Unknown reference variable capacitor internals 15-1100pF - Page 1
Forum section: electronics / general

author: trobbins
author_rank: Frequent Contributor
timestamp: February 23, 2023, 11:50:51 pm
content: I was given the internals of what I expect was a lab grade variable capacitor (given the construction style). It is hand marked "15-1100pF" and I measure the min and max levels at 11 to 1218pF at 1kHz as is. There are no other markings, and it is physically 150mm long with a 90mm diameter. It does need a clean as there is some fine dust showing, but otherwise looks to be in good working order. Any thoughts on who the manufacturer was would be appreciated, as I couldn't find any teardown/inspection threads with relevant internal photos.Ciao, Tim

author: JohnG
author_rank: Frequent Contributor
timestamp: February 24, 2023, 02:04:19 pm
content: Maybe GenRad (General Radio)? Just a guess.John

author: TimFox
author_rank: Super Contributor
timestamp: February 24, 2023, 05:11:55 pm
content: My guess would be the guts of a G-R 722-D precision variable capacitor.https://www.radiomuseum.org/r/gener_raco_variable_capacitor_722_dq.html

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 24, 2023, 05:25:48 pm
content: It doesn't look like any GR thing I've ever taken apart. It also doesn't have certain GR construction features. The value is certainly similar to things GR built. Could it be a tuning cap from some oscillator or filter, GR or otherwise?

author: Whitefoot
author_rank: Contributor
timestamp: February 25, 2023, 01:16:52 am
content: It doesn't look anything like the guts of a GR 722-D, which is built like a tank with a cast frame.

author: coppercone2
author_rank: Super Contributor
timestamp: February 25, 2023, 01:22:37 am
content: Yeah I have that genrad its not it. The feed through is interesting, and so is the red marking compound or whatever the hell it is.I have seen very similar feed through in a russian military variometer w/ similar ink, feed through and ceramic shaft, but the rest of the construction looks different and does not have soviet engineering characteristics .What is the deal with those plates on top and bottom. The pattern. I can't figure out what that is.Can you show me a picture of the shaft bushing, i.e. the method by which the ceramic shaft is attached to the rest of it?Are those spheres glass or plastic?Maybe its just a gut feeling but could that thing be east german?

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2023, 03:21:26 am
content: I appreciate the comments - thanks.The red spots are on the wire-wrapped external terminations and appear to be a QC marking. Similarly the locking nuts on the bottom plate have a grey daub marking (ie. a torque mark).The spherical standoffs are glass.The top bearing incorporates a ball bearing, and the bottom bearing appears to use a metal end-cap to the ceramic shaft with a small diameter shaft that fits in to an open metal bearing in a spring steel thin plate.

author: coppercone2
author_rank: Super Contributor
timestamp: February 25, 2023, 03:26:40 am
content: I think I read about glass standoffs some where before, perhaps in reference to a capacitor. I know they had some kind of connector that had a glass bead also IIRC (like a development that lead to APC-7 connector and such). Somewhere on the forum I think, or a news group, or other forum.

author: coppercone2
author_rank: Super Contributor
timestamp: February 25, 2023, 03:28:57 am
content: BTW wtf is that pattern on the plates? I still cant tell what it is. Cosmetic? Indexing marks? Some kind of machine marks?Kinda seems like something a specific company might be put on their plates to make them cosmetically appealing. Is it something so the plate do not stick when stored (i.e. if you put two plates with grease on them they get really stuck like glue if they are flat).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 25, 2023, 03:34:09 am
content: I've seen that pattern before on stampings, not uncommon though I've no idea where you buy the stuff. The glass might be fused quartz. Gently poke it with a sharp X-Acto knife- it might also be polystyrene.

author: coppercone2
author_rank: Super Contributor
timestamp: February 25, 2023, 03:34:57 am
content: does the bearing have a collet that clamps on the ceramic rod? Still Cant tell. Looks glued on.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2023, 04:37:37 am
content: The sphere's certainly seem to be glass, from the clarity and lensing of light, and a scalpel blade skating on the surface .The patternation is from impressed square indents with a regular square grid spacing (abt. 1/14" or 1.8mm spacing).The bottom dry bearing is part of a metal cup that is glued to the end of the ceramic rod. The top roller bearing has an inner metal collet that is fitted on to and clamped to the ceramic rod.

author: coppercone2
author_rank: Super Contributor
timestamp: February 25, 2023, 04:47:33 am
content: the picture with the sharpie mark has a missing indent on the plate. must be the signature of the manufacturer. And the screw I think are similar to genrad, because they seem familiar, I liked the screws that genrad picked for their units

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2023, 05:07:50 am
content: Yes there are a few missing indents, and some mal-formed indents on that and the other 3 surfaces - I'd guess due to wear on the roller or whatever was used to impress the indents.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 25, 2023, 02:11:37 pm
content: Quote from: coppercone2 on February 25, 2023, 03:28:57 amthe pattern of indentations is to dissipate stress in the metal sheet after stamping, in order to restore flatness. it is common, for example, in the brass plates of an old-fashioned wind-up mechanical alarm clock. i've had a dig around online to find out a good reference, but without much luck. the best i could find was this one:https://www.thefabricator.com/stampingjournal/article/stamping/regaining-flatness-in-stamped-parts...until i found this:https://www.practicalmachinist.com/forum/threads/sheet-metal-straightening-flattening-plate-die.305994/"The device that was used in production of precision instruments like clocks, counters etc was a flatting plate. This is a hardened and ground steel plate with a pattern of miniature pyramids. Once pressed into the sheet metal, the sheet metal surface undergoes a controlled plastic deformation and comes completely flat and work hardened with a fine pattern of dots on one surface."cheers,rob :-)

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2023, 11:16:03 pm
content: Thanks Rob - very interesting to read that practicalmachinist thread, and also one of its links: "Stippling the blank–a process that addresses internal stress after it has already been created–involves coining a crosshatched or alternate pattern into one or both surfaces of the part after all cutting–and extensive metal deformation–is done. Trapped stress is the result of permanent metal deformation, and it is the fundamental reason for a loss of flatness." So it seems that seeing such a pattern on main structural parts is a sign of top level manufacture expected within a reference part.