EEVblog #461 - Genrad Decade Resistance Box Teardown  - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: April 23, 2013, 10:04:55 pm
content: Teardown TuesdayHow interesting can a box with switches and resistors be?Not very? How about a multi thousand dollar Genrad metrology grade decade resistance box?IET's video on the box:

author: ddavidebor
author_rank: Super Contributor
timestamp: April 23, 2013, 10:19:48 pm
content: I will clean all the old grease Befor use wd-40

author: krivx
author_rank: Frequent Contributor
timestamp: April 23, 2013, 11:52:00 pm
content: It's a pretty similar construction to the decade boxes I have, one of which also has milliohm resolution. I posted some pictures here:https://www.eevblog.com/forum/projects/h-w-sullivan-dual-dial-non-reactive-resistance-box-restoration/

author: JackOfVA
author_rank: Unknown
timestamp: April 24, 2013, 12:02:41 am
content: General Radio spun off part of its business to a new company, QuadTech in the early 1990's -- so QuadTech would have been responsible for new sales and repair of many older design GR products such as the decade switch boxes.IET then picked up QuadTech around the year 2000.That pins down the date of repair, something between 1991 and 2000 or so. (Probably parts were still marked QuadTech for some time after 2000.)

author: EEVblog
author_rank: Unknown
timestamp: April 24, 2013, 12:08:12 am
content: Quote from: JackOfVA on April 24, 2013, 12:02:41 amAh, that explains it, thanks.

author: Dave
author_rank: Super Contributor
timestamp: April 24, 2013, 01:10:59 am
content: "I love it when a plan comes together!"

author: sorin
author_rank: Frequent Contributor
timestamp: April 24, 2013, 02:21:42 am
content: Where is the Vishay link?

author: Unknown
author_rank: Unknown
timestamp: April 24, 2013, 02:41:11 am
content: I have a fleeting suspicion that the thick silver wires are in fact 40% silver brazing rods. Not exactly cheap but a lot more common than one might expect.

author: The_Penguin
author_rank: Regular Contributor
timestamp: April 24, 2013, 03:21:23 am
content: I don't think I heard you comment in the video about the soldering, can you tell if it is silver solder, or regular tin/lead?

author: ddavidebor
author_rank: Super Contributor
timestamp: April 24, 2013, 04:49:11 am
content: Well, all the soldering are giant blob, who cares of the tipe of the lead

author: Lightages
author_rank: Unknown
timestamp: April 24, 2013, 04:54:07 am
content: I would hazard to guess that the discolouring on those ribbon wires might have been from someone abusing that selection with a bit too much current. It would take quite a bit of current though so maybe it was a way to trim them by heating them.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 24, 2013, 04:54:18 am
content: heydave - pleas explain me something , can't someone like me make something like that ?not tallking to that sort of "quallity" . I mean , can't I buy simple fine quality switch ( link below)and wound my own resistors from nichrome ?nichrome is not that costy , let's say 1$ per resistor I make out of it . so assuming I have got precision reference to compare with ( I do because I might go to work on Cal lab ) so then trimming the resistors is just the right angel tongue and some fine tuning skills ,so overall say I'll invest 100+ $ . not that bad for say 0.01% or even better 0_0for the case I thought about nice plywood made box and perhaps just for fun I'll plate the internal with some shielding plates and connect them to "Guard" terminalso please tell me - is this nonsense or possible ? can you guide us and do one yourself be4 I'll manage ? ( I can't see when I'll be able to do that . perhaps within month or more)switch :http://www.ebay.com/itm/Rotary-Selector-Switches-4-Pole-11-Positions-4-Deck-220V-/390509100965?pt=AU_B_I_Electrical_Test_Equipment&hash=item5aec27ffa5*I thought about that switch due to the tade off between quality price and contact resistance , so 4 pole added together should provide relaibillity and pretty low contact resistance ( to the first decade or so )nichrome :http://www.ebay.com/itm/Nichrome-wire-32-Gauge-60-series-100-FT-0-29oz-Resistance-Resistor-AWG-/261119375395?pt=LH_DefaultDomain_0&hash=item3ccbed7823thank you in advance

