Keithley 179A resistance measurement is slowly degrading, is there a way to fix? - Page 1
Forum section: electronics / testgear

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 03, 2023, 04:45:04 am
content: Hello.Got this bench some time ago, working fine (just 7905 was busted but quickly replaced and everything is fine).But recently I started to note the following - resistance measurement is giving increasingly wrong (lower) results.Say if for 20K resistor on 20K range it says 19.15K (when resistor is exactly 20K, 0.01% tolerance), for same resistor on 200K range it will say 13.35K, and around 10K at 2M range.And these are going more and more inaccurate, as time goes by.Also, if I touch any of contacts during resistance measurements, results will become wildly different - like for that 20K resistor, it will say 68K and so on.So what might be causing this? I haven't done calibration according to manual yet, but I just want to get prepared, to know with what to start.Volts and amperes measurement are ok, no issues so far.Any ideas?

author: AVGresponding
author_rank: Super Contributor
timestamp: May 03, 2023, 05:22:10 am
content: First thing to rule out is the test leads, the flexing over time can cause the wires to gradually harden and break, with physical movement making the reading unstable.If you can, try the leads in a different meter, and try different leads with the meter.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 03, 2023, 05:30:41 am
content: Yes, leads already replaced/tested. No change.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2023, 06:11:13 am
content: An unstable reading is nothing that a calibration can fix. One should not recalibrate a broken meter. Beeing out so much is a sign of a defect and not drifting out of calibration. To do a proper calibration one needs quite some instruments, even for only a 4.5 digit meter.It is somewhat normal that touching the DUT can effect the resistance reading, though usually down. A higher reading is unusual, but possible, e.g. with poor contacts or too much capacitance to ground that may make the resistance converter unstable so that it oscillates. Added hum could also be an issue.The switches in these old style Keithley meters tend to get kontact problems. So the question is if repeated switching of the ranges / function changes (often it makes things better if the switches or relays are the culprits) the reading.Another easy test is checking for excessive input bias current. So it the 200 mV range reading the same for an open input and a short ?Another point would than a visual inspection and check of the supply, inclucing a check for ripple.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 03, 2023, 06:21:10 am
content: Quote from: Kleinstein on May 03, 2023, 06:11:13 amMy bet would be the switches too, I have had two K 177 DMM's with unstable readings that a switch clean and careful lube fixed after treatment.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 03, 2023, 08:37:27 am
content: I did some more testing and here are the results.The measured values are lower than actual when button Lo/Hi is pressed. When it is depressed, measured values are ok. However, that applies to 200K and above ranges only.The value changes when I touch SINGLE contact with a single finger. with 22K resistor connected, if I touch the positive wire, it will show 152K instead of 22K and 86K, if touching the black wire. Multiple switching and hard pressing changes nothing. If switch to megaohm range, touching by finger will read either as 1.8m or 1.5m, instead of 22k.

author: TaylorD93
author_rank: Regular Contributor
timestamp: May 03, 2023, 10:27:36 am
content: If you have a 2nd DMM, set it to the DC current range (mA or uA ranges) and connect it directly across the input terminals of the 179A. Set the 179A to a manual ohms range and with the 2nd DMM measure the current. As you select the lower Resistance ranges on the 179A the current through the external DMM should increase.I would check these to any service/calibration manual to make sure that the test current is within tolerance.If the test currents are measuring fine, then that would indicate the 179A is not measuring the voltage across the test resistors correctly.If the test currents are out of spec, then it may help narrow down the issue

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2023, 11:09:07 am
content: Touching the resistor with a finger introduces hum and adds capacitance. Ideally the hum current should not be that large as the transformer is well isolating. Given the large effect there is a chance that the isolation of the transformer is damaged - so a good reason not to touch the inputs unless it is clear where the damage is (could well be somewhere else).How is the meter behaving with smaller resistors and especially a short ?If the problem is only with larger resistors, like > 1 K it could be K105: if not making contact the ohm converter circuit could oscillate with higher resistors and even more with with capacitive loading to ground.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 03, 2023, 12:34:03 pm
content: I measured input current of Keithley with another multimeter (battery operated) and highest current is 80 microamps at lowest ohm settings. It progressively gets lower, when changing the range and becomes around 10 microamps at 2M range.When inputs are shorted, it displays .006 on lowest setting. However, measurements are way off. I have bunch of 0.01% precision resistors and instead of 10 ohms it displays 30, instead of 24 it displays 50, 100 becomes 90, 100 becomes 950 and so on. As resistances increase, error decreases.

