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#1
02-21-2012, 03:21 PM
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junk micas
On my capehart 21n2 set everytime I put about 3-5 hours run time on it I lose an original mica or ceramic cap. Has anyone else had this kind of battle? So far I am up to about 9 mica caps and one ceramic with another ceramic that went out today. Some people have given me flack for the shot gun approach by putting in all new paper and electrolytic caps but we all have seen that for the most part they are bad by the time we get the set. But this will be the first radio that I am debateing replacing all mica and ceramic at once.
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#2
02-21-2012, 03:36 PM
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It's been years since I discovered that old "postage-stamp" micas go bad. I have seen cheap foreign-made ceramics spit molten metal
 out the side as well, thanks to a cheapskate parts buyer where I used to work. Rule of thumb here (and my best freind disagrees): replace ANY mica over 50 years old, and be ready to replace some ceramics, too, Just try and match the temperature coefficient when doing RF/IF circuits (NPO = C0G these days). It doesn't hurt to go one step higher on voltage rating, either (if size allows). There's a great seller on eBay I HAVE NO CONNECTION TO called Tracktronics that always seems to have a large selection of HV ceramics on hand. Mostly RMC brand, made in America, can't beat that!
Last edited by Einar72; 02-21-2012 at 03:37 PM. Reason: Spelling errrors
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#4
02-21-2012, 04:09 PM
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one more question, can I use the 630 volt "yellow" plastic caps in the IF circuits or should I use Ceramic to replace mica and ceramic? Also I don't know the temperature coefficient of the originals. I can go to fair radio and get most of what I need in 1000 volt new ceramics to put in or get new from Just radios out of canada.
Last edited by radio nut; 02-21-2012 at 04:22 PM.
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#5
02-21-2012, 05:21 PM
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Best to use like for like. Yellows and orange dips and the like to replace old paper/wax caps, and micas for micas as they are in more critical circuits. Radios from the sixties you're probably OK leaving the micas and ceramics alone, but older than that, some may be starting to be questionable. What happens is sometimes the seal fails where the wire enters the outer casing, or the casing splits (especially when you try to turn one over in the circuit to read the value) and moisture gets in. And some marked Micamold are actually paper caps: they'll often have their mfd. rating in figures rather than in dots. Their position in the circuit and value would tell if they should be replaced with poly or with actual mica: mica in RF duty, poly in audio and bypass locations. Personally I leave micas and ceramics alone unless there is a problem. If you do a wholesale replacement in an RF circuit you may get the alignment out of whack, not that you wouldn't tweak it in the end anyway.
__________________
Reece Perfection is hard to reach with a screwdriver.
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#7
02-21-2012, 08:14 PM
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The 'domino' ones? Those things are always going bad. I have an old military shortwave radio thats just loaded with them that i put aside because I know they ALL have to be changed.
I have never seen a disc go bad yet. Thats a trip
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#8
02-21-2012, 08:42 PM
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the ceramics that have went bad were a very hard material that looked like a yellow cap in shape but were low value caps. one was 50pf and the other was65pf i believe. the color on them almost looked like cement grey
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#9
02-21-2012, 09:56 PM
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ive seen disc caps go bad in modern equipment. they usually just go dead short, and end with a bang.
I worked on a yamaha synthesizer/midi based piano where a 22pf ceramic went leaky around 200 ohms, causing the speaker to melt-down in the unit. took me forever to find that. lol. kept putting 15vdc on the right speaker.
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#10
02-22-2012, 09:52 AM
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Quote:
I saw a few discs go bad. The lousy ones they use in Zenith's. I think they made their own. 
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#11
02-22-2012, 10:22 AM
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Silver mica disease is an issue with many radios. Thanks to this forum, I now know how much time I wasted looking for cold solder joints, junking some otherwise good radios out of frustration.
1960 GE T155 Musaphonic table radio - 6" speaker sounds super, excellent reception with 6 tubes. Volume drops intermittently with crackles. Tapping on the RF amp coupling transformer seemed as if a solder joint was cold but none were found. Per the Sams schematic, 110 and 100 pf caps are molded into the terminal bases of all three RF-IF cans in this radio. These things fail mechanically and cause major static. I will post a photo of the surgical removal of this failed cap and tucking a new one neatly into the can, soldered to the terminals across the coil.
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#12
02-22-2012, 03:57 PM
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Quote:
Last edited by radio nut; 02-22-2012 at 03:59 PM. Reason: change wording
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#15
02-22-2012, 11:56 PM
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Don't spend yourself in a hole with all these caps - capacitor mfg. techniques today produce a product far more stable and long-lived than the old wax-dipped or molded-case paper caps. These modern caps are often far better than what was available (or used) in the 30's or 40's. If you can find micas in the reddish dip at tube voltages, go for it. But you are repairing old radios, not missile-guidance or heart-pacemaker devices. Just buy (as I stated previously) temp-stable ceramics for RF circuits if you can't find the value you need in mica. Unless you listen to single-sideband hams on the 20-meter band, you won't be bothered with drift issues.
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