Keithley 6485/6487 teardown - Page 1
Forum section: electronics / testgear

author: plesa
author_rank: Frequent Contributor
timestamp: January 17, 2013, 10:34:38 pm
content: I found on my cell phone few internal photos of Keithley 6487.It is quite similar to the 6485 ( without +/- 500V voltage source - bottom board and second transformer).The front end contains LMC662, LMC6081. Voltage reference is LM399H.Sorry for poor quality.

author: razberik
author_rank: Frequent Contributor
timestamp: January 13, 2017, 09:09:27 am
content: Hello, do you remember please whether those white EE2 relays are for range switching ?Thank you

author: plesa
author_rank: Frequent Contributor
timestamp: January 20, 2017, 06:02:14 pm
content: Quote from: razberik on January 13, 2017, 09:09:27 amYes, based on clicking it is used for range switching. And electrometers 6514 and 6517 are using similar relays to Meder HI series.

author: razberik
author_rank: Frequent Contributor
timestamp: January 24, 2017, 09:30:52 pm
content: These HI Meder looks good. It is comfortable to deal with Meder ... in my native language, they manufacture some series in Prague.Today I got a good chance to do some brief teardown. I attach pics.It surprise me that they dont use better relays, but literally jelly-bean relays.

author: razberik
author_rank: Frequent Contributor
timestamp: January 25, 2017, 11:51:32 am
content: Today I was given a task to test resistivity of some materials. I was climbing with voltage from few volts to few hundreds of volts. Then suddenly relays started to clicking and OV.RFLOW:A showed on display. Totally bricked. Changing ranges does nothing.Exactly the same behavior like this:https://forum.tek.com/viewtopic.php?f=41&t=103860Any tip what to do ?

author: plesa
author_rank: Frequent Contributor
timestamp: January 25, 2017, 07:42:09 pm
content: Replace comparator U49, it should be LM339. If it is not U49, try to find LM339 in this area.What did you tried to measure?This meter should survive about 1-2kV on input. So you cannot damage it by measuring insulation resistance with integrated voltage source.If you are using external power supply or higher voltage than 500V consider some protection circuit made by gas discharge tube, resistor and 2x antiparalel low leakage diodes (1N3595, DPAD5, DPAD1 or JFET).In attachment you have minimal values of protecting resistor, it also depends on used range (lowest auto range can be selected in menu).For measurement you described use manual range rather than autorange.

author: razberik
author_rank: Frequent Contributor
timestamp: January 25, 2017, 09:22:42 pm
content: I was measuring insulation resistance with internal power supply.Yes, I had AUTO setting. I learned it the hard way.Well it depends on what boss say. Send it to repair or let me try repair it ... We have this machine for years.

author: plesa
author_rank: Frequent Contributor
timestamp: January 25, 2017, 09:32:31 pm
content: Quote from: razberik on January 25, 2017, 09:22:42 pmKeithley does not repair these meters or the repair is corresponding the new meter price. Try to replace comparator metioned above.What level of current you needs to measure?

author: razberik
author_rank: Frequent Contributor
timestamp: January 25, 2017, 09:36:32 pm
content: Hard to answer, when it was working, I was obtaining results in Teraohms. The guy who ordered this measurement told me these results seem to be correct.Where and how did you figure out that LM339 use to fail ? Is it some common failure for this pAmeter Keithley topology ?

author: plesa
author_rank: Frequent Contributor
timestamp: January 25, 2017, 10:44:02 pm
content: Quote from: razberik on January 25, 2017, 09:36:32 pmTeraohms it means that you was measuring current <500pA. If you want to measure it right way, place the resistor between DUT and meter according to table above with GDT and protection diodes.Also try to find metal enclosure for your DUT and ground it. Also monitor humidity level, this can affect your test results as well.I have some experience and it is common failure, same like DGxx switches is Keithley DMM ( 2001/2002). There are bunch of threads related to these repairsAlso If you are interested in this topic readhttp://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf

author: TiN
author_rank: Super Contributor
timestamp: January 26, 2017, 04:31:48 am
content: Thanks for photos, I might have to add some soon.

author: razberik
author_rank: Frequent Contributor
timestamp: January 26, 2017, 11:35:02 am
content: My boss agreed with trying the repair. I replaced the LM393, but no result.It seems to me that instrument is switching ranges correctly. I mean - if I set manually some nano/micro range and then I press to AUTO, it switches to the highest range 20mA.If I connect DMM to analog output, I receive som random voltage with nothing on input. If I short the input, the analog output changes, but I dont receive correct number either.Any tip what to do next ?I attach little bit more detailed photo. There are some DG411 switches. Shall they be wrong ?TiN - I have some hi-res photos, but not very good angles and ambient lighting.

