Agilent 3458A repair - Page 1
Forum section: electronics / repair

author: franklin
author_rank: Contributor
timestamp: March 07, 2020, 01:53:28 pm
content: Hi. This is my first time on this forum so I may be making some mistakes.I just got an Agilent 3458A very cheap and I knew it had an error. The unit is from 2013. After switch-on this happens:Switch-on: ERROR 110 CALIBRATION REQUIRED -- ACALACAL: ERROR 205 HARDWARE FAILURE, TEST VALUE OUT OF RANGE: 108ERROR 210 CALIBRATION REQUIREDERROR 101 CALIBRATION ERRORI left the meter ON for a few hours with a 10 VDC reference connected and it showed approximately 7.8 V. At some instance it started to display correct voltage (10 V) which lasted for about 30 minutes when the display reverted to 7.8 V. Reversing input leads to -10 V displays -9,25 V.I’ve checked the most obvious, the voltages to PCB A1 and they are +5, -18.5, +18.3, and -9.9 VDC. I’m thinking the error is caused by A1 (A1 manufactured 2013) after reading TIN’s work but I’m very much in need for advice where to start. I have only a DVM as tool to help. DCI and OHMS are not working properly, that is the values are not right. I believe there must be a common denominator. By the way, meter temperature is 7 degrees; obviously way off.I’d be grateful if someone could put me on the right track.Thanks

author: TiN
author_rank: Super Contributor
timestamp: March 07, 2020, 02:41:06 pm
content: What about self-test?

author: franklin
author_rank: Contributor
timestamp: March 08, 2020, 09:37:57 am
content: The selftest at switch-on displays this:TESTING RAMACAL DCV, OHMS RTESTING HARDWAREERR is on, ERR 110, “CALIBRATION REQUIRED – ACAL”

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2020, 10:24:02 am
content: There is also a dedicated self test to be called. This is more extensive than the power on test.Expect a few more error messages than from the failed ACAL .

author: franklin
author_rank: Contributor
timestamp: March 08, 2020, 12:01:49 pm
content: Thanks for the answer. I don't think I've come across this dedicated self-test in the manuals; I may have overlooked that. Is there a procedure for this self-test?

author: coromonadalix
author_rank: Super Contributor
timestamp: March 09, 2020, 03:08:52 am
content: check this youtube done by " the signal path "...... very good infos

author: franklin
author_rank: Contributor
timestamp: March 11, 2020, 11:29:13 am
content: I think I misunderstood the thing about SELFTEST. Hopefully I’ve done this right this time. The self-test goes like this:ROM TESTRAM TESTSLAVE TESTTEST DCV 10V (duration about 3-4 seconds)SELF TEST FAILEDERROR 205, Hardware Failure, Test Value Out of Range: 108I’ve downloaded the CLIP but the copy is not very clear and the writing is rather small. I’m not at all sure where to start. I’m guessing PCB A1 but don’t have a clue where. For those that may be interested in pictures I’ve enclosed images of the boards. The DMM is as far as I can see just like new both outside and inside. There was no dust on the filter or the fan blades so I’m wondering has it ever been used? I’ve not discovered any scratches or other signs of mishandling so I’d say it’s in “as new” condition. The display is bright and for those interested, I’ve swapped the snaphat batteries on A5 from 2001 (2,88 V) to 2019 (3,40 V) just to be safe. See images.The last calibration was in 2015 so I suppose it has been in service at some time and in an environment with little dust. When the DMM is fixed (if it’s possible) I would end up with a “brand new” 3458A. But for now, I don’t know where to look for any problem components.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 11, 2020, 01:44:58 pm
content: There is a button on the front panel, labeled TEST.Please press the blue button and then this TEST button for extended test.Have you ever attempted to perform an ACAL ALL?PS: seems to be an intermittent error, maybe a temperature fault of a component.Try different NPLC settings, especially short aperture like 10..100µs, if this problem persists. Also check 10V in 100V range, and lower anges (1V, 100mV).You may track the DC signal (10V) through input circuit to the multiplexer, DCV low voltage input (sheet 4/5), then beyond the MUX towards the input amplifier sheet 3/5, TP100, and further on to ADC board.. there exist two differnt paths, a slow, and a fast one, depending on aperture.Do Ohm mode and ACV sync modes work?PPS: Sorry that I stitched that schematic so badly, years ago, but there was nothing better available.You can zoom the pdf, though.Frank

