FIXED : Keithley 199 ohms readings never settle - Page 1
Forum section: electronics / repair

author: DIPLover
author_rank: Regular Contributor
timestamp: May 07, 2018, 07:42:49 pm
content: I just got a Keithley 199 for 40$ and although the body is a bit roughed up, the face which was covered in sticker residue (including the actual display area...?) cleaned up nicely with 99% alcohol and now looks new without even a scratch.I have started to compare it to my other meters to get a feel for it's calibration state and so far DCV and DCA are very good for the ranges I can test. (For example, after appropriate warm-up time etc. the 2.5V output of an AD584 reference reads 2.49945 vs my calibrated Keithley 196 which reads 2.499484, 10V out reads 10.0001 vs 10.00036 on the 196). Those are the ranges I use the most by far so I am quite satisfied.DCA on the 3A scale is also very close to the 196 so no complaints. I got this for the superb display and don't intend to make any critical measurement with it.But there is a problem with the ohms.I picked a series of 1% resistors in 18, 180, 1.8K, 18K, 180K and 1.8Mohm values and measured them with several meters.I compared the 199 to the average of all my other good meters, and to the 196 alone.The 199 always reads high and is off by 8%, 0.3%, 3%, 4.5%, 5.5% and 6.8% on the different ranges.That's a lot, but what is most alarming, is that the readings on the 199 never settle. Even on the 196, they will settle after several seconds down to the last digit. On the 199, the last 3 (sometimes 4) digits are never stable and will continue to change forever.Except when I measure the 1.8M resistor using the 30Mohm range. Then the value is bang on (within 0.012% of the 196) and stable.What could be the cause of this?****** EDIT on 2019-03-20 ******In the end my choice of 18 series resistor for the original test was very bad, since the calibration point for this unit are at 190, 1.9K, 19K etc. Thus being very close the errors didn't look too bad and I was more concerned with the jumpiness. It turned out all ranges were WAY out of wack except 30M and 300M.In the end I replaced transistors Q16 BJT NPN with a 3904 (it was leaky). This fixed the 300, 3K and 30K ohms ranges.I also ended up replacing Q11 N-Channel JFET with a pn4901 (failed short, all three pins reading the same 4.5+ Volts when it should have been closed and gate was receiving -14.8 V control signal). This fixed the 300K and 3M ranges.I also had to recalibrate all ohms ranges except 30M and 300M since they had been calibrated (some by me) with the faults.*************************************It was a fun repair and I owe my thanks to Kleinstein.

author: Vgkid
author_rank: Super Contributor
timestamp: May 08, 2018, 04:29:55 am
content: Have you tried using your K196 to measure the voltage across the resistor that is being measured by the 199. That could help point you if there is a current source problem. Likewise have you put a resistor across only the voltage terminals ou the 199 , in order to see if there is a leakage problem(doesn't seem like it is , but doesn't cause any harm).

author: DIPLover
author_rank: Regular Contributor
timestamp: March 02, 2019, 07:32:04 pm
content: Well, after a very busy year, finally got some time to investigate this properly...It turns out the Keithley 196 uses the same circuit as the 199 for measuring ohms, with the same reference voltage and resistor values, so we can use it to get expected values. (see attached simplified diagram).According to the manual, measured resistance should be equal to :Rref * Vsense-----------------VrefWhere Vref is 5 V and Rref depends on selected range :300Mohm : 10 M30 Mohm : 10 M3 Mohm : 1 M300 Kohm : 100 K30 Kohm : 30 K3 Kohm : 2 K300 ohm : 2KI setup a new test with a 5K precision resistor (4.9984K according to vendor).I measured the voltage across the resistor for both the 196 and the 199 for the different ohms ranges of both DMMs.Range ohmsOhms 199Voltage 199Ohms 196Voltage 196300 M000.004 M0.0025 V000.005 M0.0025 V30 M00.0049 M0.0025 V00.00508 M0.0025 V3 M1.19xxx M0.2525 V0.005006 M0.0252 V300 K123.xxx K0.2942 V005.0000 K0.2183 V30 K13.xxxx K0.2641 V04.99874 K0.6965 V3 K2.75xxx K1.1543 VOverflow3.019 V300overflow1.1543 VOverflow3.020 VI replaced digits with "xxx" in the table when they fluctuate and never stabilize.We can see that the two highest ranges on the 199 give proper stable readings with correct values and correct voltage.Voltage readings on 199 suggests effective VRef is measured as :~99 K instead of 1 M on 3 Mohm range~85 K instead of 100 K on 300 Kohm range~95 K instead of 30 K on 30 Kohm rangeWhat kind of failure could cause behavior like that?What should I investigate next?

