IF Cans

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IF Cans

Post by Rickey on Sun Dec 23, 2012 3:02 pm

Hi ,I got a question about the IF transformers. Does the signal go from the primary, to the secondary, thru the magnetic field. Just a little confused!
Thanks, Rickey
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Re: IF Cans

Post by Resistance is Futile on Sun Dec 23, 2012 4:53 pm

Yes. that's what transformers are designed to do, take an AC signal and induce it to the secondary through magnetic induction. Whether it is a steel, ferrite, or air core as the induction medium.

I.F. Transformers are used to isolate Stages and help select band width of I.F. frequencies. This is also where the I.F. capacitors come into the design. When you have a coil and a capacitor either in series or parallel it acts as a resonant frequency at one designed and desired frequency. In the case of I.F. transformers for Broadcast band, the most general frequency of selection is 455Khz.
This will help in removing frequencies on either side of the selected frequency so they do not get amplified and cause problems.

Now as an interesting sidelight when you have two coils next to each other, the signal gets induced from one coil to the other. If the coils were physically next to one another you would get a large induced frequency that would be full of harmonics, but when the coils have separation by some distance the voltage induced falls off but you get better selectivity of the desired main frequency. Since harmonics for the most part are not as strong as the primary frequency they have a harder time of being induced to the secondary coil. You do sacrifice signal strength but you gain selectivity so stations can be more easily separated.

As a basic concept any voltage that is by nature an alternating current can be transferred by magnetic lines of force called fields.
This principle was first recognized during Edisons work by a man who's name was Faraday. Edison was bull headed in supplying DC to homes but His method caused many distribution problems because of huge wires needed to carry current.
Eventually, Edison's system was replaced due to the work of Nicola Tesla, who had first worked for Edison. He devised alternating current (AC) generators. AC current can be easily and efficiently sent over long distances with little power loss because it is "transformable."
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Re: IF Cans

Post by Rickey on Sun Dec 23, 2012 6:11 pm

Thanks Cliff, good explanation. To check an IF coil for opens,just check each side, correct. Do they ever short out?
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Re: IF Cans

Post by Resistance is Futile on Mon Dec 24, 2012 2:04 am

yes, do an Ohm meter check for both shorts and opens. Some of schematics have the DC resistance shown.
they will short when the voltage gets to high or a cap shorts out (not common)
More apt to be broken wires to the terminals and corrosion.

Usually the biggest problem with I.F. Transformers is with owners playing with the trimmer caps and causing the misalignment of the I.F. Cans.
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Re: IF Cans

Post by Bill Cahill on Mon Dec 24, 2012 3:27 am

Sorry, but, I must interject here. Since starting witht the 1949 sets, Zenith, at first with the large transformeres, and, thenm slowly with all the small transformers, they had built in caps in the base. They regualrly go bad, though not always, due to silver mica disease. And, unless the short is bad enough, an ohm meter will not show the leakage between the two sindings.
A good eye should usually tell. You should see a clear silver underneath thebase, and, rivet clean. If rivet is coroded, and, you see darkenned areas, that's the leakage caused by corosion from the rivet.
An ohm meteer will not always show this.
Not too often that the coils themselves will short, though occasionally one will get away.
That, and, usually open windings will be caused from dampness that caused the winding to get at least one spot of corosion, then, open.
But, meter testing, as Cliff states, is great for testing for shorts on individual coils, and, opens. Warning. There are coils that are only 1-3 ohms, and, that can fool you.........
Sadly, not all that many schematics give the resistances, either.
It can be frustrating, at times.
Bill Cahill

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Re: IF Cans

Post by Dr. Radio on Mon Dec 24, 2012 8:59 pm

Cliff and Bill,

Excellent responses.

Just to add my 2 cents, I've encountered some that go "open" where the coils are good inside, but the silver mica capacitor part looses connection to the metal connection tabs due to oxidation and mechanical/temperature fluctuations. Drove me crazy as it acted like an intermittent solder joint on a the circuit board.
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Re: IF Cans

Post by Bill Cahill on Tue Dec 25, 2012 7:51 am

Thanks. I forgot that one. I think I know someone that had that happen once. That can be very frustrating.
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