General Radio 1403A 1000pf capacitor adjustment - Page 1
Forum section: electronics / general

author: charliedelta
author_rank: Regular Contributor
timestamp: May 18, 2021, 10:25:23 am
content: I have a General Radio 1403A 1000pf capacitor. Its supposed to be a 1000pf capacitor with 0.1 % tollerance. When i measure this capacitor with several accurate LCR meters I am getting values readings of around 750pf.I cant believe that such a well constructed capacitor can be so badly off. There appears to be no easy way of adjusting this capacitor.I seem to think I am misunderstanding how this capacitor was calibrated or measured. Can anyone shed some light onto why this capacitor is so badly off?There is no signs of physical damage to the plates.Thanks

author: antintedo
author_rank: Contributor
timestamp: May 18, 2021, 11:55:28 am
content: What is the DC resistance between terminals? There might be whiskers shorting some of the plates.

author: Stray Electron
author_rank: Super Contributor
timestamp: May 18, 2021, 12:51:54 pm
content: I might be wrong, but it looks like there is white corrosion on the plates and around the top where it joins the cylindrical shell. And it looks like there are dents and dings in the plates!!

author: Stray Electron
author_rank: Super Contributor
timestamp: May 18, 2021, 01:03:14 pm
content: The connectors and the plates are simply stacked on metal rods and are held in place with nuts. Have you checked to see if all of the nuts are still tight? You probably need to check the torque on them with a torque wrench instead of just tightening them with a pair of pliers. But I have no idea what the torque requirement is. But you could probably find a recommended torque value for that thread size, pitch and material in Machinery's Handbook or other reference.To me, it looks like there are clear signs that the inside of that unit has been exposed to moisture. If so I suspect that there's some dissimilar metal corrosion between the aluminium plates and the stainless steel shafts and nuts and I expect that that's what is altering your readings.

author: Stray Electron
author_rank: Super Contributor
timestamp: May 18, 2021, 01:09:11 pm
content: Did you already take one of the plate supporting rods out of the capacitor assembly? it looks like there's one missing. There is a empty hole shown near the top OF THE PHOTO in the bottom photo.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 18, 2021, 01:38:12 pm
content: It's hard to believe the connections would be so bad as to disconnect a plate. It's a solid screw system. Was it ever right? Did somebody repurpose it in the past by removing plates to get a new value? I think that's a trimmer screw on top, but it won't find you that much extra capacitance! If you have some decent vernier calipers you could measure the area of the plates and the spacing and maybe determine if the number of plates is grossly off. Did you get it cheap? That might be a clue! I assume you're measuring from center pin to center pin? I think you can also short one side out and measure across the other connector, but it will raise the value a bit.

author: chuckb
author_rank: Frequent Contributor
timestamp: May 18, 2021, 10:50:24 pm
content: Like someone else mentioned, the capacitor has been modified. It is missing a large number of plates. See attached Pix.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 18, 2021, 10:56:00 pm
content: chuckb was one minute faster4 plates are missing so it looks like someone needed a custom value.You can still make use of it by converting it into 100pF DIY standard.The plates and standoffs are made most likely of aluminum so don't worry about the whiskers.They were a problem in tin plated can of GR1404 nitrogen filled standards.Here is mine:

author: charliedelta
author_rank: Regular Contributor
timestamp: May 19, 2021, 09:01:28 am
content: Thanks all for the replies.1. DC resistance, its open circuit. I tested it with the Fluke insulation tester.2. DINGS, dents and corrorosion. There was a anti oxident goop that and it was corroded in some places. The dings and dents were causes by me trying to open it to break the badly corroded aluminium.3.There may well be corrosion. I will dissassemble it completely after first soaking it in isopropyl alcohol. Maybe thats what will fix it.4.It was a surplus acquisition that was marked as faulty from the lab. But the fault was that the connector wires were broken from spinning connectors. It may well be that someone has removed plates to optimise the value for a specific application.5. I have not dissambled or removed any plates, its as is received. The original owner may have modified it.6. Thanks for the pictures. Mystery solved, the unit has been butchered. Anyway I got a 10pf in the deal for 10 dollars so I have some value left in the deal. Thanks all for the help.Its really sad when people butcher such excellent engineering. Nothing worst than ignorant butchers or tinkerers who think that they can reinvent the calibration wheel.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 19, 2021, 03:35:02 pm
content: Quote from: charliedelta on May 19, 2021, 09:01:28 amLook at it this way. If it doesn't do what you need but you can make it work for what you do need. Modify. I don't think it's a collectors item.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 19, 2021, 04:51:13 pm
content: There was a parts unit on eBay, maybe create a single correct one out of two!

author: Henrik_V
author_rank: Regular Contributor
timestamp: May 26, 2021, 10:44:23 am
content: C-D : I have a opened and ruined 1404 ... but shurely enough parts (plates, screws, spacer) to bring yours back to value... drop me note , would be shipping costs from Germany:)Then ask Conrad to hook it up to his 1615

author: manganin
author_rank: Regular Contributor
timestamp: May 27, 2021, 12:20:45 pm
content: Quote from: Conrad Hoffman on May 18, 2021, 01:38:12 pmProbably requires a long storage in bad conditions, but I have seen that more than once with the 1403 fraction of a picofarad types. In those a small 3-terminal capacitance is created by an aperture in the guard plate between the electrodes.Aluminium oxide is a good insulator.

author: charliedelta
author_rank: Regular Contributor
timestamp: May 29, 2021, 09:02:40 am
content: Hi HenrikI would be interested in your parts unit. I have sent you a PMThanks

author: Henrik_V
author_rank: Regular Contributor
timestamp: June 14, 2021, 04:35:31 pm
content: CD here is what you hopefully get...Maybe check the torque while dismounting and use a torque wrench to mount ....