author: franklin
author_rank: Contributor
timestamp: March 11, 2020, 06:00:17 pm
content: I’ve just tried the extended test. Here’s the result:ROM TESTRAM TESTSLAVE TESTTEST DCV 10V (duration about 3-4 seconds)SELF TEST FAILEDERROR 205, Hardware Failure, Test Value Out of Range: 108ACAL ALLERROR 205, Hardware Failure, Test Value Out of Range: 108ERROR 210, Calibration requiredERROR 101, Calibration errorDCV1 VDCRange AUTO, displ. 068…Range 1 V, displ. 068…Range 10 V, displ. 068…Range 100 V, displ. 1.0..10 VDCRange AUTO, displ. 7.77….Range 10 V, displ. 7.77….Range 100 V, displ. 10.000 00..Range 1000 V, displ. 10.000 00..DCISeems to work fine (disregard the March 7 text)ACV12 VAC (the only voltage available) works fineOHMSRange AUTO, does not displ. OV.LD (with no input), displ. stuck on 21 kOhmRange, manual, does displ. OV.LD (with no input)Does not measure correct values of resistorsI will track the DC signal as you suggested later on.About the CLIP: I’m actually not complaining just stating facts as I see it (my vision is not the best) and it’s the only CLIP I’ve found so I thank you for that.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 11, 2020, 07:12:13 pm
content: So obviously you are already tracking down the fault.. it's very close.It will be at / before the MUX, i.e. problem with the driver or the FET of the 0.1 / 1 / 10V channel, which is used for Ohm also.DCI, ACV, 100V/1kV are all using different ones, as you can see for yourself on the schematic.PS: I forgot the smileyswhen I mentioned the stitching of the schematics.. no problem whatsoever, I'm always happywhen it's of use.I have to drive right now to get some cheap gasoline, and I'll be back later for further analysis.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 11, 2020, 08:34:37 pm
content: Please check first, if DCV 10V reads something useful, when AZ OFF.Then check, if in DCV 10V mode, the input draws excessive current, should be about -2pA only.Then check if -14.7 and +14.7V of the input protection circuit is ok.Apply Uin=10V at input in DCV 10V mode.If AZ ON, then use a scope to check U11A, its steering voltage pin5, should be 0 or +5V, then FET driving voltage pin 2, should be -20V or about Uin, everything in comparison to U8C in 100V range, which seems to be ok. If AZ Off, you should measure steady values with a DMM there, i.e. +5V for pin5, about Uinfor pin2.I suspect, that either the driver U4 is faulty, (temperature effect) or CR10 (?), this CC diode.Maybe there was an overload which damaged the input protection, therefore the check of these +/- 14.7V.Frank

author: franklin
author_rank: Contributor
timestamp: March 14, 2020, 01:41:01 pm
content: Sorry about the delay on my part. Just recovered the PC after a crash.AUTOZERO OFF.Instrument displays 7.8 V (+10 V input to instrument).Excessive current: I can measure down to 10 nA but not 2 pA. However, it seems ok since there is no obvious change in the measurement when connected or not.+14.7 V and -14.7 V are both measured to +15,0 V and -14,9 V. It seems there are two zeners, CR6 and CR8 (not quite sure about that) but I don’t know if this increased voltage is significant (maybe not).At U11A, pin 5 is +5 V and at pin 2 it’s 7.8 V (same as display; +10 V input to instrument).When in 100 V range, U8C pin 9: +5 V and pin 14: 0 VWhen in 10 V range, U8C pin 9: 0 V and pin 14: -20 V.I’ve located U4 (16-pin right?) and CR10 on the print but not in the schematic. I guess that there is a system/logic in the schematic which I’ve not yet fully recognized. What would I expect to measure at U4 and CR10?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 14, 2020, 06:58:09 pm
content: Hello franklin!This pA measurement is done as follows:put a short on the input jacks (short clamp or cable)Set DCV, 10V (fixed range), AZERO OFFFIXEDZ OFF (default) => 3458A has GOhm input imepdance by defaultremove short and watch the display, reading should go slowly downwards, at a rate of about -0.01V/sec. The input bias is about Ib~ 200pF x dU / dtexplanation: The input has a capacitance of 2 x82pF (C22, C23), plus some pF from the PCB, so about 200pF. The bias / leakage of the input MUX / amplifier @ AZERO OFF will charge this capacitance with typically -2pA and so you can easily estimate this bias by observing the rate of the voltage.If this input path had been damaged, i.e. Q26, Q27, or Q13, you would probably see an increased rate, maybe also of another polarity.These -14.9 and + 15.0V are fine.Another possible failure mode is the breakdown over temperature of one of these CD4094 shift registers.These steer all kind of FET switches, and my unit, which was then 9 years old, had a similar failure message as yours, but only over 45°C inner temperature (on a hot summer day), because of a defective 4094.U4, pin 11 directly steers the comparator, pin 5, and so switches FET Q13 on and off.These signals seems to be ok, I assume you have measured with a DMM and AZERO OFF.If AZERO ON, Q13 is continuosly switched on and off, as Q17 will make alternatively zero measurements.When in 100V range, with 10V applied, you should measure about 0.1V at pin 14, because this is the 10V divided by 100. Sorry to have described that incorrectly.-20V at pin 9 is fine, you should see also -20V at U11A, pin 2, when in 100V range.CR10 is this strange component with an arrow inside a circle, sitting directly at a the output of U11A, and serving as a fast pull up current source for the OC of U11As output.The voltages seems to be ok.So now please apply 10V in DCV 10V range, and measure the voltage at label "DCV", or at this input side of R17, with another DMM. There you should see these 10V.Please measure also behind R18, i.e. at the two points indicated in the schematic by the green arrows.Watch the display of the 3458A also, when you measure at these two points.If that reading is not ok, then we will have to inspect the input relays circuit, especially K3, which feeds the input voltage to R17. These relays also sometimes fail.Frank

