Keithley 175A not working - Page 1
Forum section: electronics / repair

author: MikeK
author_rank: Super Contributor
timestamp: June 29, 2022, 01:01:13 pm
content: So I got a Keithley 175A multimeter off eBay for cheap. It powers up, but won't properly measure anything. I found that the 6.4V reference Zener diode only measures 4.47V. This could be the problem. But before ordering this part ($8 plus shipping) I'd like to do more testing. Any reason not to do either of these?1. Remove Zener and apply 6.4V with a floating power supply.2. Or, remove Zener and use 5.1V Zener (1N4733) plus two small signal diodes (1N4148) to achieve the 6.4V.The problem with the Zener solution is that the datasheet test current for the 1N4733 is 49mA, which is much more that what the 6.4V reference Zener is spec'd at (2mA). And indeed the reference Zener appears to run at 2mA in-circuit (based on the math). I ran the three-diode chain at 2mA, however, and it is almost dead on 6.4V.Is the Zener solution reasonable to allow me to do further testing? What could have caused the reference Zener to fail in the first place?...I know the multimeter is probably 30-40 years old, but there is a 5.1V Zener on the board that tests dead on 5.1V.Attached is the part of the schematic showing the reference Zener.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2022, 03:08:22 pm
content: The zener combination would be OK for testing, even if not very stable (low TC). It is very rare that a 6.4 Zener difts towards 4.5 V. Chances are there is a different fault, that causes a low voltage.A first point to check would be the supplies, if there is excessive ripple (e.g. due to failing electrolytic capacitors). Another possiblity would be something loading U103 too much.

author: MikeK
author_rank: Super Contributor
timestamp: June 29, 2022, 03:23:25 pm
content: Yeah, I thought the failed Zener was odd, especially given it's low operating current. All the supplies tested good, though...Except for the ones involving the 6.4V reference Zener of course. I'm going to remove it and test it out of circuit. And I think I found confirmation for the 5.1V Zener in the datasheet: Izt2 appears to be the minimum current, which is 1mA for the 1N4733.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2022, 04:28:13 pm
content: The 1N4577 ref. zener is supposed to run with 2 mA to give a low TC. It would work with a higher / lower current, but normally with a higher TC.The 1N4733 is a more normal low cost zener and the test currents in the data-sheet are more like example values. The 1 mA are more like a common point for comparisons not a real minimum for the diode.Testing the diode sepately is a good idea, though unsoldering is some stress to the diode. So this may effect the calibration.The +5 V supply in the K175 is quite important. The ADC raw conversion result is relative to the +5 V supply and than a seprate measurement of the -2 V ref. value is used to measure the scale factor ( 5 V supply relative to the -2 V ref and some resistor ratios). So a noisy 5 V supply can cause quite some extra noise. It is only a 4.5 digit meter - so it may be good enough.IF the -7 V ref. part is way off this can cause quite some trouble for the ADC, as this is used to shift the zero.

author: MikeK
author_rank: Super Contributor
timestamp: June 29, 2022, 06:02:05 pm
content: Quote from: Kleinstein on June 29, 2022, 04:28:13 pmIt is indeed way off. It measures -4.5V instead of -7V.How about if I inject current at that op-amp output? I could apply 5.1V across the output Zener, but allow for more current. If the voltage across the reference Zener then picks up I'll know that something is loading that op-amp. Good?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2022, 06:58:10 pm
content: -4.5 V at the OPs output and -4.47 V at the zener points to a problem with the OP, like too much loading there or maybe the negative supply to that OP not high enough.One could apply more current to the zener (an extra resistor from the negative supply). Chances are some part is drawing too much current from the -7 V.So it may be worth at looking how the measurement functions behave. This may give a hint on what additional parts are not working properly.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 29, 2022, 07:28:34 pm
content: It's quite unlikely that the Zener is defect.Check first the supply voltages of the OpAmp, then check if the OpAmp itself shows a big offset, by measuring on on its + and - inputs.It's much more probable that this OpAmp broke down, so replace it with an OP07 or something similar, just for testing.Check the datasheet of this OpAmp, if it has something peculiar, before replacing it.Frank

author: MikeK
author_rank: Super Contributor
timestamp: June 29, 2022, 07:28:46 pm
content: Yes, the negative supply is not high enough. Since the output should be -7V, the 5.1V Zener indicates that the negative supply should be -12.1V, but is actually -9.73V. I'll hunt down the negative supply, since it's not a 12V 3-term regulator.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 29, 2022, 07:36:08 pm
content: also check capacitors for shorts, which might pull down supply or the OpAmp, like C116.