author: Labrat101
author_rank: Unknown
timestamp: June 14, 2021, 10:23:02 pm
content: Hi I would not try opening the nuts they are torque set .If its that far out I would wash it very care fully in IPA . even better an ultrasonic bath .If dirt dust has got between the plate it will have become static charged to the surfaces .so while cleaning it make sure the out terminals are shorted .If you have loosened any of the clamping nuts . Find one that you have not opened and with astainless steel torque key measure the release pressure and the reset all the lock nuts to the same.when these are assembled they put shims between the plates so they are all parallel to each.That Last picture looks like it has been knocked and bent .I would get hold of some Plastic feeler gauges and measure the good plate distances.These unit are also dependent on air . so altitude is also part of the calibration & temp.

author: Henrik_V
author_rank: Regular Contributor
timestamp: June 15, 2021, 07:07:02 am
content: The one pictured is one of a batch made by Quadtech and cans only, most of them failed due to wisker and in some you hear something is jingling inside while shaking. I harvested them out of the dumpster.We opened some cans (the 1404 cans are sealed and filled with nitrogene to avoid the influence of humidity) hot air didin't do the job, we used a torch(and vented the cans first)... the one showed dropped on the floorsome of the plates are bended. However still enough parts for CDs cap.Maybe one day we rebuild some without solder plating in the housing and polish the plates (the genuine GR ones definitly look better than the successors)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 15, 2021, 01:50:55 pm
content: I thought the 1404 plates were invar, but not the 1403. Have to check that.

author: Labrat101
author_rank: Unknown
timestamp: June 15, 2021, 01:56:33 pm
content: I am glade you rescued them from near death . ..There are a few rules when working with this type of aluminium . Most people don't realises thataluminium oxidizes in air .Oxygen just scratching aluminium under a high power microscope thesurface of the scratch looks like its alive and the oxidisation starts happening in a few seconds.aluminium oxide is not a true insulator more like a semi conductor and the particles are very small andare always particle charged . Under clean room involvement they can cause a lot of problems in the hyperfilter as the static charge created by the dust can under certain frequencies produce 10kv plus.so the the plates should only be handled with cotton gloves as not to add oils or other contaminates tothe anodized surface . aluminium is the metal form of Ruby and we all know when ruby is excited we getlight Lazar .So polishing the plates will cause more harm . clean them with ultra pure DI water . Or IPA .And when reassembling at home I use to do it inside a large plastic bag which helps to keep dust andmoisture limited . (heavy breathing & coffee )One of those units looked very nice probably the one that was nitrogen filled The pressure is normally1 bar (14.7 psi) which will give the calibration @ Sea level and wont be influenced by outside pressurechanges .I'm sure you will get these back within spec with a Lot of Love & tender Care .

author: Henrik_V
author_rank: Regular Contributor
timestamp: June 15, 2021, 03:43:28 pm
content: Quote from: Conrad Hoffman on June 15, 2021, 01:50:55 pmDon't know for the 1403, but the 1404 and 1406 are invar.I don't think there is any aluminium in the 1404 ... haven't found someOur plan is to polish the invar plates and maybe have them (and the bolds,spacers...) gold plated, more important is the removal of the tin/solder plating of the brass(?) housing parts to prevent whisker. Maybe will be tricky for/around the throug-hole plating ... But a very good workshop and galvanic is inhousealso vacuum/ Nx refill / clean room/ temper all there... but time and motivation ....since we have nice&stable working 1404 .I attached a picture of one of those and marked a wisker... probably posted before ..

author: Labrat101
author_rank: Unknown
timestamp: June 15, 2021, 04:22:04 pm
content: Invar is magnetic check with a small magnet

author: Labrat101
author_rank: Unknown
timestamp: June 15, 2021, 04:49:14 pm
content: Quote from: Henrik_V on June 15, 2021, 03:43:28 pmInteresting idea about Gold plating . I would check that this is feesable as in the use of 2 Dissimilar MetalsI think that the extra cost of gold plating during manufacture would not increase the cost of these units very muchI guess that this process is not good when it comes to high tolerance caps . I would check on what the overall effects are.I have never seen gold used in this way .. except in Aerospace and that's for a different reasons.

author: Henrik_V
author_rank: Regular Contributor
timestamp: June 16, 2021, 09:24:11 am
content: Quote from: Labrat101 on June 15, 2021, 04:49:14 pmI expect that one or more additional layers of some other elements will be needed, but that up to our galvanic experts

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 16, 2021, 08:09:08 pm
content: I used to work for a place that made optical equipment out of invar and had the parts plated. Only plating shops that really know what they're doing are successful and I still don't trust the stuff. Even with a good flash of copper or whatever else they use, adhesion over time can be tricky. I've seen a lot of lifted and peeling plating. Invar has so much nickel that it should be pretty good if the environment is good. Even lying around in the uncontrolled shop, the stuff would slowly develop a haze, but not really corrode. I suspect a polished surface, something we never did, would be resistant to oxidation.

author: Labrat101
author_rank: Unknown
timestamp: June 16, 2021, 10:50:37 pm
content: Quote from: Conrad Hoffman on June 16, 2021, 08:09:08 pmYou right what you say also Invar & Gold is in the wrong place on the periodic tablewhich is why copper or any plating will peal as the dielectrics of Invar has a very good temperature stability and copper or silver are the opposite and will peal off.separate due to the chrome content .Invar as you said has a very resilient surface and pure chrome is almost as tough as titanium .