Component substitution query - Page 1
Forum section: electronics / projects

author: nagz
author_rank: Contributor
timestamp: September 25, 2025, 06:17:46 am
content: Hi folks,I’m having a rough time finding these.Can anyone suggest a substitute that ticks the boxes?Thanks in advance

author: jwet
author_rank: Frequent Contributor
timestamp: September 25, 2025, 07:00:31 am
content: There is a company called RF Parts that sells these parts- they are pretty rare these days and are not cheap. Check them out at the link below. Their stock varies by what they can find- its all old stock or pulls (removed from equipment) but they are an honest and reliable vendor. Only one I know that sells these old compression caps.https://www.rfparts.com/capacitors/capacitors-certrimmer/capacitors-46series.htmlGood Luck

author: nagz
author_rank: Contributor
timestamp: September 25, 2025, 07:23:15 am
content: Yeah,I saw them.Unfortunately, the cost of shipping makes these a no go. I got quoted $35+ aud to ship one capacitor:(Keen to find a local source or a readily available alternative if possible. I haven’t had any luck matching specs yet, which is why I’ve turned to the brains trust

author: jwet
author_rank: Frequent Contributor
timestamp: September 25, 2025, 07:51:39 am
content: These things haven't been made in 60 years! Maybe Hamfest as they're called here- some old dude who hoarded them in this shop for a few decades.Good luck.

author: Whales
author_rank: Super Contributor
timestamp: September 25, 2025, 08:28:21 am
content: There I was confidently thinking I could find something. Nope, only values up to a few 10's of pF are easily available :|Is the construction of these complicated (stacked/interdigitated layers) or do they just use magic & thin materials?

author: nagz
author_rank: Contributor
timestamp: September 25, 2025, 09:12:44 am
content: i have no ideamight be made of unobtainium knowing my luck.i can't even ID a potential substitute

author: jwet
author_rank: Frequent Contributor
timestamp: September 25, 2025, 05:07:42 pm
content: They use mica as a dielectric. Its er (relative dielectric constant) can be over 6 to almost 9, vacuum is 1. C=er*e0*A/d. You can also make it very, very thin. You get a lot of C per unit volume. These little pads can go up to over 1000p. They were popular in tube stuff where you wanted to neutralize big tube inter-elelctrode capacitances. Mica is fragile and difficult to work with and has become a bit expensive- its a natural material that has to be mined and is not cheap like modern alternatives. Due to the diminishing need and hassle of working with it, its been mostly replaced by various plastics, etc. These meet most of the needs but can't produce the big C's.Modern trimmers are seldom over 50 pf or so. Is the application in restoring old gear or something new?. The only alternative that does almost the same thing are poly varicon type caps. These are a stacked rotating cap with plates and poly dielectric that can get to these capacitances. They're still made or can be harvested from old am transistor radios for a few.

author: nagz
author_rank: Contributor
timestamp: September 26, 2025, 05:53:18 am
content: I'm planning an amplifier build for HF radio.I'll look into the poly varicon type caps, thanks heaps

author: Terry Bites
author_rank: Super Contributor
timestamp: September 26, 2025, 11:34:26 am
content: https://www.surplussales.com/categories/2598-trimmer-capacitors-compression/

author: jwet
author_rank: Frequent Contributor
timestamp: September 26, 2025, 02:06:25 pm
content: The other way to get big variable C's is to use banks of little relays and a binary weighted pile of capacitors. If you look at ham antenna tuners, this is very common, they're tunable under software control. If you use mini latching relays, they can draw zero power. Look at the manual for Elecraft Mini Antenna Tuner, schematic is included in the manual- very clever and small. Good luck.

author: mawyatt
author_rank: Super Contributor
timestamp: September 26, 2025, 02:18:47 pm
content: Back around 2000 there was a company (can't remember name) that was founded (partly funded by IBM) to produce antenna tuning chips for Smartphones. These were based upon switchable capacitance for dynamic antenna tuning and fabricated by IBM in CMOS.Best

author: jwet
author_rank: Frequent Contributor
timestamp: September 26, 2025, 02:48:32 pm
content: Maxim did a part like this- Max1474 that could do 6 pF to 14 pF in 200 fF increments. Still sold. They were for key fob antenna tuners- a special problem that automotive guys had- the antennas were made on cheap FR-4 and varied a lot. Internally, they used 5 switches/caps and came in a flea sized SC-70 package. They had an SRF of 960 MHz, so short of modern cellular. There was uWave stuff made by a little company, the sexiest used MEMS relays internally- I think the app was military radar- phase shifters and tuners- exotica. All this this was pretty tiny capacitances though. Someone like TDK make diplexers and tuners etc for cell antenna matching- modern cell phones might operate at 5 frequencies- bitchy problem. This got fancy. Volumes were BIG.These big old mica trimmers filled a weird applications problem. Not only did you need a big C but it was highly variable. In most modern problems, you might have a big C but it doesn't vary so much. You can use a big fixed C in parallel with a small trimmer. Tubes had this unusual problem that depending on manufacturer, you could end up with wildly different terminal capacitances and needed not only a big C but a big fully variable C. HF antenna tuners as used in Ham gear have a similar issue because they cover a 10x+ or more range of relatively low frequencies

author: mawyatt
author_rank: Super Contributor
timestamp: September 26, 2025, 06:52:50 pm
content: Found the company, it's WiSpry.https://wispry.com/about/corporate-overview/Best