I heard that micas may suffer death or disease at some point yet other people say they last forever. The ones in my set are 67 years old, should I replace them to have years and years of trouble free operation?

Hi, John. No, the micas that are prone to the disease are the dual ones that are in IF xfmrs of some post-war radios. Even then, no sense replacing them unless they short out due to silver migration.

Other stand-alone micas should last indefinitely.

:I heard that micas may suffer death or disease at some point yet other people say they last forever. The ones in my set are 67 years old, should I replace them to have years and years of trouble free operation?

Interesting. So are you saying that some sets have micas inside of the IF cans? That must be a real pait to get in there with an iron to un-soldier them.

All the micas should be checked, bad ones are starting to show up rather frequently.

Dave

Bad ones are typically actually paper inside. I have rarely found a molded mica condenser that has gone bad.

In my experience, the mica sheets with silver deposited on them inside of IF transformers more often corrode, and form a bad contact, especially when in the open air. If enclosed in plastic, they tend to last longer. Corrosion is hard to detect unless the set pops into and out of good sensitivity (due to corrosion). This can be found by rocking the IF transformers. Sometimes moving the cans will make and break the connections. A shorted mica will usually be obvious (no reception, and sometimes causes irratic voltages). If your IF transformers are slug tuned, they most likely have a silver plated mica sheet inside, and can be corroded. For optimum reliable performance, open each can and examine. The plastic enclosed ones, in my opinion, tend to be a bit more reliable, but I've had shorts problems with these. If you are at all suspicious, find a way to destroy the condensers so that they do not touch any of the contacts, and are no longer a part of the circuitry in any way. Replace with new molded micas (epoxy dipped). Select a value between about 50 and 200 pF for AM IFs, and lower for FM IFs, that'll allow resonance with the slugs just outside of the center of their travel. Start with the upper slug all the way up, and the lower slug all the way down. Never turn the slugs so that they are so far in that they come close to eachother. This'll ruin selectivity of the set.

Thomas

Quote by Thomas "Bad ones are typically actually paper inside. I have rarely found a molded mica condenser that has gone bad."

Thomas, time to wake up and realize the mica's are not lasting forever, I am not the only one running into this problem. And yes, I am talking about the ones with mica in them and not the Micamold imitations.

The problem with the IF caps is the silver starts to migrate and grow tendrils which shorts them out and causes a noise like lightning strikes in the radio. The value of the caps are important for selectivity. I will usually remove some of the bad area and measure them, and replace with the same value. There are a couple of sites with pictures and instructions to do this procedure.

Dave

Dave
what is the nature of the mica cap faliures you and others have experienced recently? Leakage?.. or value change (and how much % change have you noticed?)

I assume leakage has to be the primaruy failure as most of the RF ciruits with micas are able to tolerate value changes reasonably without noticeable performance differences that I have noticed.

You may want to read this

http://antiqueradios.com/forums/viewtopic.php?t=59472&highlight=mica+capacitors

Dave

Interesting read... the consensus seems that very few go bad and if after replacing paper you seem to still have trouble... then... you continue to troubleshoot keeping in mind that mica's are not sacrosanct.

I did find it interesting that Philco's parts lists shows they had two types of mica caps... standard-mica... and their newer "silvered-mica"

Personally:
In the last year I've restore about 45 to 50 radios of every possible brand and variety and mostly from 1930-49 vintage. And I have not yet stumbled on a single bad mica that I can recall..

Bravo Peter! That's a lot of radios! You're like your own factory!

I agree about micas, though. I don't see how it is possible for them to go bad if they are the foil/mica kind. Unless there's bad construction, and the foils touch, there's no way a short can occur through the mica unless it's struck by lightning or something big like that. If the mica swells with age, value can change.

I totally agree that silvered mica in IF transformers can be nightmarish.

T.