author: amspire
author_rank: Super Contributor
timestamp: April 24, 2013, 07:19:17 am
content: Quote from: eevblogfan on April 24, 2013, 04:54:18 amSure, but there are big problems.1. Nichrome, like the one you referred to, changes by 2% going from 20C to 100C. That is much worse then many off-the-shelf resistors. Genrad probably used selected batches of magnanin or Evanohm wire to make those resistors.2. You have to get different gauges wires for different resistance ranges. You will probably have trouble finding wire thin enough to make the 10K resistors. The wire used in the decade box is coated while most of the cheap nichrome wire is uncoated so you cannot let adjacent turns touch. The formers Genrad use is ceramic, so they may even be able to anneal the wires to improve stability.3. It is very hard to get switches that are any good. The ones you have found are probably rated at something like 20 milliOhms resistance when new, and they don't have the self-cleaning property of the solid silver alloy switches. You can parallel the poles, so that switch you found is probably a good find. You might be able to get the contact resistance down to 5 milliohms. Whereas the switches in Dave's decade box tend to have a very consistent resistance, your $10 switch may have contact resistance that can vary each time you change the position, particularly at very low currents. You can probably make a good decade box of up to 0.01% accuracy in the upper ranges if you use commercial high stability resistors. However, you may be looking at a price of over $60 a decade for decent 1/2W precision low drift high stability resistors.

author: mikes
author_rank: Regular Contributor
timestamp: April 24, 2013, 08:06:04 am
content: You can buy Genrad 5-6 decade boxes off ebay for <$100. Even if a few resistors are burned and need replacing, the switches and box are better and cheaper than what you suggest.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 24, 2013, 08:18:22 am
content: heythe only one who goes to 100K is too costy ( on ebay) he wants 400$ and that's before the shipping cost to my country , so I guess Unless I'll begin working and having sallary - the right choice is the DIY one made out of nichrome or whatever sort of wierany way . it shouldn't cost in the order of 1000$ or more . that's stupide 0_0I rather buy fluke 87V rather then good decade boxPs , I don't worry about that silly 5mOhm , I don't need better then 0.01% 250PPM either , for 20$ a daced I have more then good enough . don't mind 10,000.035 ohm . that's not that big of a deal for me 0_0 ( 7 decads at 5 mOhm each rotary switch *when new* )Cheers

author: nitro2k01
author_rank: Frequent Contributor
timestamp: April 24, 2013, 09:30:59 am
content: With the risk of getting another video addressed at me, proving me wrong () why didn't you connect the guard terminal to the shielding of the decade resistance box? Maybe it would have the least significant digit of the measurement a little more stable at least...

author: EEVblog
author_rank: Unknown
timestamp: April 24, 2013, 09:37:29 am
content: Quote from: nitro2k01 on April 24, 2013, 09:30:59 amThere were a few things I could do there to improve the measurement, this was just a quick'n'dirty.I was thinking maybe another quick video on that to see how much difference it makes.

author: EEVblog
author_rank: Unknown
timestamp: April 24, 2013, 09:39:24 am
content: Quote from: eevblogfan on April 24, 2013, 08:18:22 amThe prices aren't aimed at you!They have sold 10's of thousands of these things, so plenty of demand for niche high end products like this.The average hobbyist just needs 1% decade box with Tricky Dick switches.

author: Fezder
author_rank: Regular Contributor
timestamp: April 24, 2013, 03:21:37 pm
content: woa, nice value for such box.....but, i understand.but, what are these used anyways? sorry, newer heard about usage of these....

author: c4757p
author_rank: Super Contributor
timestamp: April 24, 2013, 03:30:33 pm
content: Quote from: Fezder on April 24, 2013, 03:21:37 pmOne this good can be used for calibration of ohmmeters.

author: Fezder
author_rank: Regular Contributor
timestamp: April 24, 2013, 03:43:16 pm
content: Quote from: c4757p on April 24, 2013, 03:30:33 pmhmm, sounds tricky :S...

