Air gap variable capacitors - buy cheap? - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 19, 2019, 02:16:40 pm
content: 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-/192610325869at 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?

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 19, 2019, 04:46:06 pm
content: Does the fact that the product was probably made in China account for the lower price?

author: soldar
author_rank: Super Contributor
timestamp: March 19, 2019, 06:17:37 pm
content: 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_capacitorI guess they still find and sell new old stockhttps://www.ebay.ca/itm/392248490930https://www.ebay.ca/itm/263233681710They can ask whatever they want for them but there no way I consider them to be worth more than 10 USD.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 19, 2019, 06:24:49 pm
content: Quote from: bsfeechannel on March 19, 2019, 04:46:06 pmI'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 boughtfewerChinese-manufactured air gap capacitors, do they? I'm gonna live life to the max!

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 19, 2019, 06:28:45 pm
content: Quote from: soldar on March 19, 2019, 06:17:37 pmDon'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?

author: james_s
author_rank: Super Contributor
timestamp: March 19, 2019, 06:33:36 pm
content: 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.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 20, 2019, 01:53:47 pm
content: 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.

author: Jwillis
author_rank: Super Contributor
timestamp: March 21, 2019, 08:12:13 pm
content: I recently bought these and they turned out to be very well built.https://www.ebay.ca/itm/ALPS-Air-Variable-Capacitor-for-FM-AM-SW-Crystal-Radio-Antenna-Tuner-Ham-Radio/143064320855?

author: coppercone2
author_rank: Super Contributor
timestamp: March 21, 2019, 08:20:56 pm
content: that is the kind of stuff you take out of old televisions.

author: bd139
author_rank: Super Contributor
timestamp: March 21, 2019, 08:22:27 pm
content: 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.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 21, 2019, 08:43:32 pm
content: Quote from: bd139 on March 21, 2019, 08:22:27 pmNot really the bag I'm into, but maybe something I'll consider if I can find one near enough. Thanks.QuoteMicrophonic?QuoteThat'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.

author: bd139
author_rank: Super Contributor
timestamp: March 21, 2019, 08:47:06 pm
content: 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.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 21, 2019, 08:58:42 pm
content: Quote from: bd139 on March 21, 2019, 08:47:06 pmI see. I can live with that though, as I'm only a pseudo-noob playing around.QuoteI 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.QuoteSure. Anything specific that I should check?

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2019, 12:18:29 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: March 22, 2019, 12:22:48 am
content: Why not just buy a cheap variable capacitor from eBay? They're not that hard to find yet and often not very expensive.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 22, 2019, 10:23:01 am
content: Quote from: rhb on March 22, 2019, 12:18:29 amI 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.QuoteThanks 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.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 22, 2019, 10:30:52 am
content: Quote from: james_s on March 22, 2019, 12:22:48 amI already have one - it was quoted as having a max. capacitance of 125 pF, but I'm finding thata) 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.

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2019, 12:40:45 pm
content: Are these too expensive?https://www.ebay.co.uk/itm/ALPS-Air-Variable-Capacitor-for-FM-AM-SW-Crystal-Radio-Antenna-Tuner-Ham-Radio/132962614313https://www.ebay.co.uk/itm/ALPS-Variable-Capacitor-20pf-320pf-FM-AM-Crystal-Radio-Antenna-Tuner-Balun/132992365516

author: CJay
author_rank: Super Contributor
timestamp: March 22, 2019, 12:48:41 pm
content: Quote from: aneevuser on March 19, 2019, 02:16:40 pmWhy 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)

author: soldar
author_rank: Super Contributor
timestamp: March 22, 2019, 04:24:21 pm
content: 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.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 22, 2019, 04:40:12 pm
content: Quote from: CJay on March 22, 2019, 12:48:41 pmI'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.Quotereceiving

author: james_s
author_rank: Super Contributor
timestamp: March 22, 2019, 06:06:42 pm
content: Quote from: soldar on March 22, 2019, 04:24:21 pmCertainly 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.

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2019, 06:50:23 pm
content: 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.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 22, 2019, 07:05:41 pm
content: Quote from: rhb on March 22, 2019, 06:50:23 pmBut 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.

author: james_s
author_rank: Super Contributor
timestamp: March 22, 2019, 11:08:59 pm
content: Quote from: rhb on March 22, 2019, 06:50:23 pmThe 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.

