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.

It's been years since I discovered that old "postage-stamp" micas go bad. I have seen cheap foreign-made ceramics spit molten metal :yikes: 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!

I understand paper and electro's going bad, but what causes a mica or ceramic go bad. I thought that was rare. My radio has probably 30 more caps for me to replace now

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.

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.

ok thanks!

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

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

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.

The old domino type in the BC348 were actually paper and foil caps. If it says 'Micamold", they have to be replaced.
I saw a few discs go bad. The lousy ones they use in Zenith's. I think they made their own. :thumbsdn:

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.

ok, replace poly for paper on capacitors. But can ceramic replace mica?. Fair Radio has some mica caps I need but They have new ceramics for every value I need.

Micas are more stable than ceramics so in a frequency-determining circuit think I'd stick with micas.

thanks! there is a lot more to this hobby than I thought when I first started.

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.

I guess I am alittle confused here. Fair radio has new manufacter ceramics but how do I know that they are temp-stable? I am putting more effort into this set because it is my first FM set and its a capehart and high fidelity."Maybe" I am sweating the small stuff too much but in my area higher end sets just don't come up for sale often. I am just trying to "get it right" I have been working on radios for over a decade but this is the best radio ( and the most complex) I have come across.

I guess I'm not done with my Zenith after all.

I have used ceramic on IF capacitor replacement in the past but now that I have been able to find new mica caps I use them. I have never had any trouble with ceramic though. 22 pf for FM circuits and 100 pf for AM circuits has always worked for me. I am seeing a lot more bad mica caps in IF's, probably 75% of the 50's-60's tube radios have these bad caps. The radio might start to work good when all other caps have been replaced but after a short while of playing bad mica caps began to show up, or too often you replace all other caps and there is no sound and it is bad caps in the IF cans. Hammarlund HQ-180 has been the most difficult to repair radio with the bad mica caps as there are so many IF stages.

Easy rule for Ceramics: For RF work, ask for temperature coefficient NPO when buying older parts on eBay or C0G when buying new on Digi-Key, Mouser, Allied, et cetera... Yes, that's a zero between the C and G.