author: TiN
author_rank: Super Contributor
timestamp: January 26, 2017, 11:41:35 am
content: Feel free to upload all or any pics to my ftp, I'll reshare. There are not many photos of there online. Also if you have more photos of bottom voltage source board, would be cool to see. Access info is in my sub link.Did you check obvious, input protection parts?If I be you, I'd check voltage source operation. Set meter to get small output, like 1V or 5V and test if it's present.Or opposite, source small stable signal to source and check where it disappears on circuit, assuming problem is somewhere between input connector and ADC chipCheck input voltages on front end opamps (DIP8 fella's). Be careful not to contaminate things, to prevent leakage (using unpowdered gloves helps, also protect you a bit from high voltage shock from FE supply).

author: razberik
author_rank: Frequent Contributor
timestamp: January 26, 2017, 01:49:35 pm
content: I was wrong. There is no DC voltage at the Analog Output. There is 2VAC for odd ranges and 6VAC for even ranges. It is 100Hz frequency, looks like two combined 50Hz. WTF ?Everything is disconnected. When Zerocheck is active, zero at output.I was wandering about the input and I simply dont understand the purpose of the LMC662.Is this the input ? And what is the LMC6081 for ?AD) I am getting the exact AC signal from AD829 video amplifier output.

author: plesa
author_rank: Frequent Contributor
timestamp: January 26, 2017, 03:52:27 pm
content: OK, so it seems that other components has been damaged. Another components in the chain is LMC6081. This will also explain the weird analog output readout.During zerocheck is input shorted, so you can check input impedance. Analog output is direct output from input stage.Are you sure that on input was only voltage 500V?I have few 6485/6487 for service and all of them has damaged comparator only. Never input stage blown. On their input were voltage between 1-5kV. This meter do not have any protection on input. And all input protection needs to be external.For theory check chapter 4 in manualhttp://www.tunl.duke.edu/documents/public/electronics/Keithley/keithley_485_picoammeter_manual.pdfThere is also schematic but 6485 is similar, but different.

author: razberik
author_rank: Frequent Contributor
timestamp: January 28, 2017, 12:19:14 am
content: Hello. My boss gave me more time on it. I made detailed pictures of it and they are being uploaded to xdevs. Check it out TiN ;-)I was lurking around the preamp section. I was trying to reverse the schematic. There is quite small amount of components around, however I was doing it in work with board installed, so I was unable to track the traces under relays precisely. So I have some garbage schematic, attached. What is interesting for me is using positive inputs of opamps, but hey, they did it the same way in Kei 485. Anyway, no feedback loop is closed now in my schematic.What I have so far found, that AD829 video amplifier is outputing rail-to-rail (+/-15V) swing in 100Hz frequency. There is no corresponding input, however I dont know the feedback network of it.If this was some non-precise circuit I would definitely give it a try to desolder it, but I have to think twice at this delicate instrument. Any tip ? I have strong belief that this AD829 is the driver of the feedback loop.AD) plesa - yes, I am absolutely sure that 500V was not exceeded. It happened while sourcing about 300V from internal supply and suddenly relays were clicking like crazy and then OV.FLOW.

author: TiN
author_rank: Super Contributor
timestamp: January 28, 2017, 08:07:51 am
content: Thanks for photos, nice to know that same voltage source board used in 2501 too.Did you check inputs on AD829, what voltage is there?Here are all photos fromrazberik, moved at permanent location:

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2017, 10:40:37 am
content: Seeing a 100 Hz signal makes me think about the supply: could be a capacitor problem and thus too much ripple. At least check the supply voltages with a scope.

author: razberik
author_rank: Frequent Contributor
timestamp: January 29, 2017, 09:34:34 pm
content: Here you go. The preamp schematic.It was hard to track some tracks. Some of them I didnt find out.I wonder how the U25 LMC662 is supposed to work.Need some time to rearrange the schematic into more readable form.

author: plesa
author_rank: Frequent Contributor
timestamp: January 29, 2017, 09:55:44 pm
content: Did you check the power supply for AD829 and also check the output of LMC6081 by scope or DMM.If you needs some measurement of functional meter I can help.