author: MikeK
author_rank: Super Contributor
timestamp: June 29, 2022, 08:38:53 pm
content: I'm not certain about the -12V rail. I measure -9.5V, and that's what the resistor values of the regulator indicate that it should be. But then the op-amp can't produce -7V?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2022, 08:55:50 pm
content: The 12 V zener is not for regulation, but more for protection in case something goes wrong. The 9.5 V look about right with the resistors at the LM337. The manual calls for -10 V +-10%.A bad U103 is very real possibilty. It could also be U103 oscillating - so just a DMM may not be enough to check. If U103 does not provide any current, VR103 (5 V zener) could provide about -4.5 V. This may be intended for start-up, but should not be the normal operation current path. This makes the op-amp U103 (and maybe a solder joint / socket) a prime suspect.

author: MikeK
author_rank: Super Contributor
timestamp: June 29, 2022, 09:03:21 pm
content: Yeah, with a -9.5V negative supply U103 should be able to achieve a -7V output. If C116 is leaky, like Dr. Frank suggests, that could bring the output down. These are almost 40 year old electrolytics afterall.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 30, 2022, 04:56:08 am
content: C116 may be bad, but a leakage in the 20 mA range with some 5 V would be an exceptional case. Tantalum caps usually fail short and Al cap are more failing open or maybe with leakage in the µA range.C116 is loading the OPs output and could be an issue in choosing an OP to replace U103.

author: MikeK
author_rank: Super Contributor
timestamp: June 30, 2022, 09:45:49 pm
content: The 6.4V reference Zener does appear to be okay. Without pulling it I can apply 7.0V across it with 20mA limiting current and it does regulate to 6.35V which is within spec. Anything less than 20mA, though, and the Zener voltage is less. So something stealing current does make sense. Still may pull it to see if it regulates at 2mA, which appears to be what it's supposed to run at in circuit.If this had previously gone bad and someone had replaced it with a not-exact replacement the 2mA wouldn't be enough for a more generic Zener.

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 01:49:30 am
content: Curiosity got the better of me and I pulled the 6.4V reference Zener. At 2mA it's right at 6.35V. Even at 1mA it was still in spec. Rats.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 01, 2022, 08:25:30 am
content: As expected. Have you already checked the OpAmp if its inputs are on the same potential?Some types (esp. low bias bipolar ones) break down on their inputs and then latter might draw current.If the output of the OpAmp is not working, or is pulled down by a defective component, it will not provide enough current via R120A to supply the zener.. that could also be the root cause for the excessive current draw from somewhere.Please do these simple measurements as proposed!Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 01, 2022, 11:11:41 am
content: Quote from: MikeK on June 29, 2022, 08:38:53 pmI confirm: The LM337 seems to be programmed for about -9.4V supply.The OpAmp probably provides rail-to-rail input and output, therefore, is capable of handling -7V output.Frank

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 11:49:47 am
content: Quote from: Dr. Frank on July 01, 2022, 08:25:30 amSorry, I thought I had checked this. OpAmp output has a voltage so it must be working? Inputs are off by too much. I just took some measurements shown in the image.

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 12:00:56 pm
content: And with a 3mm blue LED at the op-amp output there is enough current to make it bright.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2022, 02:54:19 pm
content: Besides the OP, there is also the 5.1 V zener toward the 9.5 V supply that can provide current as long as the voltage is lower than some 4.4 V. Given that the voltage at the OPs output is at 4.5 V and thus just at the limit the zener can drive, chances are the OP is not providing much or any current and the current from the zener it that sets the voltage.So probably time to pull the OP or at least check it's supply (pin 4) directly at the chip.