author: franklin
author_rank: Contributor
timestamp: March 15, 2020, 06:14:20 pm
content: Hi FrankI did find the CR10 finally; I was just looking for the symbol for a CR and to start with, didn’t recognize this as a diode.pA measuremenet.I might be doing something wrong here since there is no -0,01V/s change; it’s about -7 µV in 15 minutes.Now, the DCV input was connected to 10V (see schematic); 8.89V after R17 (display shows 7.79V and then drops 40 mV when measuring); 7.79V after R18 (display shows 7.79V and then drops 70 mV when measuring). About R18, can’t see the designation on the PCB but it’s adjacent to R17 so I guess I’m right. You will of course notice that the voltage 7.79V is the same as the display. Looks like there is an excessive current draw of about 220 µA. Should I suspect Q13 or Q26/Q27 for pulling down voltage?How come that you are so familiar with this instrument and have all this experience if I may ask such a question?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2020, 07:23:41 pm
content: All 3 paths are possible: Q26, Q27 and Q13. As a first test one could measure the gate current at Q13, though this is only one relatively unlikely path.The fault is not necessary with one of the fets, the actual fault could be bind Q13 too, so the current drawn somewhere there.220 µA are enough that this should be visible as a voltage drop across Q13. The on resistance should be in the 20-80 Ohms range - so a measurable voltage if the current flows there.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 15, 2020, 09:29:20 pm
content: Hello franklin,something is drawing the input towards 0V, not towards these +14.7V, neither -14.7V.As the input is @7µV after 15min, that's another indicator for leakage to ground.It's quite improbable that this happens behind Q13, because the 100V range works correctly, and there is a 9.9MOhm resistor in series with the input, instead of only 10kOhm.You could at first check, if Q13 is leaky, by measuring the voltage across its gate resistor R13.220µA * 2k = 0.44V should be well visible.The idea to measure across Q13 from S to D is also fine, but I don't expect anything here.You can do another test, to rule out that the fault is behind Q13: What happens in current mode DCI, if you connect nothing and switch from highest to lowest range, i.e. 1A down to 100nA?Is there an overflow, or 220µA, or simply near zero reading?If all that gives no result, then try something odd:The only components which could draw the voltage exactly to GND are these 82pF capacitors C22 and C23. From the direction of the current flow, C23 would be suspect.multi layer ceramic capacitors can be mechanically damaged => cracks, maybe also by over voltage, and then they might also have high leakage currents.So what happens, applying 10V again, if you tap gently on C23, or cool it down a bit with ice spray?PS: Well I've got an education as an electronic technician in a German Air Force calibration lab 40 years ago (already) where I repaired and calibrated hundreds of all kind of multimeter gear, mostly hp.That habit I kept at university, where we also had a warehouse full of old, abandoned instruments, which I repaired plenty. So I learnt a lot especially from the old hp schematics.I also bought 'my' first hp3458A in 1989 for my PhD studies, but I did not have the CLIP at that time.When I finally got my really own 3458A in 2009, I dug deeply down into the CLIP at that time, and also thanks to TiN, I think I have now understood most of the schematics of the 3458A.Same story goes for the 34401A, which I also bought in 1989 for the university institute, and also another instrument myself, as a reward for my diploma. That 30 years old instrument still works fine, and I was able to help others to repair their 34401A remotely.Frank

author: franklin
author_rank: Contributor
timestamp: March 16, 2020, 01:43:23 pm
content: Hi.Q13: there is a 11 mV drop between S and D if I have measured correctly (AUTOZERO OFF, 10V range and 10V input.Q13 gate resistor R 13: drop about 6 µVCurrent mode DC, 2358A displays: 1A range: 30 µA; 100 mA range: 3 µA; 10 mA range: 0,5 µA; 1 mA range: 40 nA; 100 µA range: 12 nA; 10 µA range: 1 nA; 1 µA range: 0,1 nA; 100 nA range: 30 pACeramic capacitors, 10 V applied, 10 V range: tapping or ice-spraying C 23 does not affect voltage at R 17 and R 18

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 16, 2020, 02:56:22 pm
content: So it's necessary to search for one of the FET switches not working properly.Please check all comparators, if they receive the correct steering signals , and if they all produce these -20 / + Ubootor GND levels for driving their respective FETs, i.e. U7A-D, U8A-D, U20A-D, U11C-D.I don't see any other possibilities, where this current should be sunk.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2020, 04:42:18 pm
content: 11 mV over Q13 and some 270 µA would be some 40 Ohms - about right for JFET switch like 2N4392.6 µV over R13 is too little to explain the current.So the problem is likely to the right of Q13.A point to check would be TP10, the voltage to control most of the FET gates.Q14 would be possible candidate for failure, as it is relatively exposed to the outside.A possible failure mechanism could be JFET gate at something like Q14 to Q15 to break down at some 28 V (- 20 V at the gate and +8 at the input).Checking the current ranges is a good idea, however this is with a very small voltage. The problem seems to be more like with higher voltage only. This could also explain why 10 V in the 100 V range is working. This would lead to some 0.1 V behind Q13.So I see nothing suspicious from the current readings.For a test one could also check if -10 V work.

