Keithley DMM6500 repair - Page 1
Forum section: electronics / repair

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 07, 2020, 03:24:05 pm
content: I've decided to post a thread as I don't want to spam the DMM6500 thread with my repair issues for simplicity's sake.My meter's 3.5 A port is no longer working after I had a 220V AC short by falsly connecting the test leads to mains while I had forgotten to switch the leads to the AC V terminals and blue both fuses and apparantly other components within the meter itselfI've managed to take the unit appart and you can see some pictures of the main board.Problem is, there is no visible damage and I've tried testing the small resistors which seem to be the current shunts as well as that large diode, the briudge rectifier and the precision shunt facing it and they all seem to be in working order.I'm stuck now and I'm affraid it could be a non-passive component such as an IC or such.Any ideas on what else could be the culprit here?The white wire is the one coming from the port with the damaged circuit just FYI.The unit is still under warranty but the problem is I need to make low current measurments and with the country on lockdown until further notice, I am not able to post it back and have it repaired.Also, I would have to pay custom fees once again and with shipping that would come out to be around 300$For that reason, I'd like to have a crack at it and see if luck is on my side.

author: Manul
author_rank: Super Contributor
timestamp: April 07, 2020, 04:11:52 pm
content: Try tracing the circuit and consider some creativity.One idea would be to apply a current on the current input, so you get a small voltage on a shunt. Then start looking where else is this voltage appearing. That can help tracing.More radical idea is to remove shunt (and destroy calibration!) and apply current limited (like 1mA or so) voltage injection on the points where this shunt was. Efectively making a controled input overload scenario. If you observe the resulting voltage of your limited current injection and go with hot air over the components - you may find a component which will make this voltage go down when sligthly heated. If you are really lucky, this component may be the component which absorbed the real overload.This is very generalised. I don't know a lot about meters, never had them fail. Anyway, wish you good luck.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2020, 05:59:55 pm
content: Some crude reverse engineering / tracing of the current measurement part would be a first step. Here it can help to look at the plans for a few similar grade meters (e.g. K2000, 34401) to get an idea how the circuit could about look like. At least the higher current part is probably quite similar. The low current part uses an TIA and is thus different. The circuit looks rather obvious.A next point would be to have a clear picture how the broken meter behaves. So a few readings from the meter and also measurement of the total drop.Following the signal path is also possibly. If one has a scope, an AC current e.g. from a low voltage (e.g. 6 V) transformer may be even better than DC, as it tests the whole range in one run.Possible culprits could be a relay (e.g. fused or broken contacts), a shunt and maybe the rectifier+large diode, the front/rear switch the initial amplifier (if there is a special one for the currents), a resistor for protection, a MOV and maybe others. Knowing what is still working and what not may help to narrow it down.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 07, 2020, 09:03:11 pm
content: Thanks for the replies,How do I test the large, white ceramic resistor? What is it anyway it says Fluke 2918 I am assuming it is a current shunt resistor?How do I trace the fault component by component? Don't have much experience in electronics repair.

author: E-Design
author_rank: Regular Contributor
timestamp: April 07, 2020, 09:14:48 pm
content: Quote from: drummerdimitri on April 07, 2020, 09:03:11 pmThe White resistor is the 10Meg divider.. its not part of the amps pathways.If the unit can be reassembled and powered up:Put fuses back in and then measure the input resistance AMPS to LO as you click through the ranges to see if all the shunts are in tact.Also report any measurements for the amps function (with external equipment disconnected)

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 07, 2020, 10:58:23 pm
content: Quote from: E-Design on April 07, 2020, 09:14:48 pmI was able to put the unit back together and measured the resistance between the LO and the front AMPS terminals and here's what I got for the ranges from smallest to largest: 5.3612 k\$\Omega\$, 1.0337 k\$\Omega\$, 130.45\$\Omega\$, 12.39\$\Omega\$, 1.34\$\Omega\$, and 0.4\$\Omega\$ for the 1 and 3A ranges (limited accuracy due to 2 wire measurment (Fluke 289).The amps reading without any input are 6.9 uA, 69.16 uA, 0.690 mA, Overflow A, 70.12 mA, 0.7A, 0.7A for the respective ranges from lowest to largest.Also, when should I hear the relay clicking? When it switches between which of those ranges?