author: GregDunn
author_rank: Frequent Contributor
timestamp: March 02, 2019, 07:39:55 pm
content: I had a similar issue with an HP 34702A. I would look at Q12; it seems to be the main current control for the ohms section and if it's leaky or damaged it could cause varying issues on multiple ranges.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 02, 2019, 07:57:40 pm
content: The problem could be due to a problem with the control signals (e.g. gate to Q3, Q11, Q12). There is also a chance that Q3, Q11 or U22 might be broken.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 03, 2019, 01:04:26 pm
content: Thanks guys, I have started by checking Q12 and its control, U42B (which is an LM339 quad comparator).Please see attached page of the actual schematic.Q12 is located in zone C4.U42 GND pin 12 is set to -10VU42 Vcc pin 3 is set to +5VU42 Input 1 pin 6 is set to +2.6V correctlyU42 input 2 pin 7 is controlled by the set Ohms function and rangePin 1 of U42 (Q12 gate control) vs range300Mohm : ON (-15V)30Mohm : ON3Mohm : ON300Kohm : ON30Kohm : OFF (0.5V)3Kohm : OFF300Ohm : OFFQ12 drain to source voltage vs range300Mohm : 0.0010V30Mohm :0.0010V3Mohm : 0.0010V300Kohm : 0.0010V30Kohm : 2.43V3Kohm : 1.98V300Ohm : 1.98VTo me this seems to indicate Q12 and U42 are working fine together.Or am I missing something obvious?I am a digital engineer and have very little experience troubleshooting individual transistors so any pointers as to what should be tested and how to test it is greatly appreciated.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2019, 02:22:19 pm
content: Some of the readings are a little odd: Assume the gate voltage of Q12, when ON should be at +5 V and when OFF should be at -10 V.The drain to source readings at least make sense and suggest this FET is working.In the 3 M range seems to provide too much current to the resistor. AS the 30 M range kind of works Q13 and Q3 seem to be able to isolate.Chances are that the extra current comes from Q13. The voltage-follower U20A could be a source a problem that would effect several ranges: too high a voltage here would cause gate current to flow.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 04, 2019, 01:51:45 am
content: Indeed I transcribed my notes wrongly.I was measuring pin 1 of LM399 relative to pin 12 (-10V)Correct Table is :300Mohm : ON (+15V) (i.e. +5 Volts from analog ground)30Mohm : ON3Mohm : ON300Kohm : ON30Kohm : OFF (+0.5V) (i.e. -9.5 Volts from analog ground)3Kohm : OFF300Ohm : OFFThe actual gate voltage, measured at the transistor side of R52 is300Mohm : 4.625V30Mohm : 4.625V3Mohm : 4.625V300Kohm : 4.625V30Kohm : -8.754V3Kohm : -8.754V300Ohm : -8.754V

author: DIPLover
author_rank: Regular Contributor
timestamp: March 04, 2019, 03:30:01 am
content: From the simplified schematic and list of Rref for the different ranges, I built a truth table for the transistors :Range Q12 Q13 Q3 Q11300 Mohm 1 0 0 030 Mohm 1 0 0 03 Mohm 1 1 0 0300 kohm 1 1 1 030 kohm 0 0 0 13 kohm 0 0 0 1300 ohm 0 0 0 1Q12 measurements agree with the TT. Moving on to Q13 as suggested by KleinsteinQ13 is controlled by U2A, another LM339. This one uses -15V for pin 3 (gnd)Measured U2 supply voltages :Pin 12 (GND) : -15.058VPin 3 (Vcc) : 5.005 VPin 4 is a correct +2.6 V reference voltage (measures 2.59078V)Pin 2 of U2 (Q13 gate control) vs range300Mohm : -15 from analog ground)30Mohm : -153Mohm : 0.310*300Kohm : -15*30Kohm : -153Kohm : -15300Ohm : -15Interestingly, Q3 is controlled by another comparator in the same LM339 chip (U2B).Pin 1 of U2 (Q3 gate control) vs range300Mohm : -15 from analog ground)30Mohm : -153Mohm : -15300Kohm : 0.362*30Kohm : -153Kohm : -15300Ohm : -15So although the supplies of LM339 designated U2 are correct, it seems incapable of giving astrong positive output. (Looking at the datasheet, I see It outputs logic low or High-Z and needspullups for logic high. R93 was an obvious pullup for U42, nothing like that for U2)R20D, R20B and R20A pull the outputs of U2 to pin1 of U20A, an LF442 dual opamp.Pin 1 of U20A vs range300Mohm : 0.391 V30Mohm : 0.3913Mohm : 0.313300Kohm : 0.36530Kohm : 0.3283Kohm : 1.436300Ohm : 1.436So U2 seems to operate correctly given what pullups it's got.V+ of U20A (pin 8 ) : +14.991 VV- of U20A (pin 4) : -15.057 VPositive input (pin 3) :300Mohm : 0.391 V30Mohm : 0.3913Mohm : 0.313300Kohm : 0.36530Kohm : 0.3283Kohm : 1.436300Ohm : 1.436Voltage follower U20A appears to do it's job. But why are the inputs so low?There is a lot of stuff going on that node, not sure I can trace it

author: DIPLover
author_rank: Regular Contributor
timestamp: March 04, 2019, 03:51:42 am
content: I notice Q5 and Q6 in zone D5 are on the same line.I do not know this configuration.Q6 has it's gate permanently wired to -4.3V and Q5 has both source and drain permanently to 5V.Since it's the closest I could find to a +5V on that line, I measured :Q5 gate voltages for ranges :300Mohm : 0.00218 V30Mohm : 0.002183Mohm : 0.248300Kohm : 0.28930Kohm : 0.2593Kohm : 1.134300Ohm : 1.134These are very close to the voltages measured across the tested resistor in post #3.

