Silver mica disease questions

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Silver mica disease questions

Post by Dion on Mon Apr 04, 2011 6:35 am

All: I've seen some posts discussing and defining silver mica disease. I'm working on a 1955 GE 6J05 chassis that works okay for the first few seconds after warm-up, but then starts losing its sensitivity while at the same time developing loud crackling noises. The audio amp tube tests very weak, and I've ordered a replacement, but I don't believe that's going to solve the crackling problem. From the other posts complaining of such static I suspect silver mica disease. The schematic shows capacitors in the pre- and post-IF transformer cans, but when I remove the cans I cannot find them. Are these integrated into the base of the can? If so, how can they be replaced? Do I unsolder the transformers and disassemble the cans to remove them? And if so, if I solder in replacements beneath the chassis, will the circuit still work properly? I'm guessing not. If anyone has experience with and suggestions for successful repair, I would appreciate the advice. Thanks! -Dion Morehouse

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Re: Silver mica disease questions

Post by Bill Cahill on Mon Apr 04, 2011 9:13 am

In my opinion, it would be easier to look for proper new old stock replacement transformers. It's a very tedious job. You would have to carefully remove the pop rivets on the bottom, remove caps, and, clean. Or, just disconnect caps, and, install new mica caps inside. The problem there is most schematics don't show the values of those, and, they vary widely from .0002mmf-500 mmf.
It could be done, but, is experimental.

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Re: Silver mica disease questions

Post by Dion on Wed Apr 06, 2011 1:06 am

Bill Cahill wrote:In my opinion, it would be easier to look for proper new old stock replacement transformers.
Bill Cahill

Thanks, Bill, for the reply. I'll see if I can locate NOS transformers. If not, I guess I'll learn something new. After I wrote the original post I did find the photo story (linked from TRF) of someone who did remove the integrated caps from his cans and installed outboard ones to the lugs below. He used a capacitance meter to determine the values of the original caps --that surprised me because I would assume that the originals can't be trusted to read at their original values. Anyway, I'm going to try that if I can't find replacement transformers.

Dion study

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Question about NOS replacement cans...

Post by Dion on Thu Apr 07, 2011 3:36 pm

Bill Cahill wrote:In my opinion, it would be easier to look for proper new old stock replacement transformers...
Bill Cahill
Bill, after my previous reply it occurred to me to wonder: Would the silver-mica caps in NOS RF & IF xfmr cans still be good? I don't know if the degradation comes with use, or just over time, but for these items I'm a little concerned that its related to time, regardless of usage. What do you think? -Dion

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Re: Silver mica disease questions

Post by Bill Cahill on Thu Apr 07, 2011 7:43 pm

First, not all the transformers had built in caps. Some of the better ones, including the early Miller had seperate caps soldered inside.
The thing that makes them go bad is enviroment.
Cold, damp enviroments tend to make the rivets corode faster. Now, I'm not saying they never corode otherwise, as they certainly can.
But, dampness is certainly a factor.
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Re: Silver mica disease questions

Post by fifties on Sat Apr 09, 2011 3:57 am

Dion wrote:All: I've seen some posts discussing and defining silver mica disease. I'm working on a 1955 GE 6J05 chassis that works okay for the first few seconds after warm-up, but then starts losing its sensitivity while at the same time developing loud crackling noises. The audio amp tube tests very weak, and I've ordered a replacement, but I don't believe that's going to solve the crackling problem. From the other posts complaining of such static I suspect silver mica disease.
Before going to all the trouble of fooling with the IF Cans, why not use a signal tracer at the point just before the detector, to determine whether the problem is in the RF/IF stages, or the audio section?
If the audio from the tracer remains clear while that from the radio's speaker deteriorates, it clears the front end of the radio.
If not, then on the next warm-up, try the tracer at the plate of the converter tube, so you can narrow down with more certainty which of the three stages is mal-functioning.

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Re: Silver mica disease questions

Post by Bill Cahill on Sat Apr 09, 2011 8:05 am

Actually, the correct way to check IF's is to put a voltmeter on secondary of the transformer. There should be 0-a low negative voltage. If you get any positive voltage, that IF is bad.
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Re: Silver mica disease questions

Post by KB8CWB on Sat Apr 09, 2011 10:55 am

Guess I am too old fashioned. Been using the noise function on my signal tracer for years. I can find the noisy little buggers very fast with that thing while the unit is powered down. I find that is one function that many don't understand or use often on their sig tracers that I find as useful as it's primary mode! Very Happy

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Re: Silver mica disease questions

Post by Bill Cahill on Sat Apr 09, 2011 4:56 pm

While your device is very useful, I don't think it will tell about leakage between the two capacitors.
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Re: Silver mica disease questions