author: E-Design
author_rank: Regular Contributor
timestamp: April 08, 2020, 02:47:38 am
content: As far as relay clicks,1 & 3 A are on a relay and 100mA is on a relay. So going into or out ofthose ranges, you should hear the relay.Example: 10mA switch to 100mA = CLICKExample: 1A to 3A = NO CLICK (same relay)Example 3A to 100mA = CLICKExample 100uA to 10mA = NO CLICKPlay around with it, that can let you know the relays are still working (seems likely to me) - 220V won't get from contacts to coil and the fuses hopefully opened before contact damage due to arcing (if there wasnt very high inductance on your wiring path)Your shunt values indicate you've got some leakage current going on -- likely from a damaged semiconductor (or shunt resistors) We can find out what it is (if you're willing to follow along)Remove U42 - its a quad analog switch (DG444) and is quite likely damaged / leaky.Remove U52 - its an amplifier that could also have damaged / leaky input.With U42 removed, bridge with solder pins 2 and 3 (of the U42 footprint left behind) or short them with a small piece of wire... this effectively forces only the 10uA range to be enabled (provided your relays are working)With those 2 semiconductors removed, you now have only the bridge rectifier and the shunts on the path.Now select 10uA range and measure input impedance (AMPS to LO) with your 289 and report.One more thing, Next time you have the board out of the chassis - look at the bottom side of the board where the bridge rectifier pins are soldered, if you see any component on bottom side bridging pins, it might also be leaky (this was an optional component, not on all boards and I dont know what you have so just check visually) - its a tiny component. if its there, it might also be leaky

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 10, 2020, 07:29:10 pm
content: Quote from: E-Design on April 08, 2020, 02:47:38 amI have done as you asked and the resistance between LO and AMPS is now 190.16 k.Also, I checked the bottom and there doesnt seem to be any small component near the bridge rectifier so I assume this is a later revision of that board?I have attached a picture either way for your reference.

author: E-Design
author_rank: Regular Contributor
timestamp: April 11, 2020, 11:30:13 am
content: Ok that seems too high of a total value.Try remove Q70 -then measure the total again. (leaving that 10uA solder bridge that connects the series stack of the shunts)At that point you can also try measure individual shuntsR224 ~ 0.1 OhmsR225 through R229 are in parallel and ~ 0.91 OhmsR230 and R231 in series ~ 9.1 OhmsR233 ~ 90.9 OhmsR234 ~ 909 OhmsR235 ~ 9.09 kOhmsSee if those are looking correct.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 11, 2020, 04:04:21 pm
content: Quote from: E-Design on April 11, 2020, 11:30:13 amIt's taking me way too long to find these parts on a board with hundred of components.Could you please indicate where they are located? It would make the task a lot easierI was unable to find Q70 so please assist with this thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2020, 08:25:34 pm
content: As far as I can see Q70 is the TO92 part right of the red relays, right next to the resistors in question.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 12, 2020, 06:55:28 am
content: Quote from: drummerdimitri on April 11, 2020, 04:04:21 pmSeriously ?? it took me 2 secs from your photos to see them ? check near the 3 orange relays, you have an to92 transistor standing up near an pcb hole ??on your 20200407_180632.jpg image.And yes as Kleinstein wrote its Q70 i've put an red circle around your parts.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 12, 2020, 10:41:00 am
content: Quote from: coromonadalix on April 12, 2020, 06:55:28 amI was looking for a component that looks completely different like other QXX parts on the board that's why it took me so long and couldnt find it.Thanks for the help.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 12, 2020, 03:25:20 pm
content: Quote from: E-Design on April 11, 2020, 11:30:13 amI have checked all the above shunts and their values are correct.With the Q70 removed, I am no longer able to get a proper resistance reading between the LO and the AMPS ports.I am getting values in the Mega and Kiloranges and even negative ones that change when the leads are switched around.