author: franklin
author_rank: Contributor
timestamp: March 17, 2020, 08:13:39 pm
content: I tested with reversed polarity (-10V) and to sum it up, display on 3458A shows:-10 input: -9.2….V . +10V input:7.8….V.So, there is a difference but I can’t see why it should be.For U7A-D, U8A-D, U20A-D and U11C-D I’ve made the measurements and hopefully got it right. For the values, see a copy of schematic 4 (of 5) enclosed.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2020, 08:41:20 pm
content: The signal at U8A looks suspicious. This should be more like -20 V at the output. So there is a good chance that U8 is broken - which is good as this is a pretty standard part. One could douple check with a scope or the DMM in AC mode to see if the odd reading is some AC effect, but it does not look like, as the 7.8 V are the expected on level.With more current flowing the JFET switches may not be fully symmetric anymore. This can lead to the asymmetric error.Also the broken LM399 may still be able to sink a limited amount of current and thus still have some effect with a -10 V input.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 18, 2020, 12:39:25 pm
content: To me, U8A is strange, these 0.6V instead of exactly zero, as well as U7D, that it has 5V, as I would expect that it should be zero, but I don't know, why the 1V output, divided from the temperature sensor is used during DCV measurement.So their respective steering Shift register U9 could be faulty also.Do you see any activity or oscillations on the respective outputs of U9?Could be, tat you measure these voltages only due to some multiplexing going on, despite AZERO OFF (I assume, that you measured in that state).If it wouldn't be that big, sensitive PCB, I'd say at this point, just replace the 4094 and the comparator chips.Maybe you can do very carefully another test, if the 3458A then reads 10V.As these comparators are OC, it's safe to short pin2 of U8 to pin 12 (-21V), to shut off Q15, and to short pin13 of U7 to its pin 12 (-18V) to shut off Q21.Frank

author: franklin
author_rank: Contributor
timestamp: March 18, 2020, 12:45:41 pm
content: Thanks for all the info. I have a question about CR10 and BOOT; I’m not aware of what BOOT imply here but guessing it might be a “sort of” ground. The reason for asking is if I understand schematics page 4 correctly, CR10 cathode is connected to A11 pin 2, +7.88 V in my case. There is only a few mV drop over CR10 so I’m thinking it’s not OK. What do you think?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 18, 2020, 01:35:28 pm
content: Hello franklin,there's a Guard Ring around many of the different FET switches and resistors, i.e. that whole dotted line as indicated, it's usually used for cancelling any leakage currents on the PCB.This guard is held at the signal voltage behind the MUX chain by amplifier Q7A, Q7B, and U12.I suppose that hp therefore called this 'boot' like bootstrapping.So it's a virtual voltage follower of the measurement signals, but you can draw some current out of it, because of U12, in contrast to OpAmp virtual grounds (i.e. at - input in inverting configuration, or + input of non-inverting configuration) where you shall not draw any current.I can not decide, whether CR10 is ok, or not, out of your observation. Reason is again, that its anode is connected to this virtual boot potential, therefore latter just follows the output of FET Q13, so the cat bites its tail. As this FET seems to be ok, a defective CR10 would either fully shut off this FET, or would fully open it, and that seems not to be the case.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2020, 02:12:14 pm
content: As far as I understand it CR10 is a constant current diode, so actually a selected JFET with gate connected to source. So essentially no voltage drop when the switch is on is correct.I support the idea of using a wire bridge at U8 to bring the gate of Q15 to -20 V. One may have to use a switch for this, as the turn on self test could need Q15 to be turned on.

author: franklin
author_rank: Contributor
timestamp: March 19, 2020, 09:07:01 pm
content: I see I’ve overlooked several things on the schematic; boot is one of them. I’m still learning (I hope). I understand that CR10 is a constant current diode (regulator?); to me it looked just like another diode although the symbol is different.I think I don’t have the necessary overview to understand how U8A influences the measurement but are you suggesting I should FORCE the U15 gate to -20 V by a direct connection to -20 V?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 19, 2020, 09:27:19 pm
content: Hello franklinWell, I'm also still learning from your case.Yes, CR10 is a constant current diode, or FET. As the 10V channel is able to digitize @ 100kHz, this boot has to follow as fast as this, so they needed a fast pull-up component also.We still have not identified the failure, so at the moment it's simply looking for oddities and then trial & error.To my understanding, U8A should not be engaged if you are using AZ OFF, and the level should be a straight zero volt. U7D is strange as well.The outputs of the comparators are Open Collector, so it should be possible to force their output to the negative supply voltage.Maybe you use a 1..10k resistor for safety reasons, and take care not to produce a short to other pins..Do you have an oscilloscope to check the steering signals on all of the comparators and FET switches?Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2020, 09:48:05 am
content: When reading low DC volts Q15 / U8A should be set to off (that is some -20 V from U8A). The low voltage at the input (though not all the way to 0 ) also suggests that the 4094 sends the command to off. Seeing 7.8 V at the output suggests that U8A is actually not sinking significant current, as this is voltage read and likely the voltage from the BOOT buffer.So the readings at U8A alone (inputs and output) highly sugggest that U8A is broken.If in AZ mode Q15 would still be off. The AZ mode usually (maybe except for reading the internal reference) uses Q11 / Q12.For the current ranges and 100/1000 V Q15 is on - so these ranges could work OK even if A8A is broken.There is a gate resistor for Q15, so there is no real risk in typing the output of A8A to the -20 V supply of the chip. There may be a extra error message on boot up though. I doubt one would need a switch to enable the connection only after boot up. The test would be more to see if there are additional defects.One could even do a test with just an extra LM339 just put directly on top of (in parallel) the broken one. Chances are this could fix the error. With open collector outputs the 2 chips would not interfere with each other. The comparator is only used as a kind of level shifter.

