Krohn-Hite 522 DC calibrator. - Page 1
Forum section: electronics / testgear

author: grumpydoc
author_rank: Super Contributor
timestamp: March 01, 2015, 11:56:03 am
content: I'm not sure I count this as a "score" - it still cost me £94 (= USD 145 or AUD 186) but I spotted a Krohn-Hite Model 522 calibrator on ebay and (as usual) couldn't resist.Sold as "doesn't power up" - well, not sure about that, it powered up just fine when I unpacked it. There was no output at the front connectors but that was a simple matter of changing the jumpers from rear panel output to front panel outputOf course it would be nice if the Keithley read 10.00000V but, in fact the above reading is in spec.The 522 has an accuracy of ±0.002% of the setting plus ± 0.0005% of range plus ± 0.0005% peroC plus ± 2uV giving, for all intents, ± 25ppm in total (ignoring tempco and the 2uV which is 0.2ppm on the 10V range)The Keithley's 1 year accuracy is 50ppm of reading plus 35ppm of range giving 85ppm at 10V (and this one hasn't been calibrated recently) so any reading from about 9.999V to 10.001V would be acceptable. Sadly the 522 is not quite accurate enough to check the Keithley and vice versa.My 2015 says 10.00022V which agrees pretty closely with the 2000 (just 6ppm difference) so I suspect that the 522 is slightly high and eitherjustwithin it's spec or just out but I can't really tell.Tear down pics later.Does anyone know what the difference is between this model and the 521 - the few pictures and specs I can find on the web don't really seem to show any difference at all.Finally is there a service manual - the manuals on the web all seem to omit the theory of operation and schematics. There's a521 manualavailable, but I'd quite like the correct manual if possible.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 01, 2015, 12:11:52 pm
content: Nice score

author: matseng
author_rank: Frequent Contributor
timestamp: March 01, 2015, 12:26:58 pm
content: Oooohhh.... That's a beautiful oldskool display.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 01, 2015, 02:38:58 pm
content: Quote from: JohnnyBerg on March 01, 2015, 12:11:52 pmBetter than the Fluke 343A which was supposed to be working and cost me twice as much.It is still, somewhat optimistically, in the repair pile. If I were honest with myself it would be in the "too shagged to be fixable" pile but I live in hope

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 01, 2015, 02:46:47 pm
content: I am looking forward to the teardown.Especially the reference.Perhaps, it doesn't need repair? Only calibration?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 01, 2015, 04:15:18 pm
content: Wonderful instrument, flat case and just beautiful display!!Did you already check all ranges, including current vs. your Keithley?Frank

author: Vgkid
author_rank: Super Contributor
timestamp: March 01, 2015, 04:32:49 pm
content: Nice score, teardown requested.I have noticed quite a few of the 522/21 have appeared on ebay in the last several months.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 01, 2015, 05:14:18 pm
content: Quote from: JohnnyBerg on March 01, 2015, 02:46:47 pmThe reference itself is pretty much identical to the one in the MV106J that Dave had in EEVBlog #210, a low tempco zenner fed from a constant current source, remarkably unremarkable given the spec of the unit which is similar to the MV106J (0.002% of setting rather than 0.003%)This is the circuit from the 521 manualQuote from: JohnnyBergI don't think it needs any repair at allAs for calibration, I will probably get one of the Keithleys calibrated (I've been promising myself this for about a year) and then check the EDC against the calibrated DMM.Quote from: Dr. FrankA quick go/no go check, but both of the current ranges and the 100mV and 100V ranges seem to work OK as well as the 10V range.

author: Vgkid
author_rank: Super Contributor
timestamp: March 01, 2015, 05:34:54 pm
content: I would love to see the schematic for the 100v range.

author: saturation
author_rank: Super Contributor
timestamp: March 01, 2015, 05:56:06 pm
content: Great buy. This is the upgraded version of the MV1xx series, but the basic accuracy is the same, in the 20-30ppm range.Its a more reliable way to keep a reference in your lab, easy to transport to anyone with a calibrated DMM for a check, and later serve as a transfer reference to bring the corrected DCV to your other devices.You can email KH for full schematics:UM:http://www.krohn-hite.com/htm/ServiceSupport/PDF/Manuals/522%20Manual.pdf

