Author Topic: (An attempt at) turning a pile of e-waste into a weather radio.  (Read 261 times)

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Offline Vincent

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(An attempt at) turning a pile of e-waste into a weather radio.
« on: October 20, 2021, 03:30:59 am »
Back after several years of absence!

I'm one of those weirdos who finds building things out of trash more satisfying than with brand new parts. Anyway...

I used to have several large boxes full of old circuit boards, now down to one smaller box, but I still have enough of them to try and build a proper weather radio (that is, one that doesn't grab all the interference sources around and make me go  |O  every time).

Now I only have very, VERY basic knowledge of radio receivers, so trying to build an RF amp, local oscillator and mixer with discrete components probably wouldn't be a success LOL. But it turns out I still have the boards from an old hi-fi system, and there's this FM tuner module on one of them, Y'know, the little tin canister. With a bit of reverse-engineering (in the form of an incomplete, rather mediocre schematic) and the datasheets of the surrounding ICs I was able to figure out how to make it spit a 10.7 MHz IF signal. At least I think so.  ???

It's a Mitsumi part (EAF 2203-001) for which I couldn't find any information.

If all goes well, a TDA1220B (from some other AM/FM radio) would receive this IF and generate audio for an AN5265 audio amplifier, to a 2W speaker, both from an old TV.

The one big question remaining is whether or not I can make this FM tuner module reach ~162 MHz. It uses 1SV103 varactors which have a capacitance ratio of 2.6-2.8 over their voltage range. At first sight it seems to allow a pretty wide range of tuning frequencies. But maybe it's not that simple...  :-// It's probably not even designed for anything outside the broadcast FM band. I guess it won't harm anything to give it a try, unless it starts emitting interference or something.  :scared:

Otherwise the only brand new component is the prototyping board I'll solder everything on. Even the enclosure is one from an old cable TV terminal LOL.

Maybe some of you guys know a bit more about those varactor-based tuners? The rest of this project looks relatively straightforward (unless I'm trying something that's bound to fail LOL), so not having a deal with RF and mixers would save me quite a bit of work! I don't feel quite confident enough to try to design RF circuitry just yet.

Offline bob91343

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Re: (An attempt at) turning a pile of e-waste into a weather radio.
« Reply #1 on: October 20, 2021, 03:42:07 am »
You seem to know what varactors are.  Now you want to raise the local oscillator and front end tunings from about 99 - 120 MHz to something around 175 MHz for the first and from 88 - 108 to 163 MHz for the second.

That's not really such a big stretch, so go ahead and try it.  You may want to pad the varactors to achieve both the ranges, by adding a series capacitor to each of them.  Remember that to increase the frequency by, say, 50% you must decrease the capacitance by quite a bit more than that, unless you want to muck with the coils as well.

Good luck; I think it's possible.

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