author: razberik
author_rank: Frequent Contributor
timestamp: January 30, 2017, 12:52:40 am
content: I replaced the AD829 and BUF634, but no change.When AD829 is removed and I short the GND/Guard and output pin on AD829 footprint, I receive some value on display. When BUF634 is removed, it reads 0.00001nA.Any idea ?

author: razberik
author_rank: Frequent Contributor
timestamp: January 30, 2017, 01:13:54 pm
content: Some news.plesa - Yes, I have checked all power supplies, these are all OK. I didnt reply because this was the absolute first thing to check. No ripple, no significant noise, just nominal voltages.I have tracked the voltages around U25 LMC662 and U27 LMC6081. This differs between mA+uA and nA ranges. On mA+uA ranges, I receive saturated rail-rail swing at the U26 AD829 output, on nA ranges, it saturates to negative supply.IN+ on U26 is -17mVDC all the time. I receive -11.2mVDC on fully functional Keithley 6485 I lent from my colleague. I believe this is OK.I believe that saturation is driven by the input at IN-.On nA ranges, the U27 LMC6081 behaves strange to me. Output is -30mV all the time. IN- is at zero. When I touch the DMM probe IN+, the DMM reads some miliVolts ramping up. Starting value is random, but it ramps towards few miliVolts. No change at the output.I used HP 3478A for reading voltages. (When I read e.g. 10VAC, it corresponds to +/14V rail-rail swing)All is referenced to GND/Guard. The instrument has shorted Analog Output GND with chassis.I attach the input area schematic with voltages and waveform of the rail-rail swing at the U26 output.

author: razberik
author_rank: Frequent Contributor
timestamp: February 02, 2017, 12:13:01 am
content: As I indirectly promised, here is the rearranged schematic. I also found out the relay switching matrix, noted in schematic for ZCHK OFF and nanoAmps range.K9 is operated during startup and switch off only.What is interesting is the LMC662 circuitry. It seems that only the input FETs are used to compensate input current of LMC6081. Thus these two opamps are matched ?

author: TiN
author_rank: Super Contributor
timestamp: February 02, 2017, 06:57:14 pm
content: razberikdoing great job on schematics reversing, my respect!Keep it up.To contribute at least something, I took few ugly photos of my latest toy, KI 6485. As expected, very very almost sameObviously no point of having triaxial on 6487, as guard is not driven actively.

author: lukier
author_rank: Unknown
timestamp: February 02, 2017, 07:05:14 pm
content: Quote from: TiN on February 02, 2017, 06:57:14 pmHmm is that the right front panel?I have the old 485 from the ugly-brown era and it also has just BNC, no triax. I wonder how similar is the front end circuitry, maybe I'll take some photos and compare (AFAIR no LM399 in 485).

author: plesa
author_rank: Frequent Contributor
timestamp: February 03, 2017, 01:53:58 am
content: 6487 does not have guard at all.Input stage of 6485 and 485 it different, 6485 use the opamps and 485 use discrete FET.Razberik, did you tried LMC replacement? I do not think there is any matching nor preselection of them for this entry level meter.

author: razberik
author_rank: Frequent Contributor
timestamp: February 03, 2017, 11:17:51 pm
content: Hello there.A big credit goes toSKE. After a few talks he came with a theory.Lets analyze the most simplified loop for nanoAmpere range. I omit capacities and use most simple opamp model.Firstly we apply Vsource+resistance to give about 2nA.U26 creates about 1V across 500Meg resistor. This is the way it should be. When I place 1TOhm at the input to GND, it simulates open input, the U26 output settles aprox. at zero.We can now believe that U26 and U50 are OK. So the wrong part can be propably U25 or U27.If we assume that the instrument died after crazy relay clicking and U27 works probably as an input protection (I was told that the only input FETs are used, it is easier to buy this cheap opamp than low-leakage diode itself).So If I bring some leakage into input, I receive the exact behavior like on real instrument.Shortly ... this was true. When U27 LMC662 was desoldered, instrument started to work. I attach pictures to prove it.One thing interesting. I soldered new LMC662 and washed the soldering area using IPA in few cycles, but there is still some residue remaining at the pins (tiny flux ring around pin). This doesnt affect the input current.

author: plesa
author_rank: Frequent Contributor
timestamp: February 04, 2017, 12:11:06 am
content: Congratz to successful repair. In your box is lot of space for input protection so I will consider to add it.