author: grumpydoc
author_rank: Super Contributor
timestamp: March 01, 2015, 08:35:24 pm
content: Quote from: VgkidThere isn't anything special - the output buffer amp is fed by ±150V and gain adjusted as appropriateHaving found a few old ebay auctions with better pics, and having had chance to look through the schematics for the 521 I can see a couple of differences between it and the 522- The 521 has a 7-segment display, the 522 actually has an alphanumeric display- The 522 doesn't seem to have provision for the 1000V option.A few teardown pics:Inside the unit (click on picture for larger version).Generally it's pretty tidy inside. There is evidence of repair to one of the voltage regulators but otherwise it looks factory fresh. Date codes on ICs suggest a 1999 or 2000 build date so it's actually not that old.The unit is built around the same voltage reference as Dave's MV106J but instead of the range switches forming a voltage divider the reference voltage is fed into a 24-bit DAC.The DAC architecture is a bit odd in that is seems to be built as a decimal rather than binary circuit. First of all there is a 10-tap voltage divider formed from 10x precision 1k 0.01% resistors, these can be seen in a row under a set of trimmers in the centre of the picture. Each voltage tap can be tweaked with the trimmer. Then pairs of 4051 analogue multiplexers, which can be seen above the row of trimmers, are used to form 1-of-16 switches (of which 10 inputs are used). Finally the voltages from the 10 taps are summed via a six input decade weighted mixer and then passed to the buffer amp and then to the outputs.The whole thing is controlled by a 6502 microprocessor.It really looks like they took the basic design of the older units and just built a digital version of the front panel switches.This is the voltage reference.As with Dave's unit there is a little sticker with the diode parameters. Again a 1N829A is used for a 0.0005% peroC temperature coefficient - presumably aged and hand picked. What I can't quite figure out is why the temperature coefficients of the divider chain aren't thought to worsen the diode figure. The 1K resistors are probably low tempco ones but even if they are measured and matched they aren't exactly taking care to ensure a consistent thermal environment. Trimmers have, of course, notoriously bad thermal coefficients but I suspect the saving grace with them is that they are being used as voltage dividers so any change in absolute resistance will affect the ratio of resistance between the wiper and each end much less.Unlike Dave's the diode is just soldered into the boardI imagine the bent leads are for strain relief.The CPU sits on a long plug in board at the front of the unit - very old school 65C02 with external ROM and RAM.the reverse of the main PCB is not especially interesting, top right are the links which select front or rear panel output.On the whole the design appears to be much more straightforward than the Fluke 343A, for much the same spec

author: codeboy2k
author_rank: Super Contributor
timestamp: March 02, 2015, 01:36:37 am
content: Quote from: saturation on March 01, 2015, 05:56:06 pmEven that manual is incomplete, and has a pop-up notice in the PDF that requests to contact KH for a manual. Maybe that's what you meant by email for full schematics?In any case, when someone has them, please postthanks!

author: dacman
author_rank: Frequent Contributor
timestamp: March 03, 2015, 01:04:49 am
content: Quote from: grumpydoc on March 01, 2015, 02:38:58 pmYour 343A may be 45 years old and it has some capacitors that tend to go bad. Check the silver looking TE series capacitors (or all the aluminum electrolytic capacitors). There are two or three under the shield in the right front, in addition to the ones that are in plain site.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 03, 2015, 07:17:16 am
content: QuoteA good tip but before I get that far there are a couple of things I need to sort out including some mechanical damage, broken and shorted out indicator bulbs and several chewed up screws to extract.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 03, 2015, 03:10:48 pm
content: Quote from: codeboy2k on March 02, 2015, 01:36:37 amKrohn-Hite were very helpful, but they did point out that they normally charge for the schematics so I do not think that they would want me to post them.However the 521 manual is available with a schematic. The quality isn't great but it is readable and appears to be substantially the same as far as the reference, DAC and output is concerned. There is a link in my first post.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2015, 02:25:04 pm
content: With respect to the tempco of the 1N829 reference zener: one trick we used in grad school was to measure a set of reasonable reference zeners in an oven at different current values. Each individual zener would have 0 tempco at an idiosyncratic current value (while the catalog value was typically at 7.50 mA).

author: grumpydoc
author_rank: Super Contributor
timestamp: March 09, 2015, 03:27:12 pm
content: Quote from: TimFox on March 09, 2015, 02:25:04 pmI'm pretty certain that is what is done with all of the EDC voltage standards - if you look there is a small sticker on which is written a specific current/voltage so I think this is the zero tempco current for the diode.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 03, 2020, 08:26:24 pm
content: I know this is a very old topic, but has anyone cleaned the decade switches on the 522? Mine is developing some poor connection inside the switches and sometimes needed some wiggle for it to settle on a digit.

author: nidlaX
author_rank: Frequent Contributor
timestamp: March 04, 2020, 03:33:08 am
content: Quote from: NoisyBoy on March 03, 2020, 08:26:24 pmHey, funny you ask, I spent a Saturday doing this two weeks ago on my unit. I removed the entire front panel PCB from the chassis and did the following for each switch:Sprayed a bit of Deoxit Gold into each switch through the area around the shaft where there are holes (used a paper towel around the switch to reduce splash)Actuated the switch full-range about 20 timesSprayed compressed (canned) air into the switch the same way I sprayed the Deoxit (again wrapping with paper towel)I can't say this is a permanent fix, but it did significantly improve switch contact and reduce bounce on my unit. That said, you could still have worn out switches on the most commonly used ranges depending on the age of your unit. If you need to, you might be able to find ITT Jeanrenaud switches new old stock on eBay or elsewhere.Good luck!