This guy is cool! I like this one! http://antiqueradios.com/forums/viewtopic.php?t=44436&highlight=

Regarding micas, though, I think that it would be a good idea that no further commentary was made by anyone unless the commentator(s) first cracked open the faulty component(s) to see what was inside. It is not at all accurate to simply report that a device with colored dots has become leaky or otherwise, and to assume that the device was mica. ....So everyone, please do not report any further mica troubles unless you have taken the time to examine what's inside. Micamold didn't just make papers or micas. They made both, and one cannot make accurate commentary when one assumes that the contents of the device are one item or another.

Thomas

:Regarding micas, though, I think that it would be a good idea that no further commentary was made by anyone unless the commentator(s) first cracked open the faulty component(s) to see what was inside. It is not at all accurate to simply report that a device with colored dots has become leaky or otherwise, and to assume that the device was mica. ....So everyone, please do not report any further mica troubles unless you have taken the time to examine what's inside. Micamold didn't just make papers or micas. They made both, and one cannot make accurate commentary when one assumes that the contents of the device are one item or another.
:
:Thomas
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Thomas

There are many of us who have been working on radios for many decades, we are well aware of the caps Micamold made, both mica and paper. I too once thought mica's would last forever, but after fighting a few problem sets with bad mica's I now test each and every one. I suggest you frequent ARF a little more often to keep pace with the latest developments, we would like to see you over there once in a while. IF cap failures first started showing up in the 60's. Remember, there was a time when Sprague "Bumble Bees" were going to last forever too.

Dave

I have open on my bench a David Grimes "Baby Grand Duplex," c. 1924. Since this is a reflex set, there are more caps than usual, five. They are original Dubilier micas, the flat ones about 1" square. They are all good, zero leakage.

Checking mica caps is certainly a good idea. But this thread started with someone suggesting that perhaps all mica caps should be replaced, wholesale. If that is the concensus among ARF members, then I respectfully disagree.

Unless you crack them open and determine what's inside, though, your report can hardly be accurate. Please crack them open and note what's inside before commenting on their longevity.

Thomas

Thomas says "crack them open," which I agree with for checking failed caps that may or may not be mica. Practically speaking, I slice them in two with my bandsaw. A mica cap, in cross-section, is unmistakable.

I really can't sanction wholesale replacement of mica caps -- yes, they might not last another 82 years like all five caps in my above-mentioned David Grimes set, c. 1924. (2006-1924 = 82 years.) What would I replace them with that would be sure of lasting another 82 years, and also look like they belonged in this old radio? And how would the replacements affect my interest and my joy in this set?

:Unless you crack them open and determine what's inside, though, your report can hardly be accurate. Please crack them open and note what's inside before commenting on their longevity.

:Thomas

And what about the cost of replacing an original mica cap that tests perfectly good?

Forum,
The mica/paper/Micamold/molded cap controversy goes on and on. I treat all hardware (caps, tubes, resistors, inductors, transformers, lamps) the same. In the interest of originality, "if it ain't broke, don't fix it", and if within 20% of the original stated value, it ain't broke. All failed hardware gets replaced with what is available. The obvious above chassis stuff like large canned caps and transformers are an exception. I make original looking cans when necessary for transformers (all audio so far), I make original looking speaker cones and edge suspensions and restuff canned caps. Even made softer motor mount grommets, when all I could find was the modern hard rubber stuff. Made new rubber shoe/belt for the phono idler wheel with items available at the hardware store. Don't have a strobe light, but counted rpm's with a stop watch, and it was right on. I've replaced some surely to be mica and some that look like molded mica, but only because they were broke.

marv

:And what about the cost of replacing an original mica cap that tests perfectly good?

:And what about the cost of replacing an original mica cap that tests perfectly good?

_______________________________________________________

If you read my post you would see I am trying to make you aware of the fact that the mica's are not lasting forever, I do not advocate replacing all of them, just checking them. The choice is yours.