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 04, 2019, 12:32:51 pm
content: I had some strange ohms readings on my unit and it turned out to be Q16 - a protection transistor hung off the ohms line. Lift it out (it isn't necessary anyway)and see if anything changes.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 01:38:25 pm
content: I would not expect Q16 to be the problem, as the high ohms ranges seem to work, and there is too much test current in one range.My suspicion would be with Q11.The case in the 3 M range might give a clue: having only 0.25 V at the DUT (5 K) and reading 1.2 M would suggest the reference reading to the ADC is only at some 0.2 V, so much high and with quite a lot of voltage lost, as the sum is much below 5 V.Most of the swithes are also used in other ranges, e.g. as a voltage divider for the higher voltage ranges. So if these work correct, Q3 and Q13 should work Ok.Q5 and Q6 are used as a kind of low leakage diode. The readings suggest that not much current is flowing through R13 - which is a good thing. So Q5 and Q6 are not causing trouble.The LM339 has open collector outputs and can thus only draw the signal down. To turn on a JFET it needs zero source-drain voltage. This level is usually supplied through a high (e.g. 100K, the 1 M at the gate are more for protection during transients and to slow down switching) resistor. So when on the gate voltage depends on the signal level present.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 04, 2019, 08:54:26 pm
content: I had measured Pin 13 of U2, which is the Q11 gate control. For some reason I didn't post them...Pin 13 of U2 (Q11 gate control) vs range300Mohm : -15 from analog ground)30Mohm : -153Mohm : -15300Kohm : -1530Kohm : 0.325 V*3Kohm : 1.425 V*300Ohm : 1.425 V*Again the voltages are too low when the transistor should be ON.The pullup on this line is R20A that goes to the same R20A LF442 voltage follower output, like the previous R20B, C and D pullups.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 10:02:40 pm
content: Is the 5 V reference still OK at 5 V when a low Ohms range is selected. Too low a reference could explain some of the odd behavior.The control voltage to Q11 looks reasonable. There is just too little test current flowing in the low ohms cases.So maybe instead of testing a single point at different settings it might be better to set the meter to a non working range, like 30 K ohms and than measure the voltage at the points along the current path: the 5 V (VR), PIN10 and 11 of U22, drain of Q11 and the external 5 K resistor.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 04, 2019, 10:15:33 pm
content: Thank you for staying with me on this. I'll do as you propose and be back ASAP.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 04, 2019, 10:27:46 pm
content: ... and back ...Testing whole path on 30K setting5VR : 5.09707U22 pin 10 : 0.33376U22 pin 11 : 0.34678Q11 pin 1 : 0.33348External 5K: 0.2641

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 10:57:44 pm
content: So it looks like most of the current drops across the 30 K resistor. For some reason some of the current flows a different path and not through the 5 K. The 2nd odd point is that the meter reads 13 K (from 1st. post). This suggests the voltage over the 30 K resistor does not find it's way to the ADC. the way to the ADC is through U24. U24 C should be still On all the time. So the 0.347 V should also be at pin 11 of U24.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 05, 2019, 01:11:57 am
content: Indeed pin 11 of U24 reads 0.347 V when measuring the 5K resistor on the 30k range.wn1fju :Q16 emitter and base are at 0.00002 VQ16 collector is at 0.32833 V

author: DIPLover
author_rank: Regular Contributor
timestamp: March 05, 2019, 03:36:57 am
content: Pin 14 of U22D shows 5.09VPin 15 of U22D shows 0.334 VNo voltage drop across the 2K R26 resistor.The working Keithley 196 using almost the same circuit (please see attached schematic, it is much better quality)pushes 140 uA through the 5K resistor.Our K199 only pushes 53 uA