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 24, 2013, 06:39:23 pm
content: Quote from: EEVblog on April 24, 2013, 09:39:24 amBTW : dave , I thank you A-lot for making me remember that I don't have the need for that expand , thank you for safing me that precious money- HUGE thumb up

author: nicknails
author_rank: Regular Contributor
timestamp: April 25, 2013, 12:16:09 am
content: Very nice bit of kit! I love the switches! So simple, but effective and elegant.

author: Frangible
author_rank: Regular Contributor
timestamp: April 25, 2013, 03:08:14 am
content: Here are some pics of a much older model decade box also made by GR. It's had a really hard life (including a blown out section), so it's not much use as a standard. But for tweaking resistor values in old circuits, it's still a peach!

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 25, 2013, 03:24:55 am
content: Wow, Super Lube! It's CFIA rated for incidental food contact, unlike WD-40 or DeOxit...Love the stuff.

author: LaurenceW
author_rank: Frequent Contributor
timestamp: April 25, 2013, 08:40:24 am
content: Dave, I assume with all those wound wires that this box is very much intended for DC work? There must be significant inductance in there.Fantastic engineering on the switches, though.

author: mikes
author_rank: Regular Contributor
timestamp: April 25, 2013, 11:29:54 pm
content: Quote from: LaurenceW on April 25, 2013, 08:40:24 amThey're made to handle AC, through at least audio frequencies, up to the 100K decades. The decades up to 100 ohms used Ayrton-Perry windings. The higher ohm ones do better than regular circular wirewound resistors, because of the flat card windings.They publish specific impedance info in the manual.

author: BMac
author_rank: Contributor
timestamp: April 26, 2013, 11:29:56 am
content: Dave,Don't do it! Don't spray that beutiful piece of equipment with WD-40, Use WD-40 on your lawn tractor, but not that precision piece of equipment.I am out of town, right now but when I get back I'll send you a can of Deoxit.Cheers brother, love your blog!BMac.

author: BMac
author_rank: Contributor
timestamp: April 26, 2013, 11:35:01 am
content: Dave,After you spray it with WD-40 you wont be able to stand the smell either.BMac

author: WattSekunde
author_rank: Regular Contributor
timestamp: June 11, 2013, 07:37:22 am
content: Take a look at this old beautiful wheatstone bridge on eBay (I am not the seller an I don't know him).http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=251288222202

author: ftransform
author_rank: Frequent Contributor
timestamp: June 15, 2013, 12:22:55 am
content: i want to demolish the cult that is deoxit. what the hell makes this crap so special ?wd40 is used for nuclear missiles. fucking plutonium!

author: c4757p
author_rank: Super Contributor
timestamp: June 15, 2013, 12:35:37 am
content: Quote from: ftransform on June 15, 2013, 12:22:55 amThe price.QuoteNah, plutonium doesn't do a very good job cleaning contacts. Not that I would know...

author: ftransform
author_rank: Frequent Contributor
timestamp: June 15, 2013, 01:27:40 am
content: the KGB uses plutonium to "clean" contacts

author: robrenz
author_rank: Super Contributor
timestamp: June 15, 2013, 02:37:22 am
content: Quote from: ftransform on June 15, 2013, 12:22:55 amI am the cult leaderit is special because it works. If it is snake oil they have fooled me and Agilent and a host of other major industry leaders. I especialy like it in my espresso