Post by Dion on Sat Apr 09, 2011 6:09 pm

fifties wrote:Before going to all the trouble of fooling with the IF Cans, why not use a signal tracer at the point just before the detector, to determine whether the problem is in the RF/IF stages, or the audio section?
Fifties: Thank you for your reply. I don't own a signal tracer and have to admit that I don't really know, beyond what its name suggests, what its for. I have a decent o'scope and passable RF signal generator with 400 hz embedded audio signal. I suppose I could have checked it out that way. Unfortunately I've already taken the cans apart. They look pretty bad; some kind of oxidation seems to have diffused into the mica all the way to the center rivet.
The reason I did not consider testing them is a paradigm problem: I worked as a bench technician on transistorized equipment for many years, and have only recently started working on tube circuitry. In a transistor circuit, the direct coupling between stages makes it very difficult to isolate extreme noise problems to one stage, because the signal backs up into prior stages. It did not occur to me that this would have probably been easily possible on these tube circuits. -Dion

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Re: Silver mica disease questions

Post by KB8CWB on Sat Apr 09, 2011 6:10 pm

While I admit it will not work in all cases, it will in most. And it does detect leakage by applying a 100vdc between the test point and ground. Then it is amplified and you hear just a click on a good test point, sounds like a thunderstorm if leakage is present. Works on caps, resistors, solder joints, most anything. I have found many bad components this way quickly and easily. Does it work all the time? No but what test method does? Some interesting reading on the Heath T-4 and specifically the "noise" test function.



Bill

Edit: Sorry Dion, wasn't trying to hijack your thread. And at some point you might want to consider getting one of these jewels. Personally I'd be lost without my signal tracer. And it is one of the cheaper investments you can make!

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Re: Silver mica disease questions

Post by fifties on Sat Apr 09, 2011 7:24 pm

KB8CWB
I'll be darned; I have a Heathkit 1T 12 tracer that has a the noise function switch, which I don't think I have ever used...Good information!

Dion
Even though I have the above mentioned unit, I usually use a little device that I built into a ball-pen barrel, consisting of a Diode and a cap, with two leads coming out and terminating in an earphone plug, which plugs into a Radio Shack mini-amplifier.
If you Google simple RF Signal Tracer circuit, there are a few you can build like mine and with just a few parts. They work just fine whether on solid state or tube receivers. All the conglomeration is, is a diode detector connected to an amplifier. The amp can also be connected directly after the detected signal, usually at the volume control, to verify the audio amp part of the circuit.


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Re: Silver mica disease questions

Post by Dion on Tue Apr 12, 2011 12:31 am

All: Thank you very much for info. I've learned some from just reading the exchange between you, not to mention the advice directly to me. I will be following up on all of it. -Dion

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okay, HOW do I attach photos?

Post by Dion on Sun Apr 17, 2011 1:03 pm

Dion wrote:All: I've seen some posts discussing and defining silver mica disease. I'm working on a 1955 GE 6J05 chassis that works okay for the first few seconds after warm-up, but then starts losing its sensitivity while at the same time developing loud crackling noises. The audio amp tube tests very weak, and I've ordered a replacement, but I don't believe that's going to solve the crackling problem. From the other posts complaining of such static I suspect silver mica disease.
Zenith 6J05 Update: This radio is back together and working very well now. I found that both IF transformers were indeed subject to silver oxide migration into the mica, up to the retaining stud, where (I assume) there was arcing to ground. I tried to post pictures of these parts, but was unable to figure out how. After clipping out the components of the integrated caps from the IF cans, I reassembled them with external caps. I had to re-align the radio after this, but that only took a few minutes and the radio now works probably as good as when new in 1955. I also restored the Bakelite cabinet, but will (when I learn how) post pix of that in the cabinet resto section so as to remain on-topic here. Thanks to all for your posts, which gave me the info and confidence needed to proceed with what seemed to me to be a very "iffy" repair. Oh yes: I purchased a Heathkit IT-12 signal tracer on-line. Does anyone have a .PDF or other copy of the manual they'd be willing to share? The only person advertising such a manual on e-bay was a little evasive about the quality of the scan and printing. ALSO...If anyone can tell me how to attach pictures to my posts, I would appreciate the help. -Dion


Last edited by Dion on Sun Apr 17, 2011 9:17 pm; edited 2 times in total (Reason for editing : admit that I'm clueless about attaching photos.....)

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Re: Silver mica disease questions

Post by Dion on Wed Apr 27, 2011 10:41 pm

Dion wrote: This radio is back together and working very well now. I found that both IF transformers were indeed subject to silver oxide migration into the mica, up to the retaining stud, where (I assume) there was arcing to ground. I tried to post pictures of these parts, but was unable to figure out how... -Dion
Thanks to Bill Cahill for cluing me in on how to get pix into the forum. Here are pix of the silver mica-diseased parts, and the finished radio. Click on the thumbnails to go to album for full-sized pics. -Dion

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