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 Post subject: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 7:03 am 
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This may be a stupid, foolish, and silly question, but is it possible to align a 4.5MHz IF transformer, and 4.5MHz quadrature coil to 10.7MHz? I have a LOT of spare 4.5MHz stuff, and is looking to build an FM receiver. I already have the tuner, scrapped from a radio. And if you already guessed, it uses a Quadrature (AKA Buzz-O-Matic) Detector. FM radio is not my Forte.

Thanks!

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 12:07 pm 
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No, its a good question. The stupid and foolish question is the one not asked :D

It is a stretch to move a 4.5 MHz IF transformer to 10.7. It probably could be done, but it would require unwinding some turns off each coil.

Another option would be to use them at their native frequency of 4.5 MHz in the FM radio you want to build. The bandpass is a bit narrower, but I've seen many 4.5 MHz IF systems used in FM radios, particularly before WWII.
The bandpass is optimized for TV sound, but many early TVs using the Mallory Inductuner would tune FM Broadcast through the sound channel with success.

A resistor across each winding, say 100,000 ohms would probably help broaden the response enough for proper FM reception.

-Steve

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 3:17 pm 
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It is possible to make just about anything "work." The real question is, how well?

In the IF stages of FM radios, the basic problem is the passband. The loudness of signals is conveyed by how far the carrier is deviated from its center frequency; the frequency of the audio input is represented by the number of times the signal is deviated per second. The FCC says that the maximum deviation of a broadcast FM signal should be +/- 75 kHz, or 150 kHz total. In theory, an FM IF strip would have to pass the entire 150-kHz flat to carry the full range of audio possible without distortion, but this is seldom done in practice due to the noise and interference at the band edges. But you'd probably want at a passband of least 100-kHz flat for a high fidelity radio.

Audio fidelity was never as much of a high concern in analog television, so the standard for that was set at +/- 25-kHz, or 50-kHz total. In consequence, the IF and ratio detector transformers for TV sets were only built to be flat over 1/3 the range that FM radio transformers would typically be--and that's before you attempt to make them operate at more than double their design frequencies. Aside from that, an IF transformer has to be reasonably well matched in its input and output impedances, otherwise the Q is ruined and you won't have any selectivity--or maybe too much (which would cause distortion).

All of which suggests that if you want to build a practical FM radio, it would be best to find some 10.7-MHz transformers out of a junker FM radio and let the 4.5-MHz stuff go. However, if one wanted to experiment with building an "old school" (ca. 1947) FM radio, you could do it. Many of those radios had 4.3-MHz IF strips and your TV coils could most likely be re-tuned to that point. Or you could simply use them on their existing frequencies by shifting the oscillator instead. Back in the 1940s and early '50s engineers really struggled with getting radios to work on the "new" FM band--the one we have now--but there were some sets made that covered both the new and the old bands.

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 6:08 pm 
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So I realized that I can use the IF transformers in the radio I scrapped. The only thing is that there is a dual AM/FM IF transformer, And I thought it would be a pain to work around. Now I see it is easier than doing that.

Thanks Yall!

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 6:14 pm 
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Yeah, I've seen that done, mostly to cut costs, and eliminate a can.

You could use the dual AM/FM transformer just for AM, then use a 4.5 MHz IF for the FM part.

Many ways to skin a cat, I guess :lol:

-Steve

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 8:27 pm 
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Well, I have obtained 10.7MHz If Transformers. Now the only question is now to find a 10.7MHz Quad Coil. Is it possible to reduce the capacitor on the 4.5Mhz coil to make the coil resonate at 10.7MHz?

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 8:42 pm 
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Might be possible, or use one winding of a standard IF transformer. Its simply a coil/cap resonant at 10.7 MHz

Are you using a 6BN6 or one of the other FM quadrature detectors?

-Steve

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 8:47 pm 
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A 6BN6. Well actually a 3BN6, Same construction, different filament.

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Mon 10, 2020 9:28 pm 
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ahh, ok!
The only FM radio I am aware of that uses that tube is a Sarkes Tarzian which uses a 12BN6.
Also, I have seen either a HH Scott or a Harman Kardon tuner that used one or two as limiters. I can't recall as its been a while.

-Steve

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 Post subject: Re: 4.5MHz IF transformer Question
PostPosted: Aug Tue 11, 2020 6:25 am 
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I saw a radio the other day. It had the distinctive 6BN6. Too bad I didn't take it! :cry:

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