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 03:08:00 pm
content: Yeah, I'll pull the op-amp and test it to see if the output is working. If failed, I'd replace it along with the other TLC271 on the board...Might as well.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 01, 2022, 03:12:00 pm
content: U103, the TLC271, supplies -7V to 3 other ICs, up to -30mA. (A very crude design, to use the A/D reference voltage for this).U101, 102, 104 normally should draw about -2mA each.One of these 3 ICs might as well be defect, besides C116, VR103 (less probable), and of course CMOS OpAmp U103, itself.I would de-solder U103, then at first externally supply -7V at its place, current limited, of course, and measure the current consumption of these 3 other ICs.If that is on the order of less than 10mA, U103 is probably defect.Check its functionality on a breadboard in a non-inverting amplifier configuration, and also check its input bias currents, if these are on the order of pA (or lesser than ~ nA), and if the output is able to pull -10mA at least. See datasheet for details.Frank

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 03:25:45 pm
content: I'm thinking this would be an easier test of U103, without desoldering it:Desolder one leg of the 5.1V Zener and use a DMM to see how much current is going through it. Anything significant means U103 isn't working. Yes?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2022, 04:06:59 pm
content: The -7 V are not really used as a reference for the ADC. They are used to shift the input signal, so that the actual ADC only sees a negative voltage. The actual ADC reference is the +5 V supply from a normal regulator for the logic. The divider for the -2 V used to measure the scale factor is directly from the zener. So from the noise perspective it is even worse - but this is only a 4.5 digit meter and not a 5 or 6 digit one. The better ones with a similar ADC (e.g. K193) derive the +5 V ADC ref from the reference at least.Lifting 1 pin of VR103 (5.2 V zener) is an measuring the current is another option, though it is still hard to tell how much current U103 is than delivering. So one would still not be sure if U103 is bad or not.One could try to measure the total current from the -10 V supply (e.g. turn the switch of and measuren the current there). There are not many other consumers at the -10 V. There should be no more than maybe some 10 mA for the -15 V. If U103 is OK it should take more like 20-30 mA alone to try to force the -7 V before it reaches its limit. So a low current consumption would mean a bad U103. With a high consumption U103 may still be OK.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 01, 2022, 04:16:48 pm
content: Quote from: MikeK on July 01, 2022, 03:25:45 pmNo.You will not learn anything from that.As said, the other components might draw too much current, even if U103 is working correctly.Frank

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 05:51:14 pm
content: But if I lift one leg of the 5.1V Zener and the op-amp output is zero it tells me that the op-amp is indeed faulty.

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 06:04:11 pm
content: I lifted the 5.1V Zener and the op-amp output goes to -3.4V,so it must have been providing some current. I measured the current through the 5.1V Zener at 20mA.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 01, 2022, 07:40:21 pm
content: Quote from: MikeK on July 01, 2022, 06:04:11 pmSee? That does not give a clear conclusion.Either one of these 3 other ICs draws too much current, or the TLC271 is defect on its output stage.It should deliver -30mA at least.I would remove the TLC271 and measure the supply current for the other ICs, first.Frank

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 08:07:39 pm
content: I was assuming that if the 3 other ICs where drawing too much that the 5.1V Zener current would be much higher. I'll pull the op-amp and test it.

author: MikeK
author_rank: Super Contributor
timestamp: July 01, 2022, 11:17:46 pm
content: I haven't tested the op-amp, but I may not need to. With it removed I provided 50mA at the op-amp output point (pin 6) and it still took it. That is, the other IC's are pulling at least 50mA, which is way too much. More searching...

author: MikeK
author_rank: Super Contributor
timestamp: July 02, 2022, 08:10:27 pm
content: I ran it for 10 minutes supplying 40mA and the three supplied IC's (U101, U102, U104) only got a couple degrees above room temperature. Using an inexpensive infrared thermometer. U101 was just a couple degrees above the other two. Not sure if that was enough current to see the culprit heat up. Temps higher still were seen in the area of Q109-Q113, but those don't appear involved in the -7V supply.