author: franklin
author_rank: Contributor
timestamp: March 22, 2020, 02:39:31 pm
content: Hi.Things have been moving VERY slowly here for a couple of days but I will try to catch up.I’m sorry, I don’t have an oscilloscope and as of now I cannot borrow one either. I’m not aware of what to look for regarding these steering signals. As for the LM339, I don’t have one at the moment but will try to get one. First, I will try to connect Q15 gate to -20V through a 10 k resistor and see what happens.Just applied -20V to U8A pin 2 (easier to access than Q15) and 3458A display jumped to 10V.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 22, 2020, 02:50:55 pm
content: If you order parts, also buy some CD4094, as these are most suspicious, due to the probable temperature error you've once seen. You may also order appropriate precision sockets, because the 339 as well as the 4094 are not inside the analoge circuit, only on the digital side.These specific strange voltages you see with your DMM may be caused by short 'normal' pulses, so an oscilloscope would tell you directly, what really happens.That way, it's really very slow to analyse and track down the fault.Frank

author: franklin
author_rank: Contributor
timestamp: March 22, 2020, 03:18:40 pm
content: I’ll see what I can do about the scope. Just tried the -20V to Q15; applied -20V to U8A pin 2 (easier to access than Q15) and 3458A jumped to 10V. About the 4094, LM339 and sockets, I’ll get that as soon as possible.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 22, 2020, 03:31:38 pm
content: Wow, first sign of the root cause!Please try the similar with U7D, i.e. connect its pin 12 to pin 13, I think...

author: Kleinstein
author_rank: Super Contributor
timestamp: March 22, 2020, 04:57:52 pm
content: The voltage at U7D looks a little odd too, but more like not turned on all the way. The control signal from the 4094 is high, so the comparator should be off at the output. It is only that the voltage is only 3.8 V and not all the way to 7.8 / 10 V. The lower voltage could still be due to gate current towards the 1 V level and a relatively high resistance FET for Q22. So this could as well be OK if U8 works.

author: franklin
author_rank: Contributor
timestamp: March 22, 2020, 07:40:56 pm
content: A7D (AUTOZERO OFF, RANGE 10VDC): Connected pin 12 (-18V) to pin 13: no reaction on 3458A display; should there be some reaction?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 22, 2020, 09:36:33 pm
content: Not necessarily. Could have been..This only excludes a fault in this comparator.We're still looking for the sink of these 220uA, aren't we?maybe another test, carefully cool down slightly the U8 and the shift register, whether you can reproduce this supposed temperature errorFrank

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2020, 07:29:00 am
content: The path for the 220 µA should be through Q15 (that should have been off) and Q22 (intentionally on) to the 1 V test-signal.So it is possible that the comparator U8A is the only fault.When ordering parts I would still get at least a 2 nd LM339, some 4094 and maybe replacement electrolytics for the supply.With U8A still turing on Q15, I would have expected a change when Q22 is turned off. When Q15 is already forced off, there should be essentially no change (maybe some pA range change in the input current).

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 23, 2020, 10:01:15 am
content: Quote from: franklin on March 22, 2020, 07:40:56 pmMaybe I have misunderstood your reply.Have you also applied the 10V at the input, when doing this test?Does the 3458A read these 10V correctly, when you connect pin 12 to pin 13?

author: franklin
author_rank: Contributor
timestamp: March 23, 2020, 11:34:12 am
content: First, I must correct this: “A7D (AUTOZERO OFF, RANGE 10VDC): Connected pin 12 (-18V) to pin 13: no reaction on 3458A display”.There IS a reaction and display show 10V! I must have been unconscious when doing this test.Also cooled down U8 and U9 but only a fraction of mV change so I feel I can rule out the temp. problem. I guess what happened in the beginning was only a coincidence.With a limited knowledge and overview of the 3458A, I’m starting to think this:1. There is no current leak through Q26/Q27 pulling the 10V down.2. The current (220 µA) must go through Q13 somehow. Something on the output of Q13 might be pulling voltage down.3. Applied -20V to U8A pin 2 again; BOOT (see CR10) raises to 10V plus some µV (haven’t tested that before) and U11A pin 2 raises to 10V.4. U8A pin 5 is 0.15V and U7D pin 13 is 3.8V; strange. I’m not at all aware of what voltages should be in this case.3458A read these 10V correctly, when I connect pin 12 to pin 13

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2020, 12:18:07 pm
content: Q13 is turned on as it is supposed to, so nothing unusual there.Q15 is supposed to see -20 V at the gate to be turned off. However U8A is bad and Q15 is on instead.Q22 is supposed to be on (control from U7D) and thus sends the 220 µA towards the 1 V auxiliary voltage. In addition with so much current, there is some gate current at Q22 and this brings the voltage at U7D to the slightly unusual 3.8 V level instead of some 7.8 V.Under normal operation Q22 is not supposed to be on with much more than 1 V at the boot signal - so the extra gate current would not be there.Applying the -20 V to U8A out / gate of Q15 turns of Q15 and things are OK (at least for low DCV).Applying the -18 V to U7D out turns off Q22 and things look OK too, though having slightly more leakage currents.So chance are high that U8 is the culprit and this could be the only fault.

