Silver mica disease
 

After getting fed up with the FM sound on my Zenith 7H822Z (Z denotes tone control), I decided to take apart the discriminator IF transformer apart for inspection of silver mica disease. The FM sounds distorted, as when I tune in a station to it's maximum level, the audio is completely distorted, and only sounds good when tuned to low levels. AM section only works (works pretty decently when the antenna is connected). When the antenna isn't connected I get no stations and a 60Hz buzz. With FM, my audio level is quiet and I get a 60Hz buzz, and distorted audio is heard over the 60Hz buss when a station is tuned.

Anyway, I took it a part and noticd the silver mica sheet is discolored arount the edges. This a bad thing? The mica looks fine, but I want to add discrete mica caps anyway, because the discolored around the edges doesn't look right.
Is this a sign of silver mica disease? And what cap values should "I use for the discriminator IF?

Thanks.

Jonathan

There are a bunch of threads and some pictures of silver mica disease over on antiqueradios.com. I think the usual symptom of silver mica disease is an extremely loud crackling static. It's essentially a short, and if it's bad enough, say in an IF amplifier for example, you might have plate voltage on the grid of the following stage, causing things to not work at all.

John

Usually if there is a mica cap problem there is no FM reception at all as this upsets the tuning of the IF circuits or causes a short. I would check for other bad capacitors first if there is just a distortion problem.

I have repaired these IF cans before but prefer to replace if a replacement is available. To repair, you need to remove the mica sheets and trim the inside contacts and insulate them from each other and the can so there is not a short. I then added 22 pf capacitors under the chassis across the IF coil terminals to replace the bad caps. The radios I have done this on did not have a problem in the discriminator circuit so am not sure exactly if those are the values to use there but they seemed to work fine on the other IF's.

However I still think in this case I would check for other problems first.

Thanks guys. From looking at the pics, it seems like the conductors are not making sufficiant contact with the silver. There are some small capacitors on the mica sheet that exhibit slight silver mica disease. But being as stupid as I am, I took apart the 1st IF transformer! AM works, so I hope it'll still work. Using 100pF for the AM IF coils should work, but does anyone know what to use for FM? Also, all paper/electrolytic caps were replaced.

Thanks.

Jonathan

88-100 pf for AM
22 pf for FM worked for me.
I had a bunch of 22 pf caps so used 1 each for FM and connected 4 in parallel for 88 pf for AM. Realigned the circuit and off to the races again.

It's just what I needed, Thanks.

Jonathan