Mica Capacitors & Silver Mica Disease.

Chas had some good info there.

I stand corrected, mica caps did use mica as the dialectic: but they made paper foil caps in the same style cases. IIRC, they were differentiated by a white dot as the first color dot. We always referred to them as 'Paper mica's" because of the case.  They also made some silver mica caps in clear polystyrene cases, and I was told they were silver micas, but never verified it. Military surplus IIRC.

They did use a ceramic style cap as an AC bypass cap. Sometimes they opened, sometimes they shorted, giving one a real, possibly deadly "surprise", just like the rolled foil paper ones. I have that tee shirt.
 Some if the X/Y caps I have recently ordered come in a "ceramic style" case, and are coated with plastic, typically yellow or blue. Some have also come as a "box style" too. Depends on the mfg.

Back in the "early days" ( Up into the early 60's IIRC), they used a standard wax/paper cap quite often as an AC bypass. They had either an "AC" voltage rating on them or a very high DC voltage rating. They didn't have X-Y caps back then as far as I know.
 
I remember seeing some in radio & TV sets that used a "integrator", that is to say a capacitor with a resistor in parallel as line bypass too. I never trusted those.
Fortunately, I seldom saw those.

Fellas,

There is a lot of info on the web about caps in the form of both WIKI and manufacturers PDF's.

The Micamold corporation got going to provide components for WW-II electronics. Old ads can be found with the drawings of these rectangular caps. For all intents they look like a molded stacked mica cap, they are not, they have a flat rolled paper/foil construction.
Micamold did make a mica cap, it is of a square body either marked with dots or print. They use a mica insulator and metal foil plates. True mica caps, any brand can be made more accurately because the plates and the insulators are of a known size and thickness. By altering the metal foil type, temperature compensation can be achieved.

As stated what has happened; Though the Micamold rectangular paper cap was touted for its reliability in a tropical environment it was still porous to some degree and the impurities in the paper insulator failed early on, but by then they had done their job and the War was over. But not the component war at home, Micamold continued to offer and there was also a military surplus that found its way into shops.

Micamold also made a wire wound resistor in a similar Bakelite case but they are narrow and almost always with color dots. These were about 3 watts and are often found right after the rectifier in between to the filter network or in cathode bypass. These will fail open like any other resistor from overload, again, no mica in it.

The defining value to more accurately determine if an in circuit Micamold cap is a paper type is 10,000pf or .001 and the shape, if it is blistered and is used as a bypass and not frequency determining.

The Micamold "effect" is not the same as the "silver mica disease". Those results have been explained but what brings it about is the silver deposited on the mica within the IF can is under electrostatic tension. Silver can't resist and actually grows dendrites under the tension of the B+.

This same dendrite has been an issue with tin plating in solid state components but it is not electrostatic it is tin itself. Early satellites failed because of this when they opted for lead free solder. The solder issue has improved and current lead free lead is improved. Hammond organs failed because of these dendrites in the vibrato scanner. You may have seen them in dormant equipment as a fuzz, not to be confused with lemon yellow fur which is cadmium flowers, yep, toxic, but unless your a hygiene slob, not a big issue, do not sand!

Some paper caps do withstand the test of time, Some Radiola blocks used in the 17, 18, 60 and so on have survived well, some not. The probable explanation is like all "smart" manufacturers they will have more than one source of supply in case the unexpected happens, therefore quality at plant "A" may have been better than plant "B"...

Ideally, metal FOIL combined with a temperature stable plastic film, hard dipped are the most stable over time. Caps like these are in timing circuits and tuning buffers where more than 10,000pf is required and long term stability. These caps are also well known by their publish specifications for there good dv/dt ratings. Any circuit that has high energy pulses with be fine with that application. Such as vibrator buffer, plate circuit tone controls or filtering. There are some metalized designs that are specified for those circuits too.

Another specialty cap that has come to notice is the AC line bypass, a specially designed cap that fails open.

There is no such capacitor made with alternate stacks of  "the paper and mica alternated in the stack with the foil?" Paper capacitors all use a rolled construction for the flat type the roll is made oblong in shape. In fact, disassembly of early paper capacitor blocks will find they are almost entirely made of oblong paper/foil windings of either .5 or 1 mf, then all connected in parallel.

What does all my blather above say?

Replace the old cap with the wrong type and one will be back to do it again all too soon. That not all caps are bad, some are.

Resistors? Now there is a part of a different stripe!

Chas

I am confused here. They used paper in Mica capacitors? I thought they used sheets of mica for the dialectric insulation. Mica is very good insulation. Did they use paper sheeting in addition to the mica sheets? What was the purpose? Was the paper and mica alternated in the stack with the foil? Reminds me of the radial tubulars that started roughly in the 1950s. They used a dual dialectric of paper and mylar. I could never figure that out. It bulked up the physical size of the cap. Did it save money somehow on production costs or had some other virtue like being more rugged at the time? Was it just a way of gradually phasing out the use of paper caps? I don't know. However, from what I've seen, those paper/mylar caps still are not too bad today although some are showing their age in terms of a bit of leakage compared to new mylar caps. Just wondering.
73, Rod  N4QNX

Basically, there are 2 types.
Paper mica and Silver mica.

Paper mica's used foil for the conductor inside, and the silver mica's used silver.

They first came in what is called a "postage stamp" or "Domino style" size, then later in epoxy covered. And they came in many physical sizes too.

The PAPER micas are failing in droves. Some had a hard coating and others bakelite. All were dipped in wax to seal them, as best as I can remember.

 The paper was acid base and has deteriorated over the decades to the point that even NOS ones are bad.

 I fought a couple of problems with them for weeks. They were  NOS MicaMold brand. They tested dead on for capacitance on 3 different testers and checked fine at 600VDC from an eye type tester. The ones in the TV horiz ckt. were for sure bad, and the replacements failed to work. 
 I finally ordered the new silver mica, epoxy coated ones to replace the old paper mica types and that fixed the set.

   They did make the silver micas in the postage stamp style cases, too. They were marked with a special color dot. 

The Silver Mica caps that are epoxy coated almost ==never== fail. They come in brown, red, reddish brown, and blue epoxy coatings.

  The Military used a silver/glass mica too. Extremely reliable and stable, but a tad more fragile. They also had a very tight tolerance too.

That brings us around to what is known as Sliver Mica Disease.
 This is prevalent in many old AA5 radios and others, (Like Hallicrafters :twisted: ).

The silver on the fixed padders in the bases of IFT's would migrate over the mica and short the IFT out.
 Symptoms were intermittent or continual popping, crackling, etc., or a "dead set" IF wise, with good audio. 

  There have been several fixes for this, mainly taking a tool and cutting the padder out of the plastic in the base of the IFT, and replacing it with a silver mica of the appropriate PF range. (Typically 175-200 pf. if memory serves me right).
 Complete realignment of the set will be needed. 
 Note: I have never seen this happen with the top or bottom of the IFT air dialectic or mica adjustable trimmers.