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City: Coral Springs, FL
10/25/20 Radio: 53-1750
One of my IF cans has mica disease. The radio sounds great until it heats up after about 20 minutes, then I get nasty static and I can see arcing inside the bottom of the can.
I want to rebuild it. I have watched videos on how to do this but need clarification. Each side of the can has 3 terminals. On a known good can from another radio, the capacitance between the 2 terminal ends on each side measures 3.5pF.
On one side, if I measure the capacitance from an end terminal to the middle, I get 116pF and on other opposite side, if I measure an end terminal to the middle I get 55pF.
So , if I rebuild this thing, I remove the mica, and then, where exactly am I soldering in the replacement capacitors on each side? Should I be using two 3.5pF capacitors and connect the terminal ends on each side or is there something I have to do withe middle terminal as well? In my radio (53-1750), one of the middle terminals on 1 side is not even used.
Thanks,
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City: Roslyn Pa
I don't really see anything that indicates the need for a 3.5mmfd cap. @ 455kc it doesn't amount much at all. Not shown on the diagram asa fixed or gimmick cap. Where you are getting your noise from is from cap across tc3, although it wouldn't hurt to replace the one on tc4 while your there. I would suspect that the caps across the coils and the two that have the series resistor (rf filter) should around 50-125 mmfd.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
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I've heard before that a distant sound electrical storm in a radio can sometimes be a bad capacitors instead of a real storm.
Question: If all the the outer capacitors have been replaced and you hear that sound is it a symptom of mica disease?
Posts: 7,295
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City: Roslyn Pa
There are transformers that are prone to have that issue.They are the smallish ones like in the pic. Sometimes you can see the arcing with the lights off in a dark room. Look by the terminals @ the base.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
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City: Roseville, MN
RF or IF tubes will intermittently cause the same symptoms.
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The way to tell if the micas are arcing/leaking is look at the grid DC voltage, preferable with a scope. If the grid shows positive voltage spikes, there you go. The K-Tran series cans are especially prone to this. They were heavily used in the AA5 series. I had one go once in a cheap AA5 and I didnt want to waste a new can as the slug was broken too. I made a 455 Kc ferrite xformer that worked well.
Peter
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