Zenith A835R

[kg4cna]

I am getting ready to recap a Zenith A835R. I recently cleaned the chassis. Before the cleaning, the radio worked perfectly....since the cleaning, the radio powers up normally but the AM is noisy (I call it "erratic static") and I can no longer receive any stations, not even local. The FM works fine...but has some noise. I was very careful not to jostle or mess anything up while cleaning. I've made sure all tubes are secure in their sockets and nothing else was disturbed.

Keep in mind, this is my first recap. I have another radio almost just like this, a C835R and it's also working fine. I plan on recapping it as well.

Any ideas on what would cause this? Thanks for any assistance

[Tom Bavis]

These set are know for "silver mica disease" in the IF transformers - odd though that it appeared suddenly. The usual symptom is loud static crashes, but sometimes just no reception. The IF transformers have two capacitors for primary and secondary on a single mica disc. The B+ voltage will cause silver whiskers to grow across the gap (like electroplating) if there's any moisture present. When it gets close enough, voltage arcs across the gap and burns a little of the silver away, all of this over and over again...

The fix for this is to take the transformer apart and remove the mica disc, and put new capacitors on the terminals outside. If you have a capacitor tester with variable voltage, you can test this before digging in by testing for leakage between windings - obviously it has to stand at least 160V, since that's what it sees in the radio.

[kg4cna]

Originally Posted by Tom Bavis

These set are know for "silver mica disease" in the IF transformers - odd though that it appeared suddenly. The usual symptom is loud static crashes, but sometimes just no reception. The IF transformers have two capacitors for primary and secondary on a single mica disc. The B+ voltage will cause silver whiskers to grow across the gap (like electroplating) if there's any moisture present. When it gets close enough, voltage arcs across the gap and burns a little of the silver away, all of this over and over again...

The fix for this is to take the transformer apart and remove the mica disc, and put new capacitors on the terminals outside. If you have a capacitor tester with variable voltage, you can test this before digging in by testing for leakage between windings - obviously it has to stand at least 160V, since that's what it sees in the radio.

Thanks Tom! Yes, what I was calling "erratic static" is loud static crashes with no reception on AM. FM receives, but only the local station clearly, all other FM stations can be heard, just not as loud as before this problem. I might have done something (albeit unintentionally) to kill it while cleaning the chassis. I used a damp cloth and a toothbrush for the stubborn places...just giving it a general wipedown. Possibly some moisture got into one of the IF transformers while I was cleaning...although I was very careful. Again, I thank you for taking time to answer my question.

[wa2ise]

Originally Posted by Tom Bavis

The fix for this is to take the transformer apart and remove the mica disc, and put new capacitors on the terminals outside. If you have a capacitor tester with variable voltage, you can test this before digging in by testing for leakage between windings - obviously it has to stand at least 160V, since that's what it sees in the radio.

I've been able to fix Zenith IF transformers by scratching lines between (dividing the space between) the cap sections. This breaks up the silver migration path. Then put the disc back in the transformer, and you shouldn't need to do a realignment.

[kg4cna]

Originally Posted by wa2ise

I've been able to fix Zenith IF transformers by scratching lines between (dividing the space between) the cap sections. This breaks up the silver migration path. Then put the disc back in the transformer, and you shouldn't need to do a realignment.

That would be great.....but forgive my ignorance here, if I change the caps in the IF transformer, the radio will have to be realigned? I may have to send this one off if that's the case. I do not have the skill or equipment to do a realignment.


Thanks guys, I appreciate it very much!