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2019, 05:01:00 pm
content: The voltage at Pin 11 of U24 suggests that there is not much current flowing this way and the signal is getting there.The next step in the voltage reading path (pin 3 of U24) might need a scope to watch. It looks like somehow the signal is not getting through from pin 2 to 3. This could be a defect in U24 or the control signal (pin1 of U24) is not right. However this signal might be switching between levels. So the first step would be checking if PIN 1 is AC or fixed. If fixed the DMM would be enough to check pins 1 and 2.For the current that is not coming to the 5 K resistor it might help to check with an infinite resistor (open). This may make it easier to see where the current goes, as the leakage would be the only path. Q16 is a possible path (though not as the only problem). Still a brake-down voltage in the 0.3 V range would be odd for a bad transistor. There are not that many other paths that could carry away some 100 µA.Q16 is used for protection as a low leakage diode, so for a test a different diode / transistor should be sufficient.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 05, 2019, 11:18:37 pm
content: hum dont want to barge in but it is the schematics for a 196 not a 199here's the 199https://doc.xdevs.com/docs/Keithley/199/199_901_01D.pdfEDIT : oh shoot they have the same measurement circuit my bad loll

author: DIPLover
author_rank: Regular Contributor
timestamp: March 06, 2019, 02:44:48 am
content: Measured U24A.Pin 2 is our familiar 0.347V DC signal.Pin 1 is not DC but a slow train of wide 5V pulses.Pin 3 passes the voltage in pin 2 when pin1 goes low.@ coromonadalix :It's OK they are not exactly the same, but K199 is clearly a reuse of the K196 circuit, with some simplifications. Also some of the switches are shuffled around a bit (for example K196's U24A is K199's U24C so the pin numbers are different) so the K196 schematic cannot be used alone. I attached the K199 schematic to post #5 in this thread.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 06, 2019, 11:31:45 am
content: I may need to recalibrate my 196, unless i face the same problem as thoe OP in the ohms, ac or dc volts it never totally goes down to zero (input probes shorted) few digits count, last cal was done in 93?I want to try understanding the data in my 196, how the pwm goes into the main logic system, but ive never done any of that, i have only a workbench logic sniffer in hand.Data, clock , strobe, and the pwm (measurements data)Would be nice to hack an 1kv input in to the meter, dont need to be (hyper precise say 0.5- 1% precision) to unlock an unused pin in the shifts registers,or even try to control the measurement board section with an arduino

author: Kleinstein
author_rank: Super Contributor
timestamp: March 06, 2019, 03:48:36 pm
content: THe signals around U24A look good (given the 0.334 V to start with). So U24 is likely good. So it is odd to get the wrong 13 K reading for a 5 K resistor. With to little current to the DUT one would normally expect to small a resistor to be read, more like 1.3 K.For the path of the current, my guess now is on Q16 - it would be a odd type of defect, but who knows. All the other switches seem to have high value resistors of 100 K or 1M in series - so no easy way to leak the missing 100 µA.There might be a possible leak at the Ohms sense input, but there is also a 100 K resistor.@coromonadalix:The zero for the ohms ranges is usually a separate parameter to adjust, that is often possible by the user and without a full calibration. It can be a problem with the low ohms ranges in 2 wire mode. With 4 wire mode there should be essentially no zero error.Adding a 1000 V or similar range has some danger to it. The other ranges of the meter don't look like they are well protected for a really high voltage. So some wrong handling could damage things. Also the switches may very well not up to 1000 V. If no high accuracy is needed, just get a cheap meter that does support the high voltage or build a separate meter (likely with a little less resolution) just for the 1000 V.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 06, 2019, 05:18:31 pm
content: I removed Q16 (a 5818 NPN)Voltage across 5K resistor with Q16 removed300Mohm : 0.00253 V30Mohm : 0.002533Mohm : 0.65407300Kohm : 0.7907330Kohm : 0.701853Kohm : 3.05337300Ohm : 3.05336All ranges below 30 Mohm now read Overflow.I then replaced it temporarily with a 3904 NPNVoltage across 5K resistor with Q16 replaced300Mohm : 0.00253 V30Mohm : 0.002533Mohm : 0.65405300Kohm : 0.7907130Kohm : 0.701843Kohm : 3.05335300Ohm : 3.05335Same behaviour.The 3K and 300 ohm ranges now show similar voltages to the K196 across the 5K.3M and 300M are much higher than K196.30K is approximately the same, but reads overflow.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 06, 2019, 06:02:01 pm
content: Things look much better now. Chances are no more current going the wrong way. The 2N3904 is not such a bad replacement, unless high voltage is applied. So currently no need for a exact replacement if the meter is not use on mains or similar.There is still the problem with the reading, which is likely from the reference voltage reading finding it's way to the ADC.However the path was test up to Q32, and a defect here is very unlikely as this would also effect most other readings.AFAIK the DMM is using the same ADC to take 4 readings: both sides of the DUT (sense Hi and Sense Lo) and the "reference" resistor inside the meter on both sides. For some reason it looks like reading of the reference resistor does not work well.One could try to use the scope to measure the voltages going to the ADC, e.g. behind the JFET mux (where Q30,Q32,Q34.. meet).The G1 signal used to drive the FETs (where all the 100 K resistors for the JFET gate drive comes from) could also be interesting - should be the same level but buffered.Is the meter still reading overflow with a 0 Ohms resistor ?