author: TaylorD93
author_rank: Regular Contributor
timestamp: May 03, 2023, 12:46:58 pm
content: 80uA maximum surely thats not right? what range is that?have you checked the service manual rating for the output current.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 03, 2023, 01:40:21 pm
content: The Hi Lo button switches the FET Q 104 or 103 on and offYou could check if they are OK

author: GigaJoe
author_rank: Frequent Contributor
timestamp: May 03, 2023, 04:35:22 pm
content: for 179 i had measure my , i really didnt use so current for:2K 85.360 uA20K 92.285 uA200K 9.824 uA2000K 0.989 uAfor Keithley 175 for example # a differ completely:200 , 2K - 811uA20K 200K - 8.494uAM - 0.121 uA

author: TaylorD93
author_rank: Regular Contributor
timestamp: May 03, 2023, 05:06:04 pm
content: there appears to be a full service manual available from Tektronix for the Keithley 179A dated Feb 1980.That states the output current for the Ohms ranges should be;"Hi" range selectedRange - Current2k - N/A20K - 100uA200k - 10uA2000k - 1uA20M - 0.1uA"Lo" range selectedRange - Current2k - 100uA20K - 10uA200k - 1uA2000k - 0.1uA20M - N/ATry replicating this test and see what values you read.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: May 03, 2023, 08:07:29 pm
content: I would agree with Kleinstein that you should check and clean the switches as well. It will be a quick check, as in my older Fluke bench DMMs, bad contacts in the switch could cause all type of weird behavior. Good luck.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2023, 08:50:39 pm
content: The meter uses trimmers for the calibration and thus very likely no extra numerical cal constants. The current should still be a little lower than the nominal values, as there is some loading to the reference.So the readings from GigaJoe look about what is to be expeced. So the 80 µA could be correct.I no longer think K105 could be the culprit - in this case the input signal would not reach the amplifier and no resistor reading at all.The is a small chance that C107 is somehow damages (e.g. from cleaning).Really cleaning the switches is quite some effort. If it is the switches I would expect it to be somewhat changing with multiple operations, but this does not seem to be the case.I think it would be worth doing a few internal measurments. As usual first the supplies.TP5 would be interesting to check with a scope, as there is the possibilty that the amplifier oscillates in the ohm mode.The point where Q103 and Q104 meet could also be interesting when in ohm mode, e.g. with a short between the inputs. This should be a little lower than the 1 V / 100 mV ref. (e.g. around 850 mV / 95 mV) and higher with the higher ohm ranges.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 04, 2023, 08:13:44 am
content: Thanks everyone!So this is fixable, that's good.I'll take one of the weekends to complete my upgrade kit for this DMM (it adds modern LED display and continuity buzzer to ohm mode) and will check these components then as well

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 08, 2023, 08:18:16 am
content: Speaking of transformer insulation. I do have up to 2000V insulation tester, which can test gigaohm ranges.Will it be safe to use it to test the insulation from AC winding of transformer to GND pin of the DMM ?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 08, 2023, 09:30:54 am
content: Quote from: LinuxHata on May 08, 2023, 08:18:16 amThis should be OK, at least if not on the Ohm mode. In the ohm mode there is additional protection circuitry between the circuit ground and the LO input, that may complicate things.

author: AVGresponding
author_rank: Super Contributor
timestamp: May 08, 2023, 11:12:06 am
content: Quote from: LinuxHata on May 08, 2023, 08:18:16 amIt's also usually a good idea to connect line and neutral together for this kind of test, as well. If there's a line input filter, this will need to be disconnected; the capacitors won't be rated for that voltage.