Dave

::And what about the cost of replacing an original mica cap that tests perfectly good?
:
:_______________________________________________________
:
:If you read my post you would see I am trying to make you aware of the fact that the mica's are not lasting forever, I do not advocate replacing all of them, just checking them. The choice is yours.
:
:Dave

I have enjoyed the discussion on the mica caps, and there were many good points in what you all said. As far as radios, I have no experience with. But what I do have experience with is television repair. Been at the same repair shop going on 30 years. Yes, micas do go bed. Do they short? Yes, sometimes. Do they open? yes, sometimes. They also (sometimes) change value. They will also leak. But not always. I (as a hobby) collect and restore the old TVs from the 40's. I have found some mica's that were bad in the old TVs. I see even more of the new micas going bad (cheap manufacture). I am sorry I have not disected any of the bad caps, but maybe next time I will. Remember, anything that is man made might fail. I do not automatically replace the ones in the old sets, because I have seen a number of them never give any problems. However, I always replace any (in the old sets) that are in the horiz. tuning ckt, because I have found more bad ones here, than in any other circuit area of a TV set

I can not go by how it is with radios, I can only go by my own experiences.

Jim.

As stated "if it ain't broke, don't fix it" As for leakage test on a mmf mica, I can't see the needle move on my old VOM during the charge cycle, so how can I see it return to zero if not leaky? Guess I'll have to fall back to my old unhandy Solar box

marv

::And what about the cost of replacing an original mica cap that tests perfectly good?
:
:_______________________________________________________
:
:If you read my post you would see I am trying to make you aware of the fact that the mica's are not lasting forever, I do not advocate replacing all of them, just checking them. The choice is yours.
:
:Dave

Even though you can't see the charge cycle of a small value condenser, leakage will show up. It doesn't matter whether or not the needle normally goes up and down. If there's leakage, the needle will swing up and stay up. Doesn't matter what value the condenser is. Ohmage is ohmage. It all looks the same.

.....And to everyone else, regarding finding micas that are problematic, but not cracking them open to see what's inside: we all know that these units come in both paper and mica. If you can't or didn't crack them open to make an accurate report, DON"T MAKE ANY REPORT AT ALL!!!!!! For all of you who say that micas fail a lot, or at all, if you didn't crack a single one open to see what is inside, none of your commentary is valid. Please make no commentary about the devices unless you can do so accurately. To do otherwise just spreads false information. I'm sure that micas do fail, however often, but you cannot make comments unless you have data to back them up. I am really anal about accuracy, and I cannot stand it when people make comments about things, whatever they be, when they never even take the time to get accurate information. It's like the people who say that all old tube radios sound like crap and that tube audio is inferior. Sure it's inferior after 60 years when all of the condensers are leaky, and no, changing tubes isn't all you have to do! DUH! GET REAL! Unless you're going to research and accurately report your findings, you shouldn't report them at all!

Thomas

:Even though you can't see the charge cycle of a small value condenser, leakage will show up. It doesn't matter whether or not the needle normally goes up and down. If there's leakage, the needle will swing up and stay up. Doesn't matter what value the condenser is. Ohmage is ohmage. It all looks the same.
:
:.....And to everyone else, regarding finding micas that are problematic, but not cracking them open to see what's inside: we all know that these units come in both paper and mica. If you can't or didn't crack them open to make an accurate report, DON"T MAKE ANY REPORT AT ALL!!!!!! For all of you who say that micas fail a lot, or at all, if you didn't crack a single one open to see what is inside, none of your commentary is valid. Please make no commentary about the devices unless you can do so accurately. To do otherwise just spreads false information. I'm sure that micas do fail, however often, but you cannot make comments unless you have data to back them up. I am really anal about accuracy, and I cannot stand it when people make comments about things, whatever they be, when they never even take the time to get accurate information. It's like the people who say that all old tube radios sound like crap and that tube audio is inferior. Sure it's inferior after 60 years when all of the condensers are leaky, and no, changing tubes isn't all you have to do! DUH! GET REAL! Unless you're going to research and accurately report your findings, you shouldn't report them at all!
:
:
:Thomas

Thomas

I repair TV sets for a living. I really do not have the time to take apart a capacitor to find out WHY it failed. Get real. I can be a perfectionist about things myself, and I can relate to your insistancy on proof. I remember an RCA TV that was famous for a 390 pf "mica" cap failing quite often. It would change value as the set got warm. I can think of many others in my line of work over the years which would fail. And these were micas. If you do not want to hear about "real-life experience" sorry about that. I meant no offense.

