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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Mon 18, 2020 7:54 pm 
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Location: Colorado Springs, CO 80917
I can confirm from a large lot (~200) of unused "domino" type caps, various manufacturers, that I bought last year that:
(a) Micamold did make good mica caps and
(b) some micas are at least as large as 0.01uF.
I sorted & tested the lot. Tested over 95% good for value & insulation resistance. Broke into examples of each value & type & confirmed all were indeed mica insulated.
Micamold company apparently started making paper substitutes during WWII as a way of serving the war effort with lower cost & conserving the strategic mica resources.


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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Mon 18, 2020 9:02 pm 
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hwhall wrote:
Micamold company apparently started making paper substitutes during WWII as a way of serving the war effort with lower cost & conserving the strategic mica resources.

I would not tend to think that's any too accurate. Did you read that somewhere that you can link us to?
Reason I ask is that mica caps were not typically / usually made in values above .01 and certainly not at .1 and above because paper was more practical, I believe.
So I don't think MicaMold was saving mica for the war effort .. (but perhaps that's true.)
However, as far as I know, ... war-effort notwithstanding, most cap makers used paper above .01uf

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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Mon 18, 2020 9:11 pm 
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I based that on advertising I saw once from Micamold in WWII aircraft industry journals. If they didn't invent it for the war, at least they marketed it during the war.
Can I find those ads now? Not sure I have the magazines anymore.


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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Tue 19, 2020 1:04 am 
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Early on there were really only three common dielectrics. Paper, Glass & Mica. From dismantling; Many waxed paper types & oil filled types contained PCB,s. Many of those principally grey metal boxed caps common in wartime equipment were paper inside, but the sealing was so good I have a tester with one in it and others that do not leak and they have paper in them. Jacketing & sealing in several turned out to leave a lot to be desired & helped in their downfall.

Prime example: Metal cased ones of BC-221-N. They were paper types inside, the seals deteriorated & leaked PCB oil and they leaked electrically: My answer was to re-stuff.

Marc


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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Tue 19, 2020 2:26 am 
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Micamold caps with rectangular molded cases, but paper inside, were in common use at least as far back as 1939-1940, you see them in Zeniths and other brands, including line bypass applications where they had a high tendency to fail.

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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Tue 19, 2020 3:26 am 
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The common comment for those that have had dealings with "Micamold", is they cannot be trusted. However, that applies to pretty much every other brand of old Waxed Paper caps and some Mica types: Resistor similar. As much as I have issues with anything Micamold, I have put 500V on some that were Mica & if it passed it was left.

It all comes down to methodology, experience & statistics. I know from servicing lots of radios that certain parts are common failures in particular models. I know, as before, that a Wax Paper caps in spec for leakage are as "scarce as hens teeth" so one gets rid of them. It is usually better & creates less stress later, to check resistors as you do this; Any Mica, or Ceramic cap that ends up with and end free, or I am suspicions of, gets tested for leakage. That does eliminate rework, which increases the risk of damage.

Marc


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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Tue 19, 2020 9:12 am 
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Just an update on this topic, I have three or four drawers of molded mica caps, so on Monday night I decided to go through and sort them, but had to look up how to read the six dot code used on most of them in a book. Apparently the industry standard, though not all companies did this, was that if the first dot on the top row was white the dielectric was definitely mica. In the case of those bad Aerovox caps with the light brown case, there was only a single row of three dots, read in the same order as a resistor, but none of those included a white dot anywhere on the case.
There were some new old stock Aerovox mica caps in those drawers, some were 470 pf/mmf, those had white dots, and the phenolic material used to encase them was a dark red or reddish brown colour, I tested two in the PACO, they were spot on for value, and no leakage up to the 400 volts the cap tester puts across them. So far I haven't found any Micamold brand mica caps in these drawers, most were Aerovox, followed by Sprague, Elemenco, Solar, and maybe Cornell Dubilier. I have seen Micamold caps in the odd radio, I think all were in U.S built sets, in one there was a Micamold wire wound resistor which I mistakenly though was a leaky mica cap years ago.
I think that maybe those light brown Aerovox caps may have been a cheap substitute for real micas, all seemed to be used in locations where tolerances were not critical, such as a bypass on the volume control wiper, or on between plate of the first audio tube and ground. In the case of the Westinghouse 501M they used an early NP0 ceramic cap in the oscillator circuit, one of those hollow ones that looks like a dogbone resistor in shape.
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Arran


