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On Sun, 12 Nov 2017 17:53:27 -0000 (UTC),
(Hank) wrote:

In article ,
wrote:
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).

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.

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.

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?

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?