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 12, 2015, 12:39:09 pm
content: I know this is quite an old thread, I would rather post in a thread which is related rather than there be hundreds of similar threads. I did try to find any other posts about this, but this was the one that came up.I have recently bought a Genrad 1433 -G (0.1 - 100K 7 decade) box very similar to Dave's. Some of the switches are quite stiff to the point that I feel they will break, some of them feel magnificent. I thought it might have needed a bit of movement to free up the switches, but even after actuating the stuck switches close to 100 times each, they still feel very stiff.The resistance is pretty darn accurate so I am not wanting to just jump in there with WD40 or anything, but I am worried about the stiff switches and am hoping someone can give me some advice on what to do.I notice (like on Dave's) there is some grease / lubricant on the contacts, if I clean the contacts, what should I put on there? The manual says to clean the contacts, but nothing about what sort of lubricant to put on after.Thanks in advance-Chris

author: Vgkid
author_rank: Super Contributor
timestamp: November 12, 2015, 11:11:26 pm
content: Have you opened up the box, and seeing if anything is binding in the switching mechanism. I doubt that there would be enough grease to foul that up.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 12, 2015, 11:41:36 pm
content: Yeah, opened the box up and the contacts didn't look too bad. I think it is in the shaft of the switch itself rather than the contacts being gunked up. If I'm going to spray a lubricant in there I don't want to mess anything up or risk getting it into the wrong places.Do you know how easy it is to disassemble the shafts? I see some Allan key grub screws around the top part that the ball bearing is against.I wouldn't mind getting the knobs off the box to give it a thorough clean to remove the dirt on the front panel that has built up over time. I'm guessing that if I get the shaft out, the know goes with it.

author: plesa
author_rank: Frequent Contributor
timestamp: November 13, 2015, 12:08:43 am
content: I suppose that your GenRad use the same switches like IET HARS series.For cleaning copper contact I used Deoxit D100 and for protection Kontakt Chemie 61, and for locking mechanism I used Kontakt Chemie WL to remove grease residues.In user manual they recomended WD40

author: dacman
author_rank: Frequent Contributor
timestamp: November 13, 2015, 12:24:44 am
content: I would spray LPS-1 down the shafts and on the mechanisms on the bottom. There are a couple of screws on top for each dial and I sometimes will loosen them up to see if the dials will then turn more freely (and try to line up the dials while tightening down the hardware), but the shafts almost always need lubricated if the dials are tough to turn.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 13, 2015, 01:40:14 am
content: That's giving me a bit more confidence. Do you know how the knobs come off? Spare parts switches don't look like there is anything holding the knob on, but I don't want to go and start prying things off and damaging the box.How is the best way to clean the contacts without getting gunk all over the resistors or even leaving gunk between the contacts. Would giving it a spray with IPA after the contact cleaner help it make things worse?Sorry for so many questions, I don't want to damage the box or make things worse, a bit of preventative maintenance now will have the box in good condition for many years.

author: Vgkid
author_rank: Super Contributor
timestamp: November 13, 2015, 01:53:45 am
content: Post up a bunch of pictures(make a thread in the repair section) i dont own a GR box. I want one though.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 13, 2015, 02:25:59 am
content: Will do. I'll link it in here when I do put one in the repair section so others can find it.

author: dacman
author_rank: Frequent Contributor
timestamp: November 13, 2015, 04:21:25 am
content: Pop off the gray cap on top. Under the gray cap is a setscrew holding the skirt assembly to a shaft. The shaft goes down through an approximately three inch long bore to the bottom of a Bakelite dial assembly and the shaft is about the same diameter as the bore (therefore there is lots of wall contact area for friction to occur). Solvents like IPA or contact cleaner will freeze up the dial and if used will need to be followed by a lubricant (I use LPS-1). I use LPS-1 on both the shaft and contacts. The only time I use DeoxIT or DeoxIT-Gold is if the unit fails residual resistance. As an alternative, loosen two setscrews on the bottom of the shaft and pull it out from the top, leaving the knob on top attached to the shaft (you may need to knock it out from the bottom).

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 13, 2015, 04:30:53 am
content: Awesome. The gray cap came off and all is revealed how to dismantle them. I wasn't sure if the skirt and knob came off as one. Thank you very much