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2019, 11:37:46 pm
content: Bothers the hell out of me too. But the cold reality of it is, there is too much stuff to test. And I can't find anyone around here who builds anything. I went to the ham meetings for a while. There was no discussion of radio or electronics. So I quit going. The public library does not have a *single* book on electronics or any other technical subject.I'm trying to organize parting out stuff for sale on eBay. Microwave oven turntables, capacitors, diodes, etc. We're getting perhaps 10% of the e-waste stream. All the rest goes into a landfill. It's heart breaking, but beyond anyone's control.I trained as an economic (mining) geologist. When I look at a small piece of copper wire I see a large hole in the ground and a barrel of diesel fuel. The staggering waste of resources has doomed future generations to living in the stone age. I'll be dead long before that happens, but it will happen.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 23, 2019, 12:10:03 am
content: Actually, landfills will be far richer ore than much that is mined today. And given the more desperate economic circumstances folks will be far less concerned about a 5% increase in cancer risk.

author: bd139
author_rank: Super Contributor
timestamp: March 23, 2019, 12:23:39 am
content: Just remember if such a thing happens the only people worth anything will be the people who can repair and turn trash into something usable. And that’s us.

author: rhb
author_rank: Super Contributor
timestamp: March 23, 2019, 02:01:29 am
content: I suggest reading the work of Jared Diamond especially his other books besides, "Guns, Germs and Steel".When sea level rose at the end of the Pleistocene as the massive, mile thick glaciers in the northern hemisphere melted it isolated the hunter gather groups on the Chatham Islands, Because these were groups of only a dozen or two individuals, the pressure to forage for food made it impossible to feed someone whose function was technological. So they lost knowledge of sewing, fish hooks and other very basic technology..Ore tenors are very low today, but I doubt the assertion that a landfill is higher grade. But doubtless there will be those who prospect for a buried washing machine with the same fervor that others have sought gold.Basic technology developed in Mesopotamia because there were readily exploitable deposits of copper, tin and iron. And there was an agrarian society which was capable of producing more food per agricultural worker than was needed to sustain them.Those of us who understand technology and can repair things will be very valuable. But our numbers will decrease because there will be few who have the free time to learn the skills.I spent several months reading all the books on the history of technology in my library. My initial intent was to write an essay in response to the "Doomsday Prepper" crowd who are hopelessly naive. After several months of study, I abandoned the project. There is no solution. The folly of human society over the last 100 years or so has cast the fate of the species in stone. At some point in time, for reasons no one can predict, over 90% of the species will die. It is an unavoidable consequence of globalization. The most likely cause will be a disease such as influenza disrupting the delivery of food. But any number of geological events such as a caldera collapse event at Yellowstone, an asteroid impact or a basalt flood like the Deccan Traps would also serve.One of the great ironies is that those who are at the bottom of human society barely subsisting, will be the majority of the survivors.The exhaustion of readily accessible resources will then lead to a slow decline into neolithic levels over a period of several hundred years. Most of the scrap will have been reduced to discolored spots on the ground after 200-300 years.In the end, the human race will join the uncountable millions of other extinct species who have populated the earth. The conceit that humans are special is born of hubris and will be our undoing. We are, without question, the most advanced and powerful animals that have ever existed on the earth. But the unavoidable consequence of being born is death for an individual and extinction for a species.That which created us and all those who went before us will also destroy us and replace us with something else. This has demonstrably been the way of things for several billion years.It makes me a little sad, but it also makes me savor the magic we have created over the last 400 years. It took over 200 years for the calculus to become useful in engineering. Even as late as 1900, calculus had very little to contribute to the construction of a locomotive at the Baldwin Works. But gradually, gentlemen amusing themselves at university produced a sufficient body of science and mathematics, that engineers could use it in design instead of relying on trial and error.Once that happened, engineering took off like a rocket. Commercial demands created by engineers led to more science and mathematics and we are now able to solve mathematical problems I was taught could not be solved when I was in school.

author: james_s
author_rank: Super Contributor
timestamp: March 23, 2019, 03:57:30 am
content: Quote from: rhb on March 22, 2019, 11:37:46 pmWhy not just do what the Goodwill store does around here and have an outlet center with all the as-is stuff in bins priced by the pound? Or have a section for as-is goods with known faults priced accordingly? If you get $1 for something that would cost money to have hauled to a landfill you come out ahead. I learned to repair things by tinkering with old junk in the pre-internet era, there is far more information available now than there ever was in the past. Offering broken stuff for parts or repair at a low price is one way of encouraging people to give it a try.I rarely even bother with thrift stores around where I live anymore but thankfully there are still some economically depressed areas a little further out where they still have interesting old junk to dig through.

author: rhb
author_rank: Super Contributor
timestamp: March 23, 2019, 12:31:41 pm
content: The issue is space. The thrift has gotten a new location, so we'll be able to do more. But the stuff accumulates at a staggering rate. The population is only 7500. So there is just not a lot of market for dead consumer electronics. In 11 years I've met one person who did component level work. He gave me a Motorola R2008C/HC in need of TLC when he moved away.I have observed that solder flux residue is a significant cause of failures in humid climates. I plan to make a isopropyl alcohol pressure washer using an electric fuel pump and a modified air brush with a redistiilation system to recycle the alcohol.I'm then going to collect a bunch of cosmetically good, non-functional stuff and do a formal experiment to see what fraction of the devices can be repaired by simply cleaning them. I've now repaired 8-10 devices just by cleaning off the flux residue with a spray bottle and toothbrush. The most spectacular example being a dead 34401A DMM from eBay.