Electronics > Beginners
Air gap variable capacitors - buy cheap?
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aneevuser:

--- Quote from: bd139 on March 21, 2019, 08:22:27 pm ---Ugh finding these is hell in the UK. You’re better off going to a radio rally and getting a few there. They seem to go for between £1 to £5 a go. Have a look here: https://rsgb.org/main/news/rallies/
--- End quote ---

Not really the bag I'm into, but maybe something I'll consider if I can find one near enough. Thanks.


--- Quote ---You need a double bearing one (one at front and back of shaft) for tuning applications. The other ones are very microphonic and drift.
--- End quote ---

Microphonic?


--- Quote ---The better option these days is a 1SV149 varactor (aliexpress) and a 10 turn pot (8 quid from RS). Much cheaper, just as stable and a hell of a lot cheaper.

--- End quote ---

That's interesting - I was unaware that you could get varactors with such a wide range. I note that Mouser says that is obsolete though. However, I've ordered one of the caps that I originally mentioned, partly out of interest. I'll report back on my findings when I get it - won't be for a while though, as it's winging its way from China.
bd139:
Microphonic. Basically they are sound / vibration sensitive. If you knock the table you can hear a bit of frequency modulation sometimes.

1SV149 probably is out of production but there's stacks of them available in the far east. I've built a sig gen and a couple of radios with the cheap ones from aliexpress and they seem to work pretty well.

Please do report back. I could do with a couple for ATU projects.
aneevuser:

--- Quote from: bd139 on March 21, 2019, 08:47:06 pm ---Microphonic. Basically they are sound / vibration sensitive. If you knock the table you can hear a bit of frequency modulation sometimes.
--- End quote ---

I see. I can live with that though, as I'm only a pseudo-noob playing around.


--- Quote ---1SV149 probably is out of production but there's stacks of them available in the far east. I've built a sig gen and a couple of radios with the cheap ones from aliexpress and they seem to work pretty well.
--- End quote ---

I shall look into these. Voltage controlled tuning would be much nicer - potentially controllable from an Arduino or similar. Still, I'm currently struggling to get a decent envelope detector working at the moment, so that's for the future.


--- Quote ---Please do report back. I could do with a couple for ATU projects.

--- End quote ---

Sure. Anything specific that I should check?
rhb:
Visit a few thrift stores and buy an old radio.  The older the better taking into account condition.

What you're looking for is analog tuning.  Get a transistor portable if you want small, a table model if you want high Q.

And if you want top of the line air variables I have an unknown quantity of NOS 1940's vintage air variables at a wide range of voltage specifications made by Hammerlund and such.  Send me a PM if interested.
james_s:
Why not just buy a cheap variable capacitor from eBay? They're not that hard to find yet and often not very expensive.
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