Domino capacitor with no color code

[o***@tubes.com]

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).

[Foxs Mercantile]

Post by o***@tubes.com
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)...

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.

[Frank]

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.

[Foxs Mercantile]

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

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

[Frank]

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

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

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.

[Terry S]

Another TV shop story....

Customer brings in a 12" B&W set. It got wet sitting near a window during a rain storm. It's not working. Typical shut-down scenario.

Shop owner takes the set in, pulls the cover, dries it out, touches up a few solder joints that may or may not have been related to the set's failure.

The set sits open on the bench for a week, running just fine. Boss calls the customer, set is ready to be picked up.

I'm at the desk that day, the customer comes in. Learns no parts were replaced. What the hell is this $22 charge for? Mad as all get-out that we had the nerve to charge him for doing "nothing".

I took his payment, and politely asked him to contact the shop owner if he had issues with the charges. The boss did eventually refund some of his money. Goodwill gesture.

Terry

Post by Frank
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.

[Foxs Mercantile]

Post by Frank

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

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

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.

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

[Hank]

Post by o***@tubes.com
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 #?
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).

Well, since you're the "expert" on restoring old radios, do you really
need to be told how to restore a run-of-the mill AA5?

First off, a 10,000 pf Micamold is a paper cap, not a mica. Most AA5's
had a .01 or .02 400 or 600 volt cap between the output tube plate and
ground. Some hooked it across the output transformer terminals or to the
output tube cathode if the output tube cathode resistor had a bypass
cap. Very common failure point, and a common "quick fix" was to dike
the cap loose, though it should be replaced. There's a reason why AA5
builders included it. You want at least a 600 volt cap there. A .01 or
.02 mike 1000 volt disk ceramic between the 50B5 plate and ground is a
good fix.

Restoration of an AA5 should include replacement of the input coupling
cap to G1 of the output tube (typically 5000 pf), and a check that the
ouput tube grid leak and cathode bias resistor (typically 470K and 150
ohms) are correct. Your choice whether to put a (typically) 20 mfd.
bypass cap across the cathode bias resistor and/or to replace the bias
resistor with 150 or 180 ohms.

Hank

[o***@tubes.com]

Post by Hank

Post by o***@tubes.com
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 #?
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).

Well, since you're the "expert" on restoring old radios, do you really
need to be told how to restore a run-of-the mill AA5?

I dont recall saying that. Maybe you need a reading comprehension
course.

Post by Hank
First off, a 10,000 pf Micamold is a paper cap, not a mica. Most AA5's

That's deceiving. It's called a MICAmold. That would tell me that it's a
MICA cap.

But yea, I know that was a time when there was a lot of confusion
between manufacturers of caps and some other components. I have a
graphic image showing all the possible color codes for those domino
caps. Each manufacturer had a different dot layout. (At least the colors
matched the resistor code for numbers). But I saved that picture so I
can look them up. Unlike most other parts, there is no simularity
between brands and I actually have not run across all that many of them.
I surely did not know they made paper caps in that style.

I sure am glad they pretty much standardized those parts by the late 50s
or early 60s.

This particular radio has almost every type of cap that was made in
those days. It's almost like they were using up whatever was on hand.

Post by Hank
had a .01 or .02 400 or 600 volt cap between the output tube plate and
ground. Some hooked it across the output transformer terminals or to the
output tube cathode if the output tube cathode resistor had a bypass
cap. Very common failure point, and a common "quick fix" was to dike
the cap loose, though it should be replaced. There's a reason why AA5
builders included it. You want at least a 600 volt cap there. A .01 or
.02 mike 1000 volt disk ceramic between the 50B5 plate and ground is a
good fix.

I'll likely use a ceramic disk for that one. I am curiuous, since the
radio likely works without that cap, what is the real purpose for it?

Post by Hank
Restoration of an AA5 should include replacement of the input coupling
cap to G1 of the output tube (typically 5000 pf), and a check that the
ouput tube grid leak and cathode bias resistor (typically 470K and 150
ohms) are correct. Your choice whether to put a (typically) 20 mfd.
bypass cap across the cathode bias resistor and/or to replace the bias
resistor with 150 or 180 ohms.
Hank

On this particular radio, I intend to replace all the caps. There are
some very nasty looking caps in it. There is a 0.1 tubular cap across
the power cord to the chassis that is literally melted. The ends are
oozing out, and it's not wax, its some sort of hard plastic or (whatever
it is). If it was not one of the nicest looking radios I have ever seen,
I would not even fix it. I love the horse on top. As soon as I saw this,
I had to buy it, even though it was in very poor shape.

By the way, I dont usually find bad speakers in these old radios, but
this one is torn, and was already glued. It's listed as a 3.5 inch. I
dont have a junked radio to get it from. Where do you guys buy
replacement speakers for these antique radios?

[Peter Wieck]

Micamold is the name of the manufacturer.

Loading Image...

Ford makes Lincolns.
Mercedes makes Smart Cars.
Micamold makes capacitors not from mica. Why should that be deceiving?


Peter Wieck
Melrose Park, PA

[Foxs Mercantile]

Post by o***@tubes.com

Post by Hank
Well, since you're the "expert" on restoring old radios,
do you really need to be told how to restore a run-of-the
mill AA5?

I dont recall saying that. Maybe you need a reading
comprehension course.

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.

Post by o***@tubes.com

Post by Hank
First off, a 10,000 pf Micamold is a paper cap, not a mica.

That's deceiving. It's called a MICAmold. That would tell me
that it's a MICA cap.

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.

[Jim Mueller]

On Mon, 13 Nov 2017 00:21:56 -0600, oldschool wrote:

big snip

Post by o***@tubes.com
That's deceiving. It's called a MICAmold. That would tell me that it's a
MICA cap.

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.

[o***@tubes.com]

Post by Jim Mueller
big snip

Post by o***@tubes.com
That's deceiving. It's called a MICAmold. That would tell me that it's a
MICA cap.

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).

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.

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

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.

Post by Jim Mueller
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.

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).

[Foxs Mercantile]

Post by o***@tubes.com
This one has no color on the dots but has [ CA-281 ]
stamped on it. I cant see how that number means anything.

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.

[Hank]

Post by o***@tubes.com

Post by Hank
Well, since you're the "expert" on restoring old radios, do you really
need to be told how to restore a run-of-the mill AA5?

I dont recall saying that. Maybe you need a reading comprehension
course.

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.

Post by o***@tubes.com

Post by Hank
First off, a 10,000 pf Micamold is a paper cap, not a mica. Most AA5's

That's deceiving. It's called a MICAmold. That would tell me that it's a
MICA cap.

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.

Post by o***@tubes.com
I sure am glad they pretty much standardized those parts by the late 50s
or early 60s.
I'll likely use a ceramic disk for that one. I am curiuous, since the
radio likely works without that cap, what is the real purpose for it?

Parasitic prevention/suppression.
Hank

[Foxs Mercantile]

Post by o***@tubes.com

Post by Hank
Well, since you're the "expert" on restoring old radios, do you really
need to be told how to restore a run-of-the mill AA5?

I dont recall saying that. Maybe you need a reading comprehension
course.

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