author: coromonadalix
author_rank: Super Contributor
timestamp: March 06, 2019, 09:15:11 pm
content: @Diplover if needed i could poke in my 196 to help ??I did the full reset command, meter seems better, had to read the manual how's to loll, and my probes are not good, tossed them for new ones.I'm on firmware b14 outch, will backup them and the calibration chip too, and do a b17 upgrade, and recheck the readings.This puppy will cost more than my recent 34401a calibration ??

author: macboy
author_rank: Super Contributor
timestamp: March 07, 2019, 02:20:15 pm
content: Quote from: coromonadalix on March 06, 2019, 09:15:11 pmI also received a very high quote for calibration of a K199. I don't know why. Fortunately, adjustment is simple to do manually, and this can be done for any desired function/range without needing to do all of them. Your recently calibrated 34401A may be just good enough to act as a reference for calibrating a K199, but maybe not for a K196 (if following metrology guidelines of >3x accuracy). Even so, adjusting the the Keithley to agree as closely as possible with the 34401A may be desirable. If the K196 isn't trusted and definitely requires adjustment, then that's what I'd recommend if you don't want to pay for a professional calibration. In my lab, I have adjusted my two K199's to match a K2001, and that's good enough for me.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 07, 2019, 03:13:03 pm
content: Kleinstein :Definitely not showing overflow measuring 0 ohmMeasuring a 60cm loop of banana cable300Mohm : 000.000 M30Mohm : 00.0000 M3Mohm : 0.00005 M300Kohm : 000.009 k30Kohm : 00.0012 k3Kohm : 0.00069 k300Ohm : 000.376 ohmReadings are even quite stable, except in 30K and 300K ranges, where they remain jumpy.Tried next with a big old 5W 51ohm resistor (meauring 51.69 according to my fluke 8050A)300Mohm : 000.000 M30Mohm : 00.0000 M3Mohm : 0.036327 M300Kohm : 004.010 k30Kohm : 00.4298 k3Kohm : 0.21392 k300Ohm : 116.589 ohm

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2019, 04:08:17 pm
content: With a current through the DUT that looks OK (at least with the 5 K). The off readings must be a problem with reading one of the 4 ADC conversions: Sense H +L , 5 V and ref. resistor low side. As the error is quite large it should be visible even with the scope.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 07, 2019, 10:17:31 pm
content: Quote from: macboy on March 07, 2019, 02:20:15 pmJust read the manual, can be calibrated as you say with the front panel, for the ohms calibration, it ask for 190ohm, 1.9k, 19k 190k, 100meg ohms values, damn i dont have thoses values.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2019, 08:46:53 am
content: A new calibration would only fix a possible problem with the scale factor. So resistors near the end of the scale reading wrong. Without a known good resistor as a reference one would not notice this.A wrong reading for a short is a different problem. For 2 wire Ohms there usually is a separate point for zero adjustment, that is simpler than a real calibration. This adjustment does not need a special reference - maybe a 4 wire short for a very accurate 0 adjustment for the 4 wire ohms ranges. If one knows how, a 100 Ohms resistor can make a good 4 way short.Unless one has really good reference values and knows well what to do, one should not do a full calibration with adjustment at home.Even than the normal way is to first just check of performance / accuracy. Only if really needed and the reference values are known good an adjustment is done. Just a check can get away with a less accurate reference, as the meter is not altered - so no harm done.