author: TiN
author_rank: Super Contributor
timestamp: February 04, 2017, 02:21:04 am
content: Very nice job documenting and getting down to a rabbit hole, thank you!Can I use your info to add in my 6485 article, with link to this thread posts?

author: Smith
author_rank: Frequent Contributor
timestamp: February 04, 2017, 07:05:27 pm
content: The 6485 is a nice device, just like the older 485. I use both, but the 485 has two main advantages: the connector is positioned on the front (just like me) and the autorange doesn't tend to keep switching between two ranges in particular cases. The latter can be solved on the 6485 by lowering the reading speed.For everyday use I mostly use the 485, it's just so easy to use and fast and stable. I think it was build somewhere in the 80's, but it's still spot on.

author: razberik
author_rank: Frequent Contributor
timestamp: February 04, 2017, 07:35:58 pm
content: Quote from: TiN on February 04, 2017, 02:21:04 amYes, of course. No problem.I expect to do reversing K2400 SMU in future.

author: plesa
author_rank: Frequent Contributor
timestamp: February 04, 2017, 08:46:46 pm
content: Quote from: razberik on February 04, 2017, 07:35:58 pmIf you have faulty 2400 you can check 23x service manual, there is schematic.Two units I needs to repair has blown power mosfets only (MTP2P50 and 3N100 or in more recent meter 7N100).

author: razberik
author_rank: Frequent Contributor
timestamp: February 14, 2017, 09:02:36 am
content: I am still not sure if I understand used relays correctly. They use EE2-3SNUH which arent special, they dont have excellent resistance.But these relays are latching. This means that when relays not operated, coil is connected to GND. Does it mean that the coil acts as shielding/guarding ?

author: pelud
author_rank: Regular Contributor
timestamp: February 12, 2019, 11:55:42 pm
content: Hi All,I have some questions about how the analog circuitry of the meter works.1. What is the purpose of the 2N3904 and 2n3906 transistors (Q4, Q19 and Q20 on kei_6487_reverse_preamp_sch2.pdf) and surrounding circuitry? It looks like some sort of compensation network, but I'm not sure what for or how it works.2. What is the null adjustment on the AD829 for? I would have thought that the LMC6081 is the "boss", and would do whatever it can to make it's inputs the same, including compensating for any null offset on the next amp.3. What are R185, C140, R50 and C141 (kei_6487_reverse_preamp_sch2.pdf) going into the AD829 for?Also, some comments:I noticed that some people mention that the triax is not necessary. However, it very much is, as you would logically expect that the exposed metal shell of the connector is at earth ground potential, while the center conductor can be at whatever voltage source you are using. Therefore, the center conductor as well as the internal circuitry can "float", and the internal feedback of the device will drive the second "guard" conductor of the cable to same as the center in order to "intercept" any current that would otherwise leak to ground.

author: TiN
author_rank: Super Contributor
timestamp: February 13, 2019, 04:55:22 am
content: You can get away w/o triax if use floating BNC with external "shell" connected to GUARD. However you would still need/want low triboelectric noise cables with good insulator material. Using coax with driven shield at guard potential is very often used inside the instruments, that don't need worry about user's touching live wires (that might be at lethal potentials!). For example Keithley 7xxx series low-current switch matrix cards use coax to route signals from triax input to PTFE-floated reed COTO relays.Caveat with this approach is obvious = your outer BNC metal shield is now under live voltage, controlled by guard driver, so if you trying to measure something with dangerous voltage levels - you need exercise great care.Also this solution may provide more noise in samples, as you don't have as good shielding as with triax or quadrax cables.

author: Minikisscool
author_rank: Newbie
timestamp: October 21, 2022, 08:25:20 am
content: Hi,I have a 6485 which is not working.There is no display of value.Error reading Read command in GPIB (timeout)I read all topics here, used preamp schematic v2 to check inputpreamp is working, I have proportional voltage on analog outputI look to check ADC, it's OKI component (2000-802A04)In my board, it's a bit different, it's a new version 2X00AD component.Does anyone have documentation on this component or old version ?Thanks in advance

author: acarleton2000
author_rank: Newbie
timestamp: October 02, 2025, 06:32:01 pm
content: Hey I know this is an old post but I'm wondering if anyone knows what the value for R183 and R184 should be.Have one here that has these burnt out.Thanks for the help!

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 08, 2025, 08:49:12 pm
content: If 6485 is meant (Rev. 10002-102J), R183 & R184 are in parallel and their writing is 10R0 -10(Quick measurement gives 5.2)