author: franklin
author_rank: Contributor
timestamp: March 23, 2020, 12:53:37 pm
content: Q22; do you mean Q21?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2020, 12:56:54 pm
content: Quote from: franklin on March 23, 2020, 12:53:37 pmI meant Q21 - the picture with the voltage readings is not very legible.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 23, 2020, 05:23:23 pm
content: I have drawn the failure current in the schematic.Q15 and Q21 are both closed (active), so the input voltage appears across R8, which usually is the attenuated output of the temperature measurement sensor.Q21 therefore should definitely be open (turned off) for all modes, apart from TEMP?I'll check this, and the sensor output voltage later.You may check whether U9 causes the problem, by applying a 470 Ohm resistor from pin 4 (Q0) to its pin 8, GND. That should pull the comparator input below 2V, and shut off Q21, if the comparator U7D is ok.That something is not correct with this channel, is supported by the fact that TEMP? delivers 7°C only. Maybe the 4094 is defective and scrambles the serial data, or has problems with certain configurations.Q15 might also be off when AZERO OFF, but maybe that's the case only when the 3458A is put in a DIGITIZING state, to remove any unwanted capacitance.This 0.15V input on U8A, if it's really a steady state, i.e. no short pulses, should also switch Q15 off, so I will check these levels also.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2020, 06:51:00 pm
content: If Q15 is off, here is no real problem of having Q21 on. It is a little odd as there would be gate current, but nothing dramatic. Q21 would give the MUX part below Q15 at least a defined voltage in stead of letting it float. So there may even be some though behind it.Having Q15 during low voltage DC measurement would be not only for less capacitance, but also for less leakage. So it absolutely makes sense to have Q15 off.The error in the temperature measurement is still a good point to check.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 23, 2020, 06:51:15 pm
content: Surprise, surprise!It's working the other way round:Q15 is (always) off, separating the lower MUX part from the signal path.AZ is obviously realized with Q23, Q24, as there is no difference with AZERO ON.pin5 of U8A is exactly zero Volt, like all comparators U8A-D and U7A-C.Then this modus operandi explains, why 100V and DCI are working correctly.Q21 is always on, obviously its Gate diode is conducting in forward direction. U7D, pin 11 has 5.0V signal. The T-sensor U21 is NOT powered on its pin1, obviously only switched on briefly for TEMP?Therefore R8 (13.5k) pulls the MUX bus towards zero, plus this odd level at, and current through the Gate of Q21.Please check accordingly with a 470 Ohm resistor between U9 pin 12, and GND on pin 8, whether the shift register is faulty, or U8A.I can't imagine, what happens, if U8A draws excessive current out of output Q6 (pin12), whether this could also influence the level of its Q0, or what might be the failure mode of this shift register.But I'm pretty sure, that either of both ICs is causing the trouble, very probablyU9U8 is it.FrankPS: Yes, in both DCI and 100V range, Q21 is turned off, i.e. 0V at U7D, Q15 is turned on, +5V at 8D, and either U7A / Q18, or U8C / Q16 is turned on, 5V at either comparator input.So you can also check these voltages, whether U9 delivers these correctly.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2020, 10:03:47 pm
content: The 100/1000 V range could be a way to check the lower MUX with Q16-Q21.Az switching in most cases should be with Q12 and Q11 alternating ON/OFF. Q11 has most of the "negative sides" to choose from and Q12 is the switch with all the extra effort to keep charge injection / switching peaks small. It also has the extra precharge path with the BOOT signal. So I would expect additional short pulses with Q10 on too.

author: franklin
author_rank: Contributor
timestamp: March 25, 2020, 10:18:36 am
content: U9: 470 Ohm resistor from pin 4 to pin 8 (U7D, p11); 3458A displays 10V470 Ohm resistor from pin 12 to pin 8 (U8A, p5); 3458A displays 10VChecked TEMP? for both cases above: Still about 6-7 ⁰CU21: Reacts to cooling. Voltages: se enclosed schematicU7A, p5 (to Q18): 0VU8C, p9 (to Q16): 0VBy the way, haven’t been able to provide for a scope yet.

author: BU508A
author_rank: Super Contributor
timestamp: March 25, 2020, 10:55:05 am
content: How about renting one?Example:https://www.4test.no/Oscilloscopes/Economy+1000X-2000X/Oscilloscopes/

author: franklin
author_rank: Contributor
timestamp: March 25, 2020, 11:29:31 am
content: Not so easy; KORONA you see.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2020, 12:01:49 pm
content: The signals seem to be static, so an oscilloscope is not required anymore.The change of U21 is irrelevant at the moment, why only 7°C is measured, we will see later, when the MUX is working correctly again; seems to be a timing problem with the faulty U8A / U9, maybe.This one "470 Ohm resistor from pin 12 to pin 8 (U8A, p5); 3458A displays 10V" I did not expect, as I supposed that U8A is defect and tries to pull U9 towards 5V.Anyhow, I see no possibility anymore from further exterior measurements to decide whether U9 or U8 is defect.Maybe if you measure these voltages, as requested :For DCI mode: U7A, p5 (to Q18): ?For 100V DCV mode: U8C, p9 (to Q16): ?So best would be if you replace both ICs, please.Have you already ordered, and do you have appropriate desolder-station?Frank

author: BU508A
author_rank: Super Contributor
timestamp: March 25, 2020, 12:27:36 pm
content: @FrankI have a question about these U7 / U8 (LM339): which one do you recommend?Here is a list from Mouser:https://www.mouser.de/Semiconductors/Amplifier-ICs/Analog-Comparators/_/N-cib1w?P=1z0z63x&Keyword=LM339&FS=TrueThanks.

author: franklin
author_rank: Contributor
timestamp: March 25, 2020, 12:31:34 pm
content: I’ve ordered the U8/U9 IC’s and sockets; supposed to be here on Monday 30th.I’ll check the voltages you requested; still AUTOZERO OFF?