author: macboy
author_rank: Super Contributor
timestamp: March 08, 2019, 01:52:58 pm
content: Quote from: coromonadalix on March 07, 2019, 10:17:31 pmYou can enter the actual value using the front panel numeric keys, to override the default calibration point. For either Keithley 196 or 199 look at section 6.4.5 step 4, 8 and 10 in the respective manual from tek.com . In this way, you only need a stable reference, not an exact value. Connect the voltage to both the K196 and the reference meter (your calibrated 34401A). Enter the reading from the reference meter as the calibration point on the meter being adjusted. Similarly for resistance, first measure the resistor on the reference meter, then connect it to the meter being adjusted and use that value (don't touch the resistor as this will cause thermal shift in value). It is best to keep the reference at close to full scale on both meters, which isn't possible in this case since the 34401A is 1000000 count and K199 is 300000 count. So for the 3 VDC range on the Keithley, you could choose around 3 V (100% full scale) and measure with the HP in 10 V range (30% FS), or choose 1 V for 100% FS at 1 V range on the HP but only 33% FS on the Keithley. You could try both ways and see where you get a better result.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 09, 2019, 02:00:55 pm
content: I dont want to hijack the OP threadthks for the infos Kleinstein and MacboyChecked for precision foil resistances at 0.02% they all come around 12$ usd each minimum, 17 weeks lead time ?, they go as high as 39$ usd ??I'll do that when i'll be on version b17, just need to erase the eeproms and recap this puppy. I'm impressed with the low pin counts for the data transfers between the meter section and digital section. Love to see the isolation coils for signal couplingThe need is : >10megs impedance at 300mv and 3 volts, its a huge help for one sensitive system at my job, i dont need absolute precision.I could choose any resistors values near the end of the scaleSay : 300ohm, 3k, 30k, 300k, 3 meg and 30 megs (If i find them) since it overrange at 3.03 ?? its a full 3,030.000 counts display for many of the old 6 1/2 digits meters series like this.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 09, 2019, 11:13:36 pm
content: Quote from: Kleinstein on March 07, 2019, 04:08:17 pmBack on target, some more measurements.Testing previous path on 30K setting5VR : 5.09727 VU22 pin 10 : 0.87887 VU22 pin 11 : 0.89050 VQ11 pin 1 : 0.87878 VExternal 5K: 0.70187 VU20A pin 2: 0.87423U20A pin 3: 0.87384Next looked at Q32 (see attachements)Note actual frequency on Q32_pin3 is 5.7Hz like all others. (176ms period)and A/D Input buffer amplifier U46 (see attachements)The 20% gain on what I understand to be a unity gain buffer, is worrying.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2019, 10:58:25 am
content: The signals so far look good, though a little odd with the long Zero phase at Q32 pin3. However this is more like a software oddity and should not be something that breaks. It looks like all the 4 signal levels (5 V, 0.9 V, 0.7 V and 0 ) get through to the ADC.The gain of the ADC input guard buffer is indeed odd. I first though it could be due to loading by the scope, but the voltage looks like 6 V_pp, so more than going from 0 to 5 V reference. It not just a simple buffer, but coupled to the bootstrapped supply for the amplifier.So the next points to check might be the amplifiers for the bootstrapped supply of the input amplifier. In the K196 schematics this is U37A+B.This could also effect other ranges, if measuring from a positive higher impedance source - but this may not be such a common case in normal operation it is only -3 V to +3 V, while in Ohms mode it is up to 5 V and possibly with high high impedance. The effect might be visible as extra input current in the 3 V range with 2-3 V at the input.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 10, 2019, 11:37:23 am
content: This whole region is much simplified on the K199.Please see attached page 3 of the schematic.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2019, 12:23:56 pm
content: The K199 circuit is indeed much simpler. The extra gain is also trivial to see: there is the 1.1 K resistor in series to the OPs (U46) output and the feedback from there. So the gain is limited to directly at the output of the chip. So nothing wrong here.So far it looks all normal from the analog side. This makes me think that the wrong readings could be due to a corrupted calibration for the ohms ranges. So maybe someone tried an ohms calibration with the defect hardware - this would have corrupted the calibration and would now give wrong readings.Ideally the calibration would be just a single constant per range (the reference resistor value). So even if the calibration is wrong it should still be linear. So one could do a short linearity test on one of the ranges (e.g. the 30 K range). Something like values of around0 / 100 / 500 / 1 K / 5 K / 10 K / 20 K .If the readings are still linear, I would consider a wrong calibration.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 10, 2019, 08:00:22 pm
content: I think you are on to something...Tried with some 1% resistors and identical leads on both meters.Remember previous tests with precision 5K showed Overflow on K199.Linearity test on 30K ohms rangeohms K199 K196 Factor0 : 00.0013* K 00.00027 K 4.815100 : 00.8310* K 00.10035 K 8.281510 : 04.1876* K 00.50569 K 8.2811 K : 08.2864* K 01.00045 K 8.2833 K : 24.8115* K 02.99537 K 8.283Linearity is very good.By this factor, the 5K would measure over 40K which explains the overflow.The last 2-3 digits are still very jumpy on the K199 whereas they are stable after a few seconds on the K196. This was my original concern (the readings were much closer to reality in spring 2018 than when I reactivated this thread in winter 2019).Could that be just an averaging setting on the meters? I will check.For calibration, I understand I could measure some resistors in the R30 series with the K196 and just enter the exact values in the K199 right? I don't need super precise magical resistors or calibration standard?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2019, 08:42:22 pm
content: The resistor used for calibration would set the scale factor. So it would effect the accuracy. The reading from the K196 could be a good value.Doing calibration would delete the old values. I don't know for sure if one can do a calibration of the ohms ranges only. The instructions should tell.The scattering could be due to the speed settings. The extra factor from the scaling would also apply to the noise - so a higher noise is also expected.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 10, 2019, 08:54:04 pm
content: The K196 is in cal so it's values should be good enough for the 199. I bought the K199 mainly for monitoring with the great display and GPIB, no critical measurements will be made with it, but still, I want it as accurate as possible.I am buttoning-up the K199 (what splendid construction, really a joy to work in, kudos to whoever did this work at Keithley in the late 1980s).I will investigate the Calibration process. If I can do just the ohms, I will do that as the DCV and ACV readings were already spot-on (need to re-check now).Did try using the front panel filter and it cleans up nicely.Will report back later.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 11, 2019, 03:01:19 am
content: Me too i'm impressed for this kind of meters "all the brown boxes of this era"