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 04, 2020, 04:24:38 am
content: Excellent! Thanks so much for the insight, instead of using the DeoxIT Gold spray, which has 0.5% cleaning action, I used DeoxIT D with 20% cleaning action. It helped, but did not completely solved the problem, I can get everything to work, but I want it to work perfectly.So, I ordered 6 decade C&K RT-A switches tonight, hopefully that will allow me to completely refresh the calibrator.Thanks for all your advice, otherwise I probably wouldn't have done anything with it.

author: AndersJ
author_rank: Frequent Contributor
timestamp: August 02, 2020, 11:46:41 am
content: Have you replaced the switches?Good or bad results?Which switches did you use?

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: August 02, 2020, 03:41:04 pm
content: Hi AndersJ,I did purchased new switches to replace all the existing switches, as the bad contacts created occasional relay clicking inside the unit and jumpy numbers and voltages on the output.Since I had the front panel disassembled, I tried the DeoxIT D while waiting for the switches to arrive. After applying it, I worked each switch about 20 times to each position, it helped quite a bit and eliminated 90% of the problem. I let it sit for another month for the DeoxIT to penetrate the switches, rotating the switches to different positions a few times a week, and positioning the front panel in various angles for the DeoxIT to flow to various part before reassembly.The end result was all the bad contact issues disappeared! Now it has been five months after the DeoxIT application, the unit is back to operating perfectly! No bouncy digits at all, no random relay clicking. I had it completed cleaned, calibrated and adjusted, and it is now well within the spec for voltage accuracy and stability.I use it for calibrating DMMs under 6.5 digit, very happy with how well it works. The part I like most is the power-on light show on the digital display.The new switches all came in sealed packaged, I will keep them in the package for now, may be I will need to use it someday.It is definitely worth it to restore your unit as it performs much better than what the spec suggests, I hope this helps.

author: AndersJ
author_rank: Frequent Contributor
timestamp: August 02, 2020, 06:24:19 pm
content: Hello NoisyBoy,Thanks for your extensive and detailed reply.I will definitely attempt a Deoxit cleaning.I have Deoxit D5.I hope it works as well as the D you used.I tried a dissassembly today but it came to a halt when Ionly had a 1.5 mm Allen key to remove the knobs.Seems I will need a 1/16 inch key.Will find one and try again.What part number do your (unused) switches have?

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: August 02, 2020, 08:10:22 pm
content: The original ITT Jeanrenaud switches is now branded under C&K after the acquisition of C&K by ITT back in 2000. I did some research and it seems like the proper replacement is what is listed below. However, since I have not had to actually replace the switches with new parts, I have not verified that it is indeed the proper replacement. Perhaps someone who has done the actual replacement can chime in.RTAP4ES10M25NSSwitch, Rotary, 10 POS, 1 Deck, Fixed Stops, Non-Shorting, 150mA, 60VAC/DC, PChttps://www.digikey.com/products/en?keywords=RTAP4ES10M25NSFor DeoxIT, I used this onehttps://caig.com/product/deoxit-d100l-25c/It is 100% solution vs. 5% on D5, also, the holes on the switch are very small, so the needle applicator will give you a lot more control over spray type for a more precise application, it prevents overspray on the circuit board.I hope this helps, sorry I cannot give you 100% certainty in the replacement switch.

author: AndersJ
author_rank: Frequent Contributor
timestamp: June 02, 2021, 03:44:02 am
content: Cleaning helped a bit, but I still have intermittent problems.Switch replacement is the next step.Has anyone tried the switches suggested by NoisyBoy in the previous post?

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: June 04, 2021, 10:19:54 pm
content: Hey, do not use that part. I tried to replace one of my switch with very minor intermittent problem on the zero digit, and that part does not work. I ran out of time trying to ID the exact replacement, but the part I provided was not the right one.

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 22, 2021, 01:04:40 pm
content: Have you figured out which switch to use?

author: grumpydoc
author_rank: Super Contributor
timestamp: June 04, 2023, 12:23:17 pm
content: Resurrecting this now rather old thread.Given that i now have a calibrated (ish) Keithley 2001 I thought I would look at adjusting the 522, especially as it seems to have drifted somewhat higher.The first step in the adjustment procedure is to use R181 to get the reference zener to the voltage on the tag - in my case 6.1606V with a forward current of 4.6mAChecking the reference voltage it was high - BUT I can't get the zener voltage down as low as 6.1606 - it's stuck at 6.1609V - measuring across the 1K "trim" resistor R1A gives a forward current of 5.114mA which is higher than the current on the tag as well.The reference circuit is the same as the one previously posted for the 521, but the 522 manual is clearer.The voltage across D101 is 6.17 so that's OK, the voltage across the combined "R1" is 6.18V (the forward drop on D101 more-or-less countering Q102's VBE)However, given that a 1N825 has a zener voltage of 5.9-6.5V I can't see how the reference circuit couldeverhave provided 4.6mA of forward current for D103 - the minimum is approx 5.9/1200 = 4.9mAI'm guessing my reference 1N829 had a very low voltage/current for it's temperature coefficient "sweet spot".What would others do - I can replace R1A with a 1.2k resistor, which should be enough to get 4.6mA or do I ignore the fact that the reference isn't quite running at it's optimum and adjust the output to be in spec with the trims on the voltage divider chain?