author: E-Design
author_rank: Regular Contributor
timestamp: April 12, 2020, 06:43:30 pm
content: You are still measuring with the DG444 pin 2-3 shorted and power up and select 10uA range ?I believe the configuration you have now should let you see the whole sum of shunt values - unless there is still a damaged part.You had said the diode bridge was ok right? - thats about all left in there.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 12, 2020, 09:36:30 pm
content: Quote from: E-Design on April 12, 2020, 06:43:30 pmI am measuring with the three components previously mentioned removed and DG444 traces 2 and 3 shorted as can be seen in the picture below.Not sure why I am still not getting a proper impedance reading from ranges 10 uA to 1 mA. The larger ranges are reading consistently impedance values of 10.083 k, 1.254and 0.335

author: coromonadalix
author_rank: Super Contributor
timestamp: April 12, 2020, 11:28:25 pm
content: Is it me or you habe a small bodge on one pin of u42 (center of the chip where two pcb vias) and a shield trace ?? please clean the surroundings properly

author: TiN
author_rank: Super Contributor
timestamp: April 13, 2020, 12:00:25 am
content: R236, R594, R593, R386 and shunt R233 look like have liquid damage or corrosion. Those resistors look very nasty to meR255 have deep scratch across surface, could be broken. These small parts require gentle and careful work, otherwise it's easy to ruin delicate instrument like DMM6500 beyond repair.Please be careful.

author: E-Design
author_rank: Regular Contributor
timestamp: April 13, 2020, 12:42:22 am
content: Check for guard bridges. Put your 289 in continuity mode and one lead on the mask relieved trace you see everywhere (somebody pointed out a bridge on pin 12 of U42). With the other lead, probe around on each of the shunts and some nodes nearby. You can cover alot of them quickly.. nothing to should indicate continuity.Also, you are measuring ~10k ohm on the 10uA range? If not, check continuity from AMPS input to one terminal of R235. You should be able to get a low resistance to this point (its just wiring, fuses and copper traces) - see what you've got.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 13, 2020, 05:52:49 pm
content: Quote from: E-Design on April 13, 2020, 12:42:22 amI checked the board and there are no shorts between any of the shunts/components and the guard bridges except for the components circled in red in the attached picture (right side shorted).I am getting a resistance reading of -109 kwhatever that means and it changes when i reverse the test leads.Resistance between AMPS port and one terminal of R235 is around 335 m\$\Omega\$.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 13, 2020, 06:40:27 pm
content: please clean your job and the ic pads they dont look clean to me, it seem to short in some place with the shield trace ... on your last photo you could have problems without knowing it ...

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 13, 2020, 07:18:54 pm
content: Quote from: coromonadalix on April 13, 2020, 06:40:27 pmI just did and it made absolutely no differenceAlso, none of the traces are shorted with the guard bridges.

author: E-Design
author_rank: Regular Contributor
timestamp: April 13, 2020, 09:38:59 pm
content: Alright, seems you have good connection up to the shunts and previously, you measured each of the shunts individually as correct so lets check from the bottom of shunts back out.So, measure the 5 parallel (R225 to R229) from the side not connected to R230 measure that node out to LO terminal. It should look low like a low value again (it just contains the 0.1 ohm shunt and low resistance wires, traces.)See what that shows us.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 13, 2020, 10:02:29 pm
content: Quote from: E-Design on April 13, 2020, 09:38:59 pmI measured the resistance from the parallel resistors to the LO input terminal and got a value of 0.123as expected.

author: thm_w
author_rank: Super Contributor
timestamp: April 13, 2020, 10:31:18 pm
content: Quote from: TiN on April 13, 2020, 12:00:25 amR225 I think you mean.R75 in the original photo has some "browning" as well.Also R283 in the follow up photo, is that a ferrite bead? It looks slightly suspicious.