author: DIPLover
author_rank: Regular Contributor
timestamp: March 11, 2019, 03:37:07 am
content: Alright, we are getting there.Fortunately, the meter allows calibration of single ranges of single functions.Manual asks for R19 series resistors for calibration (probably because suggested calibrator gives these values)But you can enter your own calibration point easily, so after letting both meters warm up for 3 hours, I carefully measured a range of R20 series 1% resistors using the K196 and proper 4-wire hookup in the low ranges.Did the 300 ohm, 3K, 30K, 300K and 3M ranges using 200 ohm, 2K, 20K, 200K and 2M respectively.Now for the good news :1. My original problem, jumpiness of readings is SOLVED.The meter now converges reasonably quickly to a stable reading on all ranges.2. The 300, 3K, 30K ranges are working fine across their full scale. 30M and 300M were already good, I did not touch them.The bad news is the 300K and 3M are only good at the calibration point (200K and 2M).Their error factor is not linear, but makes a nice curve whose amplitude is higher for the 3M range.Some raw data :K199 measurements on 300K range :ohms K196 K199 factor200 200.1315 000.357 K 1.7842 K 1.995296 K 003.538 K 1.77320 K 20.10081 K 033.272 K 1.65551 K 50.5125 K 075.213 K 1.489200 K 198.7545 K 198.756 K 1300 K 299.9202 K 244.916 K 0.8166K199 measurements on 3M range :ohms K196 K199 factor200 200.1315 0.00179 M 8.9442K 1.995296 K 0.01777 M 8.90620 K 20.10081 K 0.16720 M 8.31851 K 50.5125 K 0.37790 M 7.481200 K 198.7545 K 0.99867 M 5.025300 K 299.9202 K 1.23055 M 4.103510 K 0.509145 M 1.51577 M 2.9771 M 0.994468 M 1.80842 M 1.8082 M 2.014896 M 2.01504 M 13 M 2.959862 M 2.08937 M 0.706OPEN Overflow 2.26766 MK199 measurements on 30K range :200 ohm : 00.2001 K2 K : 01.9954 K20 K : 20.1015 KSo there is something common to these 2 ranges but unused by the lower and upper ranges that is still broken.I will get to enjoy another disassembly it seemsWhat those 2 ranges have in common is the use of R17B, which is controlled by Q13. I don't really know what to look for however.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 11, 2019, 05:11:41 pm
content: The 300 K and 3 M range look like there is still some resistance in parallel to the DUT. So a little like the original broken Q16, but to a lesser extent.Is the 30 M really working ? (e.g. checked at at least 2 points, like 100 K and 10 M).This could be the replcemant for q16, if there is one. As this is current toward the negative side we can exclude Q9.One of the FETs that are used as switches could also leak towards the gate. This would only effect those switches that are off. So Q13 abd Q3 are not candidates, but more like Q11. Q34 should be on when in 2 wire ohms mode.One might be able to see the current at the 1 M resistor at the gate (e.g. R4b).Chances are the broken part is also at the same input / line as the broken Q16 was.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 11, 2019, 06:26:10 pm
content: 30 M range is working quite well with the old calibration :K199 measurements on 30M range :200 200.1315 00.0001 M2K 1.995296 K 00.0019 M20 K 20.10081 K 00.0200 M51K 50.5125 K 00.0505 M200 K 198.7545 K 00.1987 M300 K 299.9202 K 00.2998 M510 K 0.509145 M 00.5092 M1 M 0.994468 M 00.9945 M2 M 2.014896 M 02.0149 M18.2 M 18.21961 M 18.2193 M (2x 9.1M in series)

