Author Topic: Air gap variable capacitors - buy cheap?  (Read 4348 times)

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Offline aneevuserTopic starter

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Air gap variable capacitors - buy cheap?
« on: March 19, 2019, 02:16:40 pm »
I'm looking for a variable capacitor suitable for tuning over the AM frequency band, and this:

https://www.ebay.co.uk/itm/Single-Joint-Air-Dielectric-Variable-Capacitor-12PF-to-365pf-250M-35-29-26mm-/192610325869

at about 9 quid looks suitable. However, having looked around, I've seen similar caps priced at £80 quid or more, or similar in dollars. So I'm wondering what I'd actually be getting for the £9 price - cheap junk or something usable?

Does anyone have any knowledge of these?
 

Offline bsfeechannel

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Re: Air gap variable capacitors - buy cheap?
« Reply #1 on: March 19, 2019, 04:46:06 pm »
Does the fact that the product was probably made in China account for the lower price?
 

Offline soldar

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Re: Air gap variable capacitors - buy cheap?
« Reply #2 on: March 19, 2019, 06:17:37 pm »
Wow, those used to be so common fifty years ago but I am surprised they are even made any more. They used to make two working together in tandem, one would control the antenna tuning circuit and the other would control the local oscillator. They were extremely common and extremely cheap.

There were (and are) others which were much more expensive because they were for specialty uses like high voltage for tuning transmitting circuits, etc. but for a small one like the one pictured I think the price is right and there is no way it would sell for much more.

Also note that the AM tuning types are not linear but the rotation axis is offset. 

 https://en.wikipedia.org/wiki/Variable_capacitor

I guess they still find and sell new old stock

 https://www.ebay.ca/itm/392248490930
 https://www.ebay.ca/itm/263233681710

They can ask whatever they want for them but there no way I consider them to be worth more than 10 USD.
« Last Edit: March 19, 2019, 06:19:53 pm by soldar »
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Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #3 on: March 19, 2019, 06:24:49 pm »
Does the fact that the product was probably made in China account for the lower price?

I'm not sure. They seem to be about 10% of the price of the EU/USA versions, which seems rather low even by Chinese manufacturing standards.

Still, I think I'll take a punt and see what they're like - after all, no one lies on their deathbed wishing that they bought fewer Chinese-manufactured air gap capacitors, do they? I'm gonna live life to the max!
 

Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #4 on: March 19, 2019, 06:28:45 pm »
Wow, those used to be so common fifty years ago but I am surprised they are even made any more. They used to make two working together in tandem, one would control the antenna tuning circuit and the other would control the local oscillator. They were extremely common and extremely cheap.

Don't I know it - I had two or three hanging around at home when I was a teenager, from disassembled valve radios, but that was about 40 years ago. I guess I dumped them at some point. Si la jeunesse savait, eh?
 

Offline james_s

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Re: Air gap variable capacitors - buy cheap?
« Reply #5 on: March 19, 2019, 06:33:36 pm »
They're very rarely used in modern production gear so in a way I'm surprised you can still get them at all. Fortunately they're relatively simple to make, it's not one of those technologies where layer upon layer of other supporting technology has to exist to produce them.
 

Offline rsjsouza

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Re: Air gap variable capacitors - buy cheap?
« Reply #6 on: March 20, 2019, 01:53:47 pm »
One thing I noticed about these is the price goes higher as the number of sections increases. Two or three sections used to be quite common in super heterodyne AM radios, but with the advent of FM they have four sections: two large 360pF for AM and two 20pF for FM.

As others have said, such parts have long gone out of fashion in favour of newer technologies (initially varicaps, then PLLs).

There's also a lot of junk being sold under the name "vintage", or "antique".

Sometimes it is cheaper to buy an antique broken radio and get the capacitor from it.
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Offline Jwillis

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Re: Air gap variable capacitors - buy cheap?
« Reply #7 on: March 21, 2019, 08:12:13 pm »
 

Offline coppercone2

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Re: Air gap variable capacitors - buy cheap?
« Reply #8 on: March 21, 2019, 08:20:56 pm »
that is the kind of stuff you take out of old televisions.
 

Offline bd139

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Re: Air gap variable capacitors - buy cheap?
« Reply #9 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/

I usually buy a few and sell them on eBay to pay the fuel bill for the journey to the rally.

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

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.
 

Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #10 on: March 21, 2019, 08:43:32 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/

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.

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.

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.
 

Offline bd139

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Re: Air gap variable capacitors - buy cheap?
« Reply #11 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.

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.
 

Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #12 on: March 21, 2019, 08:58:42 pm »
Microphonic. Basically they are sound / vibration sensitive. If you knock the table you can hear a bit of frequency modulation sometimes.

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.

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.

Sure. Anything specific that I should check?
 

Offline rhb

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Re: Air gap variable capacitors - buy cheap?
« Reply #13 on: March 22, 2019, 12:18:29 am »
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.
« Last Edit: March 22, 2019, 12:23:36 am by rhb »
 

Offline james_s

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Re: Air gap variable capacitors - buy cheap?
« Reply #14 on: March 22, 2019, 12:22:48 am »
Why not just buy a cheap variable capacitor from eBay? They're not that hard to find yet and often not very expensive.
 

Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #15 on: March 22, 2019, 10:23:01 am »
Visit a few thrift stores and buy an old radio.  The older the better taking into account condition.

I can't think of anywhere local to me that would have something suitable - I don't live in a big city and the only "old radios" that I've seen were in antique shops at antique shop prices. Maybe it's easier to find such stuff in the States. Back in the day I would have gone to a charity shop for one, but these days in the UK they've gone upmarket and expensive.

Quote
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.
Thanks but I suspect freight charges will make that unfeasible - anyway I have one on order, and am going to start playing around with the varactor approach too, so I'm good for now.
 

Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #16 on: March 22, 2019, 10:30:52 am »
Why not just buy a cheap variable capacitor from eBay? They're not that hard to find yet and often not very expensive.
I already have one - it was quoted as having a max. capacitance of 125 pF, but I'm finding that

a) it's more like 25 pF (maybe I was ripped off)
b) I seem to get very poor Q at the limits of its range for some reason.

So I want to try another type to see if I get better results - not yet sure how much my problems are due to my cluelessness or due to a dodgy cap.
 


Offline CJay

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Re: Air gap variable capacitors - buy cheap?
« Reply #18 on: March 22, 2019, 12:48:41 pm »
I'm looking for a variable capacitor suitable for tuning over the AM frequency band, and this:

https://www.ebay.co.uk/itm/Single-Joint-Air-Dielectric-Variable-Capacitor-12PF-to-365pf-250M-35-29-26mm-/192610325869

at about 9 quid looks suitable. However, having looked around, I've seen similar caps priced at £80 quid or more, or similar in dollars. So I'm wondering what I'd actually be getting for the £9 price - cheap junk or something usable?

Does anyone have any knowledge of these?
Why specifically does it have to be air variable and not a polyvaricon?

When you say 'tuning' is it for transmitting or receiving?

The cheap polyvaricon tuning caps are good for receive and, surprisingly, transmit too but only to a couple of watts, they're also considerably cheaper than the air spaced ones (£2 ish)
 

Offline soldar

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Re: Air gap variable capacitors - buy cheap?
« Reply #19 on: March 22, 2019, 04:24:21 pm »
Dismantling an old radio set just for the variable capacitor is bordering on criminally stupid because the old radio is worth a lot more if it is not missing parts.

A lot of those components suffered from microphonism, not only variable capacitors but also vacuum tubes. Many were improved over the years to reduce the effect. Variable caps had thick plates so they would not vibrate.
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Offline aneevuserTopic starter

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Re: Air gap variable capacitors - buy cheap?
« Reply #20 on: March 22, 2019, 04:40:12 pm »

Why specifically does it have to be air variable and not a polyvaricon?

I've got a couple of those and as mentioned up-thread, I'm not getting great results, so I want to try something different. Anyway, I've already ordered one of the air caps, so I'll have something to compare them to, at some point.

Quote
When you say 'tuning' is it for transmitting or receiving?
receiving
 

Offline james_s

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Re: Air gap variable capacitors - buy cheap?
« Reply #21 on: March 22, 2019, 06:06:42 pm »
Dismantling an old radio set just for the variable capacitor is bordering on criminally stupid because the old radio is worth a lot more if it is not missing parts.

A lot of those components suffered from microphonism, not only variable capacitors but also vacuum tubes. Many were improved over the years to reduce the effect. Variable caps had thick plates so they would not vibrate.

Certainly if it's a true vintage radio I would agree. There are lots of cheap junky radios from the 80s and 90s that are not worth anything, but I wouldn't get one just for the capacitor unless I already had one laying around.
 

Offline rhb

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Re: Air gap variable capacitors - buy cheap?
« Reply #22 on: March 22, 2019, 06:50:23 pm »
A late 70's table radio has a lot of useful parts.  And a sharp eye will get you more than you have room to store as they get thrown away.  Thrift stores test donations and stuff that doesn't work gets pitched.

The $20 LCR/transistor testers make them even more valuable as a parts source than when I was 20 something.
 

Offline CatalinaWOW

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Re: Air gap variable capacitors - buy cheap?
« Reply #23 on: March 22, 2019, 07:05:41 pm »
A late 70's table radio has a lot of useful parts.  And a sharp eye will get you more than you have room to store as they get thrown away.  Thrift stores test donations and stuff that doesn't work gets pitched.

The $20 LCR/transistor testers make them even more valuable as a parts source than when I was 20 something.

But if looking for the air variable cap you need to go a few years earlier.  The air caps started disappearing at the end of the 60s.  The more compact plastic separated ones lasted a few years longer, but by the late 70s I think varactors had completely taken over.

 

Offline james_s

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Re: Air gap variable capacitors - buy cheap?
« Reply #24 on: March 22, 2019, 11:08:59 pm »
A late 70's table radio has a lot of useful parts.  And a sharp eye will get you more than you have room to store as they get thrown away.  Thrift stores test donations and stuff that doesn't work gets pitched.

The $20 LCR/transistor testers make them even more valuable as a parts source than when I was 20 something.

The fact that they pitch anything that doesn't work has always bothered me. The non-working stuff is what I would be most interested in purchasing so long as it is priced accordingly. Quite a lot of stuff has simple faults that are an easy one afternoon project.
 


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