author: E-Design
author_rank: Regular Contributor
timestamp: April 14, 2020, 12:34:56 am
content: Quote from: drummerdimitri on April 13, 2020, 10:02:29 pmOK as a brief summary..1) you have removed all parts suspected of having damage, and have measured expected values from each of the shunts- though discolored / scratched, they are measuring good.2) Relays appear not damaged, Diode bridge seems not damaged (from previous) -3) With the temporary wire bridge forcing all shunts in the path (10K ohms), you measure AMPS to shunts then shunts to LO seems correct. Not sure why AMPS to LO doesn't see the sum of those 2 measurements.At this point, there isnt much else left in the circuit (CR20 and CR6), you could keep hunting for damageORyou could start putting parts back in (known good) - Starting with a new U52 or suitable stand-in.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 14, 2020, 06:17:39 pm
content: Quote from: E-Design on April 14, 2020, 12:34:56 amCR6 has a voltage drop of 0.520 and OL in reverse polarity so it should be fine.CR20 I assume is a 4A full bridge rectifier? I removed it from the circuit to test it under load to make sure it's in working order.Couldn't find the datasheet for this model. It is made by ON semiconductors but the QJ16 model number is not showing up on the web.Is it safe to assume this will work on 220 V AC for testing purposes?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 14, 2020, 07:01:58 pm
content: Testing the rectifier does not need hight current / high voltage. It is more about having all the diodes present (diode test from the DMM) and leakage current.For the resistance measurements, there could be interference for some bootstrapping of the protection. I am not sure of the DMM6500 uses this, but it is common in higher grade meters. So there is a high Z buffer buffer the voltage at one side of the rectifier and than apply it through 2 resistors to the + and - pins of the rectifier. If this amplifier is broken, it can interfere with the lower current ranges. (the good thing is this rectifier would usually be a not so critical, special type). One would notice such an error a voltage (e.g. up to +-0.5 V, less in the higher ranges) present between the amps terminals.Reading too high a resistance from the amps terminals could indicate a broken trace. Is the bridge really at the correct position ? It should be connected to a shunt and a input terminal (likely through the fuse and maybe a relay).

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 14, 2020, 07:14:00 pm
content: Quote from: Kleinstein on April 14, 2020, 07:01:58 pmIn that case the rectifier is working fine out of circuit.Maybe it would be helpful so probe the AC and DC side of the rectifier in circuit?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 14, 2020, 07:38:50 pm
content: In circuit the 2 DC side pins of the rectifier are connected with the diode and likely 2 resistors for the bootstrapping. with nothing connected to the inputs the voltage should be close (e.g. less than +-5 mV) to the COM terminal. If there is no bootstrapping the DC side would be kind of semi-floating only connected through the diodes in the rectifier.The AC side of the rectifier should go to the terminals, with the fuse in between at one side.

author: E-Design
author_rank: Regular Contributor
timestamp: April 14, 2020, 09:08:45 pm
content: Yes, leakage is generally the issue. The 65xx does guard out that leakage provided its not excessive (from damage)The condition of the instrument right now has U52 removed which is what drives the guard. So in circuit right now, it wont be guarded.Thats probably ok for now while we complete finding all the damaged devices.I am not 100% sure if that bridge could survive a direct connect to 220V, but external wiring impedance and the rather fast fuse opening time, it very well might have survived it.The bridge is a GBU4Ahttp://www.vishay.com/docs/88614/gbu4a.pdf

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 14, 2020, 09:30:44 pm
content: Quote from: E-Design on April 14, 2020, 09:08:45 pmSo do I need to replace U52, U42 and Q70 before we proceed with the troubleshooting?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2020, 09:01:16 am
content: If the bridge rectifier still has low leakage it should have survived the surge. If at all the lowest current range would suffer from a little leakage.For Q70 one could test is outside - chances are high it is Ok anyway. I see currently no need to put it back in. It is likely a JFET similar to J107 and likely used to the switch for the 10 mA range.One could try to check U52 outside the board and if available use a replacement. It depends on the circuit how critical U52 performance is. If just for the guard and maybe bootstrapping the diode bridge it would be a rather non critical OP, one could as well replace. What type is U52 ? This could give a hint about the circuit used.For the start I would leave out U42 and keep the bridge to kind of manually force the 10 µA range if I understand it right.The sometimes negative resistance readings in some cases earlier point to some leakage current from the active circuit, like from U52 or from the guard trace. If U52 and the rectifier removed I would expect the guard trace around U42 to be now floating, with no more connection to the main circuit.