author: franklin
author_rank: Contributor
timestamp: March 25, 2020, 12:52:11 pm
content: For DCI mode: U7A, p5 (to Q18): 0V (AUTOZERO ON or OFF)For 100V DCV mode: U8C, p9 (to Q16): 5V (AUTOZERO ON or OFF)I don’t have a desoldering station only a temp-controlled one and a handheld tin-pump. Thought I might cut the legs of the IC’s and solder them out one by one or is that a bad idea?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2020, 01:20:40 pm
content: Quote from: franklin on March 25, 2020, 12:52:11 pmSorry, obviously I accidentally tested U7A inACImode, as DCI enters differently, via Q20 and Q22.U8C level is fine, so U9 might be fully ok.If you have no better tools, carefully cutting the leads is the least stressing way for the PCB.Maybe you begin with U8 first.It's a pity, though, because you could otherwise test these ICs on a breadboard, what has been damaged.. just for academic purposes.I'm now looking forward to next week, if that's been the culprit.Frank

author: voltsandjolts
author_rank: Unknown
timestamp: March 25, 2020, 01:26:10 pm
content: I would recommend to cut the legs as close to the DIP package as possible.Attack it with snips vertical.Just to minimise risk of PCB damage...use nice small snips that get tips between legs, not electrician pliers!

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2020, 01:28:02 pm
content: Quote from: BU508A on March 25, 2020, 12:27:36 pmThese are uncritical parts, I would NOT recommend the ceramic package.Mouser specifies the cheapest for 30V only, that might be a typo. The others are leadfree, obviously, so that should not have influence on the max. power supply.Datasheets are linked incorrectly, so I can't check the parts from mouser.T.I. has no wrecked the link themselves.Anyhow, 30V max. is enough, 36V would be better.Offset and bias don't play a role.leadfree parts can of course be soldered with lead containing tin.Frank

author: BU508A
author_rank: Super Contributor
timestamp: March 25, 2020, 01:42:28 pm
content: Just for completeness, here is the link to the documentations available from Texas Instruments:https://www.ti.com/product/LM339/technicaldocuments

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2020, 01:45:33 pm
content: yeah, also found it, no datasheets available either, and whole 339 family seems to to have 30V Vcc.Really old stuff.

author: BU508A
author_rank: Super Contributor
timestamp: March 25, 2020, 03:34:53 pm
content: Found the datasheet, the one from Texas Instruments, added to this posting as PDF

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2020, 03:40:13 pm
content: The normal Vcc is limited to 30 V, but AFAIK the outputs are open collector and can stand a voltage higher than VCC. The LM399 are kind of the standard part to control JFETs switches and commonly used with more than 30 V at the outputs.I would first exchange U8. Form the measured voltages it is essentially for sure that U8 is bad and chances are it could be the only fault. So far I see no problem with U9, it seems to work just as it is supposed to.

author: franklin
author_rank: Contributor
timestamp: March 25, 2020, 05:29:05 pm
content: For KleinsteinIn one of your posts you wrote:“One could even do a test with just an extra LM339 just put directly on top of (in parallel) the broken one. Chances are this could fix the error. With open collector outputs the 2 chips would not interfere with each other. The comparator is only used as a kind of level shifter.”I’ll hopefully receive the IC’s I ordered on Monday 30th. Piggy-backing an LM339 would be a quick test so I’m inclined to try that before doing something major.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2020, 06:39:07 pm
content: I guess, paralleling another 339 is a bad idea.The expected failure mode of U8 is that its + input of 8A draws excess current, therefore putting a 339 in parallel would not give an additional hint other than with the 470 Ohm resistor.There's danger to create a short, so I would not try this experiment at all.Frank

author: franklin
author_rank: Contributor
timestamp: March 25, 2020, 07:05:29 pm
content: Duly noted

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2020, 07:41:11 pm
content: The measured voltages suggest that U8A has some positive current at the + input. To get some 150 mV at the 4094 output this would be a few mA at most. The signal level is still OK.At the output U8A is likely not drawing any current. The observed 7.8 V are just what comes from the BOOT signal. I very much doubt U8A would have a fixed 7.8 V level at the output, just the same as the reading. Drawing the pin to -20 V also works with the resistor - so not much current needed.Anyway U8 is broken - so one could as well remove it carefully and have a socket for a new chip (just in case there is something odd that kills U8). It is very unlikely to do extra damage removing U8 - so the piggy pack test is not really needed and the old U8 has to go anyway.

author: franklin
author_rank: Contributor
timestamp: March 27, 2020, 04:10:46 pm
content: Just received the parts ahead of time (nice). Removed U8 and U9, put in sockets and new ICs. It worked. I’m impressed, not of myself but of the lot of knowledgeable advice on this forum. I don’t think I had pulled it off myself. Now, the error I got is “ACAL required” so it will be interesting to see after letting the meter run for a few hours after performing ACAL.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 27, 2020, 05:19:18 pm
content: franklin, hip hip hooray!!What a great news in difficult times!It's always great fun when this community solves such problems remotely, especially on these famous 3458A.So thanks also that you stayed persistent on failure finding.After ACAL ALL, please repeat this leakage test, to briefly check whether the input path is ok.As said, if you leave the instrument open in DCV 10V range, it should typically ramp downwards from zero with that given rate, i.e. about -2pA.Another way is simply setting the 3458A to FIXEDZ ON, that gives 10MOhm constant input impedance, 1V range, check zero reading with input short (should be about < 1µV), and then let it settle with open jacks. -2pA then should give about -20µV reading.Again, congratulation for your repair!Frank

author: franklin
author_rank: Contributor
timestamp: March 27, 2020, 05:49:10 pm
content: I’ll do the pA test both ways. My previous message was a little premature though. After ACAL, I get ERROR 204 HARDWARE FAILURE – DC VOS DAC CONVERGENCE 64. However, the voltage (10V) is correct. I’ll have a look at what this message means or do you have any hints?

