Domino capacitor with no color code

I have an old radio that has a "domino cap" between the plate and cathode of a 50B5 audio output tube. The dots were never colored on it. It appears to be a Micamold brand (hard to read). It has 3 uncolored dots between arrows, and one more dot above them.

Instead of coloring the dots, there is this [ CA-281 ] stamped on it. Does that mean anything? Or is that a manufacturer part #?

The schematic says it's supposed to be a .01uf @ 600v. This is a Abbotwares Z477 radio from 1947.

Normally I dont even bother with those mica caps, since they do not normally go bad. However, this radio had one lead to that cap cut, so there is likely a problem with it. (Maybe)??? (or it was worked on by someone who dont know much)...

After I replace the nasty looking filter cap and a few other caps that look very bad, I will plug it in. Then I will clip this cap back into the circuit and see if it really is bad or not....

Anyhow, if I want to replace it, what is the best replacement. Can I use a ceramic disk, or should I try to locate a mica (probably wont be easy to find), or just use a modern tubular cap rated at 600v?

The old one is reading 9.73 NF with my meter, so that is close to the correct amount. (Of course there is no voltage on it, so it make breakdown when voltage is applied).

Reply to
oldschool
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to micamold it probably did. Probably is.

Mica had low ESL and high stability.

Why not test it, easy enough to do. Mains & a small mains LED in series would test it to 330v in 240v land.

NT

Reply to
tabbypurr

And still is.

Replace ALL the damned capacitors and be done with it. And yes, that .01 is important, it's there to limit transient spikes in the output.

A 0.01 1 KV disc ceramic will work.

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Reply to
Foxs Mercantile

The short answer is -- the .01 ufd 600V capacitor is not a mica capacitor, it's a paper capacitor in bakelite case. Just replace it with a correct value plastic dielectric capacitor.

The capacitor serves the purpose of reducing voltage spikes across the audio output transformer secondary when short pulses, such as from lightning noise, cut off the audio output tube. The cap also reduces high frequency response, but that's no problem, given the limited audio bandwidth typically used on AM radio.

It's not a super critical application and it's obvious a radio can work just fine for years without the cap. Still, it's part of the radio's original design and I'd replace it.

I've seen line bypass caps clipped out of the circuit and I'm assuming servicemen, back in the day, would clip them out as a cheap and easy way of eliminating a nasty failure point with aging paper capacitors.

Seems likely your capacitor was clipped out by an experienced serviceman a few decades ago.

Reply to
Frank

Any "technician" that would clip a part, and leave it hanging, rather than replace it is NOT an experienced serviceman. He's a hack.

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Reply to
Foxs Mercantile

Well... there's no lack of consumer products with parts missing, demonstrating that

- in some cases the initial extra parts aren't really needed

- in others more than one version exists or was planned

It's like cars, surprising how many bits you can remove and it still works.

NT

Reply to
tabbypurr

You may not have said it, but you certainly act like it. It doesn't matter what we tell you, you always have to argue about it.

Here we go again. See above. They're called micamold because that's the name of the company that manufactured them. They also made paper dialectric caps that were made to like huge variations of the standard postage stamp micas. A marketing ploy at the time to suggest that THEIR crap was as stable and long life as mica. Hint: They weren't.

You're a piss poor survivalist if you can't pay attention and learn from others.

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Reply to
Foxs Mercantile

OK, he's an experienced hack. I'm sure more than one serviceman was snipping out potentially troublesome, yet non-essential, caps back in yesteryear.

And I'm sure some customers back then would call the BBB, if not the State's Attorney, if the radio repair man replaced a cap before it had gone up in smoke.

Reply to
Frank

He probably did what was appropriate at the time. Expectations and what's considered correct do change.

NT

Reply to
tabbypurr

On the far side of the Pond, few know of Mad Man Muntz. His habit was to go into his skunk-works and when presented with a new design, he would start cutting out parts until whatever it was stopped working. AKA - Muntzing:

From Wikipedia:

He invented the practice that came to be known as Muntzing, which involved simplifying otherwise complicated electronic devices. Muntz produced and ma rketed the first black-and-white television receivers to sell for less than $100, and created one of the earliest functional widescreen projection TVs .[3] He was credited with coining the abbreviation "TV" for television,[4] although the term had earlier been in use in call letters for stations such as WCBS-TV. A high school dropout,[5] Muntz made fortunes by selling autom obiles, TV receivers, and car stereos and tapes.[6] A 1968 Los Angeles Time s article noted that in one year he sold $72 million worth of cars, that fi ve years later he sold $55 million worth of TV receivers, and that in 1967 he sold $30 million worth of car stereos and tapes.

Peter Wieck Melrose Park, PA

Reply to
pfjw

There was NO shortage of hacks that would do as little as possible.

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Reply to
Foxs Mercantile

big snip

another big snip

Micamold is the name of the manufacturer as several others have already said. They made mica capacitors that were as good as anyone else's, but they also made paper capacitors in a similar style case which are as bad as anyone else's. Then again, they also made resistors in an elongated version of the same case with three color dots. These need to be tested along with all the other vintage resistors in a set. They may have made other parts as well that I am not aware of (yet).

Other manufacturers also made paper capacitors in domino style packages; there is a color code option identifying them.

Some mica capacitors had the value stamped on them either with ink or pressed into the plastic. If the ink on a capacitor faded or rubbed off, then it would be unmarked. That's where a schematic is useful.

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Reply to
Jim Mueller

Getting paper caps into that shape must have been a trick. They are not just rolled like the tubular ones. Considering the time they were made, I am wondering what the material is for that case (coating). I'm thinking bakelite, which was one of the first plastics and a lot of other electrical stuff was made from bakelite in that time period, like light fixtures, wall switches, and more. It seemed to be a good product though. A lot of the old fixtures and stuff is still in use.

LIke I said, I have a .jpg image I found on the web showing the dot patterns for different manufacturers. Each had variations. I think there are five types. I'd post the URL if I knew it, but I just saved the image awhile ago.

This one has no color on the dots but has [ CA-281 ] stamped on it. I cant see how that number means anything. It's supposed to be .01 @ 600V. How CA 281 can mean anything about .01uf or 600v is beyond me. (Maybe just a manufacturers code of some sort).

Reply to
oldschool

Yeah sure but then, who would the triple 'B' or the D.A. have called?

Reply to
bruce2bowser

And when a ton of stuff is stolen, misplaced, burned-to-bits or just plain missing.

Reply to
bruce2bowser

They didn't print that number on there for you. Several manufactures used "in-house" numbers to discourage employee theft, or reverse engineering by competitors.

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Reply to
Foxs Mercantile

Well, I've read your posts here for a while, and it's pretty clear that you want to argue with most of us who've mastered the mysteries of the AA5 design. Time was that when I interviewed people for EE positions, I had an AA5 schematic on the wall, and asked the prospect to talk me through parts of the design. For a more advanced interview, I had a schematic of an RCA 630-TS.

We started this newsgroup back in 1994 so that a few of us who had been working with these circuits since WWII could guide newer people, as well as swap some notes with each other.

You seem to want to flame people who say that restoration of

70-year-old electronics starts with wholesale replacement of R and C passives. Some of us learned that the hard way back in the 1950's and '60's.

Oh, really!?? I'm glad you know that stuff. In the meantime, if you want to get that radio playing reliably, you'll round up a .01 mike 1000 volt cap and solder it in.

Parasitic prevention/suppression.

Hank

Reply to
Hank

I tried early on helping this individual. It soon became obvious he doesn't want help. He just wants to bitch.

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Reply to
Foxs Mercantile

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