Also, in order to check a capacitor correctly, you must check it at working voltage to be sure. I have seen many caps check good until they were under load. Again, just my own experiences.

The "duh, get real" was to the people who assume that tube audio is inferior (probably noone on this forum).

At any rate, if you don't have time to crack them open, then you can't be sure that all of the condensers that failed are definitely mica.....so, even if your intensions are good, you shouldn't report something unless you know for sure what it is....and that message wasn't just for you....it was for everyone, because just about noone who makes these reports is actually collecting accurate data. I realize that the sun will still rise and set even if the data isn't correct, but if you're going to report something, why not do so accurately?

Also, regarding loading the condenser with actual voltages, this is quite true. I do this all the time when I have condensers that arc or that cause trouble in the set, but don't show up as bad. However, for a general leakage test, a multi-meter will suffice. You'll find most of the bad condensers, and usually you'll find all of them. If you decide to put the good ones back in, and the set performs irratically, then they need to be tested under voltage. For general leakage, though, any condenser that has leakage will show up on a multi-meter, if the leakage is constant (which it usually is). It does not matter whether the capacitance value is high or low. If the condenser has ten million ohms of leakage, the meter is going to show ten million ohms of leakage....and that's the point I was trying to make.

Thomas

:The "duh, get real" was to the people who assume that tube audio is inferior (probably noone on this forum).
:
:At any rate, if you don't have time to crack them open, then you can't be sure that all of the condensers that failed are definitely mica.....so, even if your intensions are good, you shouldn't report something unless you know for sure what it is....and that message wasn't just for you....it was for everyone, because just about noone who makes these reports is actually collecting accurate data. I realize that the sun will still rise and set even if the data isn't correct, but if you're going to report something, why not do so accurately?
:
:Also, regarding loading the condenser with actual voltages, this is quite true. I do this all the time when I have condensers that arc or that cause trouble in the set, but don't show up as bad. However, for a general leakage test, a multi-meter will suffice. You'll find most of the bad condensers, and usually you'll find all of them. If you decide to put the good ones back in, and the set performs irratically, then they need to be tested under voltage. For general leakage, though, any condenser that has leakage will show up on a multi-meter, if the leakage is constant (which it usually is). It does not matter whether the capacitance value is high or low. If the condenser has ten million ohms of leakage, the meter is going to show ten million ohms of leakage....and that's the point I was trying to make.
:
:Thomas

Thomas,

OK? Leave us not forget (for the beginners) that what looks like a mica (or otherwise) could be a resistor in some cases.

Radiodoc

True that! I love those resistors! Someday I'm going to manufacture replica components so that everyone's radio can be beautiful both inside and out.

T.

:As stated "if it ain't broke, don't fix it" As for leakage test on a mmf mica, I can't see the needle move on my old VOM during the charge cycle, so how can I see it return to zero if not leaky? Guess I'll have to fall back to my old unhandy Solar box
:
:marv

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Marv

A VOM will only tell you if you have a bad cap, it won't tell you if it is good. Use the Solar, it tests for leakage at working voltage, this is the only way to test a capacitor.

Dave

Ok, I started this thread, can I have the last word. :-) without getting to technical, true if it "aint broke dont fix it" BUT.......since some of these sets are as old as dirt and you may already be recapping and re-resistering the set, why not spend the extra $2 and re-mica for piece of mind. I think everyone on this site can afford a couple of extra bucks, right? I understand orginal is nice, I like old stuff too but once the chassis is screwed down you cant see it anyway. The only problem I have found with new micas available is that the voltage rating is around 500v, not the original 600v, but as someone pointed out, most sets dont see 600v throughout the set, only up to the rectifier.