author: MikeK
author_rank: Super Contributor
timestamp: July 02, 2022, 11:30:21 pm
content: Found something promising. When I raised the -7V rail current to 70mA, op-amp U105 (another TLC271) heats up to 110F (43C). The other 3 IC's powered by the -7V rail stay at room temperature. The schematic says that U105's negative rail is provided by the -7V line! And a resistance check agrees with less than 0.1ohms between the two points. U105 is in the bottom-right corner of the image.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 03, 2022, 09:50:04 am
content: Good find, I missed that "-7" label.So then you got the stinker, probably.Check the B-C diode of the transistor behind it, Q10X (unreadable). A fail is quite unlikely , but who knows.Frank

author: MikeK
author_rank: Super Contributor
timestamp: July 03, 2022, 07:05:06 pm
content: I pulled U105 and the current draw dropped to 5mA. Cool. So what could have caused it to fail? The connected transistor (Q10x, 2N3904) tests fine, but I'm tempted to replace it as well. I tested the op-amps inputs for resistance to each of the other pins and they're all in the megohms range.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 03, 2022, 08:46:18 pm
content: Failure of U105 is likely not from the output side, but more from the input, e.g. from ESD at the input.Another possiblity would be some kind of latch-up, possibly from high energy radiation ( a muon hit or even stronger) - it is rare but can happen. Odd power supply sequencing (e.g. fast on/off cycles) may also contribute.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 03, 2022, 09:35:58 pm
content: Quote from: MikeK on July 03, 2022, 07:05:06 pmTherefore, the OpAmp broke down, of course not from its inputs, but from internal stages, because the negative supply draws too much current.This simply could happen from a timely deterioration.ESD, EOS and energy particles are extremely unlikely, compared to that.Please, simply replace this OpAmp and test the completed circuit, first.Frank

author: MikeK
author_rank: Super Contributor
timestamp: July 03, 2022, 10:03:01 pm
content: Yes, I ordered the op-amp from Digi-Key and should have it by Wednesday. I plan to install it and run the meter for a couple of hours before reinstalling U103 and the 5.1V Zener. I can set my power supply to trip and turn off at 30mA. That way I can see if something else was causing U105 to fail.Very excited to see this work. Thanks very much for the help, guys. Without the help I would have replaced the 6.4V reference Zener, saw that it didn't fix anything, and then given up. Definitely learned some things with this.

author: MikeK
author_rank: Super Contributor
timestamp: July 04, 2022, 01:33:33 pm
content: I noticed that Keithley changed the part spec for U105 between the 1983 and 1990 manuals. I wonder if this had something to do with it failing.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 04, 2022, 07:53:26 pm
content: The LF411 should be OK as well. The TLC271 would be a BOM simplification and lower power. Unless they use way more than 200 ms integration at a time the noise should not be a real problem, as the resolution of the meter is not that great.I don't think either of the OPs has an expecially high failure rate - both should be OK. Sometimes it is just bad luck.

author: MikeK
author_rank: Super Contributor
timestamp: July 06, 2022, 12:51:49 pm
content: My parts won't arrive today so I did some resistance measurements over breakfast, comparing the dead TLC271 (U105) to the good  TLC271 (U103). I think the damage happend via GND?

author: MikeK
author_rank: Super Contributor
timestamp: July 07, 2022, 07:20:41 pm
content: I replaced the parts today and it's working. I tested several ranges in Volts and Amps and it appears to be functioning as it should. Great! Thanks again, guys.

author: MikeK
author_rank: Super Contributor
timestamp: July 07, 2022, 07:22:04 pm
content: One thing I didn't mention before was this suspicious capacitor. C119. It has a black sticker on top for some reason. It's also much different than the schematic/BOM call for. BOM says 4.7uF 25V, but the installed part is 1000uF (!) and only 16V. The former owner's quick fix? Or is the larger cap needed because of the switch?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2022, 08:59:48 pm
content: 16 V voltage rating should be enough, as there is a 12 V zener to limit the voltage. For my feeling 4.7 µF may be a bit on the low side, but 1000 µF is quite large. It could add some stress to the switch from a current pulse. A somewhat larger cap may be needed if a weak battery is used. More capacitance could also help of the other 1500 µF capacitor in the other side of the switch is weal.