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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Tue 19, 2020 6:44 pm 
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Quote:
Micamold company apparently started making paper substitutes during WWII as a way of serving the war effort with lower cost & conserving the strategic mica resources.


Look at the cost of new silvered mica caps today. They are easily five times the price of any others of comparable size and voltage rating. Then if you look back through the old catalogs you realize that this was always true; mica caps were more expensive than paper ones going all the way back to when these parts were first introduced. Paper and mica dielectrics have very different characteristics in circuits, so it really made no difference what Micamold had to say about it. If an engineer or manufacturer had to use a mica cap in a circuit in order to meet a specification, they would use it. But if they could have met the same specs with cheaper paper or ceramic caps, you can bet that's what they would have done.

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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Thu 21, 2020 8:26 am 
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Chris108 wrote:
Quote:
Micamold company apparently started making paper substitutes during WWII as a way of serving the war effort with lower cost & conserving the strategic mica resources.


Look at the cost of new silvered mica caps today. They are easily five times the price of any others of comparable size and voltage rating. Then if you look back through the old catalogs you realize that this was always true; mica caps were more expensive than paper ones going all the way back to when these parts were first introduced. Paper and mica dielectrics have very different characteristics in circuits, so it really made no difference what Micamold had to say about it. If an engineer or manufacturer had to use a mica cap in a circuit in order to meet a specification, they would use it. But if they could have met the same specs with cheaper paper or ceramic caps, you can bet that's what they would have done.


Near as I can tell NP0 ceramic caps arrived on the market in the late 1930s, such as those dogbone shaped hollow tube style that I described, I have a few Rogers built sets from 1938-39 that used those, as well as the more traditional molded mica caps. So clearly the industry was working on lower cost substitutes, if it was not due to cost then it was for some other consideration, such as physical size given the value. Silver mica capacitors are not constructed in the same way as the old molded micas, the old molded micas typically had a sandwich of brass plates and sheets of mica inside. From what I remember from years ago micas were chosen because of the tighter tolerances in value, and because of stability. I don't think that it's necessary to use silver mica capacitors as a replacement in most circuits, the NP0 ceramics are just as good, and much cheaper. But it's probably not wise to shotgun replace mica caps anyhow, unless other more common faults are ruled out, not because of the cost so much but because you could introduce a fault that wasn't there to start with by something as simple as lead dress, this doesn't really apply to audio circuits of course.
Regards
Arran


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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Sat 23, 2020 2:07 am 
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We are looking at material failure; these components and the equipment that they are in are 65-90 years old now. I started to find leaky mica capacitors in early TV sets nearly 40 years ago. A good capacitor checker will weed them out.

I have been saving my mica capacitor "duds", and intend to document which brands fail the most. On a quick check, Micamold is well represented, but they aren't the only one.

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 Post subject: Re: Mica Capacitor Imposters
PostPosted: May Sat 23, 2020 5:31 am 
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I read a sizeable number of posts on this old thread tonight. I guess that's why i like the gray painted Hammarlund HQ radios. They're jam full of boring-looking tan colored disc capacitors. In fact, those Hammarlunds are actually quite boring to look at inside. Such a big box with so little in them. But you hardly ever see a post about a bad capacitor in them, do you? True, the could can have silver-mica disease, which can turn into a bear of a project. But they just keep on working. I don't have to wonder about caps, because they don't go bad.


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