10, 2011, 06:09:09 PM »

Quote
It is a very common in the American "K-Tran" snap-mount style IF transformers. The tuning capacitors are in the base,
and the mica is fully exposed.  I remember seeing radios exhibit the problem when was I a youngster in the early sixties.

Holy crap. I just picked up about 20 pairs of these K-Tran 455 KHz IF cans.
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G3RZP
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Posts: 4830




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« Reply #18 on: November 11, 2011, 12:23:52 AM »

I think Mike is referring to 'silver mica capacitor' by SMC, and not the coax connector or a surface mount capacitor.

Tom, the photon bombardment only affects it if nichrome has not been used.
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N4UE
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Posts: 299




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« Reply #19 on: November 13, 2011, 03:34:15 PM »

Great discussion. However, In testing thousands of coupling and bypass caps, one thing I have learned:
check the cap for leakage at it's rated voltage (non-electrolytic types) and if it leaks. it is NOT doing it's job. SM caps are usually rated at a rather high voltage.
I have found only 2 ceramic disks that failed (shorted) and the types like Black Beauties, leak like the proverbial sieve.
I have found a handful of 'postage stamp micas' bad, including one in my 75A-4.

These are the kind of failures a 'cap tester' or 'LCR Bridge' will NOT find. The cap MUST be tested at it's rated voltage.


ron
N4UE
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WB2WIK
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« Reply #20 on: November 14, 2011, 03:19:14 PM »

Great discussion. However, In testing thousands of coupling and bypass caps, one thing I have learned:
check the cap for leakage at it's rated voltage (non-electrolytic types) and if it leaks. it is NOT doing it's job. SM caps are usually rated at a rather high voltage.
I have found only 2 ceramic disks that failed (shorted) and the types like Black Beauties, leak like the proverbial sieve.
I have found a handful of 'postage stamp micas' bad, including one in my 75A-4.

These are the kind of failures a 'cap tester' or 'LCR Bridge' will NOT find. The cap MUST be tested at it's rated voltage.


ron
N4UE

The old Boonton (tube type) capacitance bridges could do that; you could float bias on the DUT up to a pretty high voltage (I forget what, now...maybe 500V) by applying bias to a pair of terminals on the bridge.  Watching parts drift 80% of their value under full bias was always kind of fun.  Silver micas don't do that, though...many high density ceramics do.
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WD8KDG
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Posts: 45




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« Reply #21 on: November 14, 2011, 05:01:36 PM »

In the older rigs, don't bother replacing the silver mica caps as a shotgun approach.  I have a Sprague TO-6A capacitor analyzer that will test caps at rated voltage.

So far, it has never found a bad silver mica, even when the silver mica cap would not work in the circuit. With that said; after restoring an older RX or TX and there is still a problem with a circuit & every tube, resistor, etc has been checked, only then do I replace silver mica's in that circuit.

Craig,
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G3RZP
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Posts: 4830




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« Reply #22 on: November 16, 2011, 01:45:55 AM »

Not all postage stamp micas are silver. I had one from an HRO coilpack: the value varied by 10% as you squeezed it, and it was 10% out anyway. But it did date from 1939.
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