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49-905 Mica Disease-FOUND IT! - Printable Version

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49-905 Mica Disease-FOUND IT! - Eric Adams - 03-14-2014

After finishing (or so I thought) my 49-905, It has now started to make a crackling noise on AM only that sounds like the dreaded Mica disease. From the schematic I don't see any mica caps inside the AM IF cans. Any ideas?
http://www.nostalgiaair.org/PagesByModel/674/M0013674.pdf
Thanks,

RE: 49-905 Mica Disease - morzh - 03-14-2014

C301, 302, 304 and such - gotta be micas.

RE: 49-905 Mica Disease - Eric Adams - 03-14-2014

Thanks Morzh, I'll check them out.

RE: 49-905 Mica Disease - Mondial - 03-14-2014

Eric, the problem is that the caps are integrated into the plastic base insulator of the transformer. They are not separately replaceable caps but more like the compression trimmers found in older style IF's. Just silver coated sheets of mica riveted to the terminals. As the plastic ages it compresses and contact is lost between the silver and the terminals.

RE: 49-905 Mica Disease - Eric Adams - 03-14-2014

Morzh, are you saying that there ARE micas inside of those IF cans?

RE: 49-905 Mica Disease - Mondial - 03-14-2014

They are part of the base insulator that the terminals exit from.

RE: 49-905 Mica Disease - Eric Adams - 03-14-2014

So, short of replacing the IF can is there anything that can be done?

RE: 49-905 Mica Disease - Mondial - 03-14-2014

Years ago I fixed one of these, by drilling out the rivet, removing the mica sheets and replacing the caps with external silver micas. It was a lot of work but it did solve the problem. I know of no other way, because any IF transformers of this type construction have or will have the problem in the future.

I believe they are referred to as K-tran IF transformers. Do a Google search and see if anyone has better solutions.

This is what they look like inside:

[Image: http://i132.photobucket.com/albums/q18/billynaty/micacaps/k1.jpg]

RE: 49-905 Mica Disease - Eric Adams - 03-14-2014

Well, it looks like it is possible to repair. I do have some other cans laying around I'll swap them first and see what happens. Could the noise be from one of the mica caps in the radio or is it most likely the cans?

RE: 49-905 Mica Disease - Mondial - 03-14-2014

If the noise is only on AM, then its probably one of the transformers. Of course it also could be a bad resistor, connection or loose tube socket, etc, but I assume you already checked the other possibilities.

If you wiggle the IF transformer and the noise changes or gets worse, then that pretty much confirms that its the problem.

RE: 49-905 Mica Disease - morzh - 03-14-2014

I saw an article just about 3 or so weeks ago, a guy wrote how he deals with SMD, I think he was working with some Transoceanic.

Just look up the Silver Mica Disease on the net, you will find it.

RE: 49-905 Mica Disease-FOUND IT! - Eric Adams - 03-14-2014

I got it guys! I replaced the 2nd IF can with an exact replacement and the noise is gone. Icon_thumbup

I'm glad I had a replacement. I wasn't looking forward to digging in to replace the mica.

Once again the greatest radio forum on the 'net comes through.

Thanks again, Morzh and Mondial.

RE: 49-905 Mica Disease-FOUND IT! - Eric Adams - 03-15-2014

I think this is what you're talking about. It is really helpful.

http://www.ppinyot.com/if_transformers.htm

RE: 49-905 Mica Disease-FOUND IT! - morzh - 03-15-2014

Yes Eric, this is it.

RE: 49-905 Mica Disease-FOUND IT! - Don Ferguson - 03-19-2014

Eric, don't throw that old can away. It is one of the easier ones to cut out the old mica and place new caps on the outside. Save it you may need it. Of all the cans that I repair, that type was pretty easy to get a repair style figured for it.
Don