author: E-Design
author_rank: Regular Contributor
timestamp: April 15, 2020, 11:05:58 am
content: So, if we are content with no leakage or bridges, then you can proceed with re-populating.You have also checked the Zener CR6 for leakage?When re-populating:U42- you should replace withnew one- this is likely damaged.U52- you should replace withnew one- it could be damaged. Suggestion to test it outside circuit is ok, but I am concerned might be difficult for you. The damage can be small input current leakage. You can build a low leakage setup and get the right equipment, but it may just be less hassle to replace it.Q70- I agree can leave out for now, it just a range switch.When U42 and U52 are back in, it will be enough to tell you if you have found all the damage. Remember, both U42 and U52 can have suitable stand in parts. What do you have on hand?After re-population, give the area a good cleaning with proper solvents to get rid of all that flux residue and contamination.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:52:53 am
content: Quote from: E-Design on April 15, 2020, 11:05:58 amWhen you say leakage, what exactly do you mean and how do I check for this?I always test components out of circuit to make sure they are working properly.The parts are not available locally so I will need to buy them online. Can you recommend a store that provides cheap shipping?Also, what is the exact part number for that amplifier and Q70 component so I can find a suitable replacement for them?Thanks.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 21, 2020, 02:27:53 am
content: A leakage could be :Partially failed parts : ic's, transistors, capacitorsSurface conaminants like : solder residues, solder resins, solder fluxIn multimeters, oscilloscope, rf equipemnts and many other sensitive instruments, a small leakage can cause many problems, all your electronic parts would be ok / brand new etc ... and you could still get wrong values (voltages, impedances etc ..)I work on some Rf boards around 27 mhz, small flux residues on pcb give me lots of problems, the flux we use is very strong, if we left it uncleaned it can corrode some parts overtime, sometimes i have to clean the pcb's 2 or 3 times to be sure its devoid of any contaminants, we rince the board with specially treated / filtered water.On the same board we use a relay, if the relay develop a resistance (or the leakage we talk) on the contacts on some tests points well see strange voltages values, they can't be calibrated nor compensated, we have discovered fake and counterfeit relays due to this behaviour.@drummerdimitri thats why on a earlier thread post i wrote you to clean your board properly, if the meter has an very high input impedance on some range(s) it could be affected by residues / contaminants, a very small solder short etc...And E-Design suggested the same thing ...

author: Kleinstein
author_rank: Super Contributor
timestamp: April 21, 2020, 06:27:06 am
content: U42 is supposed to be an DG444. There are quite a few similar parts that should be OK too (e.g. DG202B,DG212B,DG441, ADG....). The switch should not be used dynamically so that speed and charge injection should not matter, mainly R_on and leakage. One may get one of these locally.I very much doubt that Q70 is bad. It should be a JFET with a relatively low R_on and not too much gate leakage. One could measure R_on, even if it may be damaged. I would expect something like J107 or PN4391/2 , but other low resistance ones should work too.For U52 I don't know, but the markings should tell, what type. Chances are one could do a test with a more common type, so one could be reasonable sure there are no other broken parts before ordering.

author: E-Design
author_rank: Regular Contributor
timestamp: April 21, 2020, 05:58:16 pm
content: Yes leakage can mean contamination on the board... this is true.. when all components are put back in, the OP should clean the area up where he was working as a good practice.The leakage I had in mind for this effort though is due to semiconductor damage - seems more likely to me given the unfortunate circumstances.I agree that the Q70 is probably not bad.. leave it out for now or put it back in, wont matter - its for 1 particular range. It is a J105 Jfet.U42 is DG444 but agreed could be any of those other alternatives. If you try an alternative, please ensure the logic control polarity is the same.. these analog switches are usually offered in both flavors.U52 is an LMP2021 - a low ib, zero drift amplifier.There is a lot of sense to putting in on hand parts to see if you can get it working - but if you have nothing on hand and have to order.. might as well order the right parts. There is high availability on these.I cant suggest any particular place for you to order from with low cost shipping - thats up to you and where you like to shop.Build a parts list, post it up here, we can tell you if it will function as replacements.

author: JimKnopf
author_rank: Frequent Contributor
timestamp: September 23, 2020, 01:47:44 pm
content: How does the story ends? Did you fix your device?

author: stigskov
author_rank: Newbie
timestamp: October 16, 2025, 12:40:15 am
content: Maybe unrelated but in case of an instrument not powering on, see this thread:https://www.eevblog.com/forum/testgear/keithley-dmm6500-transformer-rfc-around-recall-experience-and-a-dilemma/msg4293643