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 Post subject: IF transformers
PostPosted: Oct Mon 01, 2018 5:14 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beaver Falls, PA. USA
In radios from the late 1930s and 1940s, it is not uncommon to find that the pigtail leads of the IF transformers are covered with natural rubber. After 70-80 years, this is usually brittle and cracked, which can cause hard-to-locate shorts. The wires are not hard to replace, after the transformer is removed. Some manufacturers, notably Midwest, put bypass capacitors and decoupling resistors inside the cans, and "tweet" filter networks as well; they should also be checked and replaced it necessary. Before doing any repairs, make sure that the coils have continuity, as they do open up occasionally.

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 Post subject: Re: IF transformers
PostPosted: Oct Sat 20, 2018 4:16 am 
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Location: Central PA 16801
absolutely

if a set has rubber wiring in the IFTs, they are the first to be removed, have wires replaced, and be reinstalled. then I'm ready for the full restoration on the chassis. while I am in the cans, if there are separate resistors or capacitors inside, they are replaced.

although it makes little difference, the 47k resistor often found in them drifts high, sometimes over 100k.

steve

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 Post subject: Re: IF transformers
PostPosted: Oct Sat 20, 2018 2:00 pm 
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I have flip-flopped a few times on this kind of thing. My normal bias is to take everything apart for cleaning, but the big issue is getting it back together correctly.....;)

With IFs where there might be SMD, I prefer to wait until the set is operating but, if it looks like there is dirt inside the IF can, I open it up. I just went through an unusual experience on one set. After some hours on the bench, it suddenly "went SMD" on me. When I finally found the (very small) silver track, it did not go from the high to low side of the built-in cap. Rather, it went from the high side to a large clump of dirt.

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 Post subject: Re: IF transformers
PostPosted: Oct Sat 20, 2018 2:08 pm 
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in my post above, i'm only speaking of the large capacitor tuned IFTs.

i have nothing to do with the little silver migration type cans.

steve

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