author: DIPLover
author_rank: Regular Contributor
timestamp: March 11, 2019, 06:30:32 pm
content: Here is page 1 of K199 schematic so we can have it embedded.Q9 is not populated on K199.Q16 is the one we replaced with a 3904.300M range is as good as 30M.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 18, 2019, 02:34:58 am
content: Finally made a man out of me and attacked this systematically.I printed 5 copies of the schematic page and highlighted the active signal path for each range.Then I started looking at each transistor and its control while measuring a 1M resistor with the 30M and 3M ranges (which had the most similar signal paths, differing only in Q13 being ON for the 3M range and OFF for 30M).I started from the right side of the schematic.Q35 was OK.Q32 was OK.Q33 was OK.It appears Q11 has failed short. All three pins read +4.68 V (30M) or +4.56 V (3M) even though it should be off in both cases. Control pin U2p13 properly reads -14.8 V in both cases.Q12 is OK.Q3 is OK.Q34 is OK.Q13 is OK.Q13 gate is -13.4 V and control U2p2 is -14.8 V for 30M range. (OK, should be off)Q13 gate is 4.3 V and control U2p2 is 4.7 V for 3M range. (OK, should be on)Now to find a proper replacement for Q11, which is Keithley part number TG-128, which I have seen in other threads is a pn4392. Unfortunately now discontinued. Pinout is Drain(1), Source(2), Gate(3). TO-92 case.Vgd 40 VVgs 40 VIg 50 mAPd 625 mWI don't have ANY jfet on hand.Closest thing I have is 2N7000 MOSFETs. Pinout is Source(1), Gate(2), Drain(3). Could be twisted. TO-92Vdss 60 VVdgr 60VVgs 20 V (plus or minus), 40 V peak.Id 200 mAPd 350 mWCould that work in a pinch? What do you suggest as a pn4392 replacement?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2019, 07:12:00 am
content: Q11 failing makes absolute sense. A MOSFET replacement would not work, as the control signal is wrong and the mosfets have an internal parasitic diode in parallel. If only used for the ohms part and thus one polarity, there would be a small chance a depletion mode MOSFET could work, but these are quite special parts - not sure if TO92 parts exist.So it should be a JFET. The exact type is not that important and for the pinout drain and source are usually interchangeable (most JFETs are symmetric and no difference between drain and source). Using wires and a SOT23 JFET could be a last resort. Otherwise check the TO92 JFETs available at your source. One wants low gate leakage and at least a 30 V (better > 35) voltage rating.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 20, 2019, 02:27:23 am
content: PN4392 are discontinued and hard to find, but Digikey had PN4391s in stock for a dollar apiece.Specs are quite similar, except for IDss and Vgs(off) which are doubled.I figured the higher Vgs(off) would be OK since the LM339 (U2 pin13) sends -14.8V when the transistor should be off, well above worst case Vgs(off) of -10 V.So I ordered some yesterday, got them today and replaced Q11.Some measurements looking for linearity of 300 K and 3 M ohm ranges.Cold start, open case without shields, 2 wire measurementsK199 measurements on 30K range :200 ohm : 00.2003 K2 K : 01.9958 K20 K : 20.1028 KK199 measurements on 300K range :ohms K196 K199 factor200 200.1315 000.083 K 0.4152 K 1.995296 K 000.828 K 0.41520 K 20.10081 K 008.348 K 0.414200 K 198.7545 K 082.525 K 0.415300 K 299.9202 K 124.409 K 0.415510 K 0.509145 M 211.459 K 0.415OPEN Overflow Overflow KK199 measurements on 3M range :ohms K196 K199 factor200 200.1315 0.00005 M 0.252K 1.995296 K 0.00057 M 0.28620 K 20.10081 K 0.00579 M 0.288200 K 198.7545 K 0.05730 M 0.288300 K 299.9202 K 0.08640 M 0.288510 K 0.509145 M 0.14684 M 0.2881 M 0.994468 M 0.28686 M 0.2882 M 2.014896 M 0.58108 M 0.2883 M 2.959862 M 0.85381 M 0.288OPEN Overflow Overflow MK199 measurements on 30M range :200 200.1315 00.0002 M2K 1.995296 K 00.0020 M20 K 20.10081 K 00.0201 M200 K 198.7545 K 00.1987 M300 K 299.9202 K 00.2998 M510 K 0.509145 M 00.5092 M1 M 0.994468 M 00.9947 M2 M 2.014896 M 02.0150 M3 M 2.959862 M 02.9618 MQuite promising, I think we nailed itNow stitching up the patient and letting it warm up before doing a new calibration of the 2 ranges.K196 is already warm and ready to go.

author: DIPLover
author_rank: Regular Contributor
timestamp: March 20, 2019, 05:05:36 am
content: SUCCESS!The Keithley 199 is now good on all ohms ranges, across their whole span.And no more jumpiness, all readings stabilize very quickly.HUGE THANKS to Kleinstein for his guidance!!Thanks also to everybody else who took the time to read this topic and/or offer advice.I have (re)learned a lot doing this repair, it was well worth it and good fun.I copy this here from edited first post, for future reader jumping directly to the end :****** EDIT on 2019-03-20 ******In the end my choice of 18 series resistor for the original test was very bad, since the calibration point for this unit are at 190, 1.9K, 19K etc. Thus being very close the errors didn't look too bad and I was more concerned with the jumpiness. It turned out all ranges were WAY out of wack except 30M and 300M.In the end I replaced transistors Q16 BJT NPN with a 3904 (it was leaky). This fixed the 300, 3K and 30K ohms ranges.I also ended up replacing Q11 N-Channel JFET with a pn4901 (failed short, all three pins reading the same 4.5+ Volts when it should have been closed and gate was receiving -14.8 V control signal). This fixed the 300K and 3M ranges.I also had to recalibrate all ohms ranges except 30M and 300M since they had been calibrated (some by me) with the faults.*************************************

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2019, 06:44:57 am
content: Quote from: DIPLover on March 20, 2019, 02:27:23 amThe gate voltage would be relative to the signal voltage. However it looks like there are no voltages outside +-5 V expected here - so the PM4391 should be OK. Also the -10 V V_of are only the extreme values.One might still consider replacing the 2N3904 with something a little more powerful: this transistor is for protection and probably already failed the first time causing the broken Q11. AFAIR the original transistor was a little high current. Still nothing to do in a hurry.