author: franklin
author_rank: Contributor
timestamp: March 27, 2020, 07:45:05 pm
content: When instrument is switched on, I got ERROR 204 HARDWARE FAILURE – DC VOS DAC CONVERGENCE 64. When running ACAL, all is OK. Now I’m starting to get suspicious; what is happening? Switched the meter off and started again. No error; ACAL and all OK. Switched the meter off and started again. No error; ACAL and all OK. It seems everything is working fine but I didn’t like how it all started. I was even checking if I had forgotten something but could not discover anything suspicious. I’ll let it run for a while and see what happens.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 27, 2020, 08:21:09 pm
content: maybe the ACAL constants have had a hickup only from the foregoing HW failure, which is healed now, after two ACAL.Let's see, how it develops.And maybe TiN can give his opinion, please, as he's got most experienceFrank

author: franklin
author_rank: Contributor
timestamp: March 28, 2020, 05:19:59 am
content: Tested temp; 35,2 ⁰C.- DCV, 10V (fixed range), AZERO OFF, FIXEDZ OFF (GOhm); 2 pA test: checks out to about -0,01 V/s- FIXEDZ ON, 1 V range (10 MOhm): Zero reading < 1 µV; raises to about 60 µV (short removed).

author: Kleinstein
author_rank: Super Contributor
timestamp: March 28, 2020, 09:19:46 am
content: The 60 µV at 10 M input resistance corresponds to some +6 pA. The drift rate is more like -1 pA, so a little confusing, but the configuration is not exactly the same, so there can be differences. This are good values - 2pA is more like wishful thinking, best case, and the sign is likely not the same with all meters. AFAIK the specs are < 20 pA with no defined sign.A second point would be to repeat the tests with AZERO on at some 10 PLC or more. AZ on is the normally used mode. Due to charge injection during AZ switching this may show a different input current.The test using the drift rate can also be used at other voltages, not just near zero. So one could start with some + or - 9 V applied for some time and than watch the drift over a few minutes. The change in input current with input voltage defines the differential input resistance.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 28, 2020, 10:15:03 am
content: Quote from: franklin on March 28, 2020, 05:19:59 amThese values of -2pA and +6pA for the different methods are completely fine, so the DCV input path for the 10V, 1V and 100mV is obviouslynotdamaged.Remark: The AUTOZERO induces current spikes on the MUX, (which are only imperfectly compensated), therefore AZERO OFF gives the least bias current, which is reproducible and typical -2pA for different instruments.These measurements require that the 3458A is warmed up for several hours, if I remember correctly I also found such different bias currents with the 10MOhm method when measuring 'cold' and 'warm'.In our internal Metrology group, we compared 3458As and confirmed the findings of other authors, like Rado Lapuh in his book 'Sampling With The 3458A', where he also describes several methods for determining the bias current, typically -2pA again, and other parameters which are important for digitizing.The bias current measurement around zero volt gives a mostly constant value, i.e. a linear voltage drift dU/dt for the capacitor method.This voltage drift will saturate (stop) somewhere at several negative volts, in my case at about -5V.In the end, you have to pre-charge the input capacitor to about that value, iterating from more negative or more positive values, otherwise this measurement will take forever.This then gives you the impedance of the input, which is specified in the datasheet to be > 10GOhm.The bias current on the other hand is only indirectly specified to be < 20pA.This impedance is mostly determined by the impedance / leakage resistance of the input jacks, the input switch, the PCB material, whereas these -2pA are given by the leakage of the FETs of the MUX and of the FET Q103A inside the pre - amplifier.So in my case I determined an input impedance of -5V/-2pA ~ 2.5*1012Ohm, or about 2TOhm, which order of magnitude Lapuh also determined for several of his instruments.Frank

author: franklin
author_rank: Contributor
timestamp: March 28, 2020, 10:44:14 am
content: Test AZ ON and 10 PLC; raises to about -55 µV (short removed). About the previous 60 µV; it should be -60 µV.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 28, 2020, 11:20:45 am
content: So the 10MOhm test is also done with AZERO ON?Then both methods agree. Great!You can also use the command APER 1, instead of NPLC 50, which would only do 1 AZ phase per measurement cycle, whereas NPLC 50 does 5 phases, as it's doing 5 * NPLC 10 in reality.That may also reduce the charge injection.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: March 28, 2020, 11:57:02 am
content: That both cases with AZERO on and AZERO off agree is good. With the error message about DC offset DAC trouble I slightly feared that this could be one of the DAC used for the BOOT voltage offset of the charge injection compensation could have a fault. If both readings essentially agree this means that this part working really well.For testing it would be more like using 1 PLC only, and maybe the 100 mV range. This would increase the switching effects even more.

author: franklin
author_rank: Contributor
timestamp: April 03, 2020, 06:47:11 pm
content: Just started testing drift (inspired by TiN and Keysight Service Note 18A). I guess “CAL 2,1” displays the A9 reference calibration voltage and not the present value since the reading is fixed. I’ve been running it for only four days so far and it’s still running. This will be interesting and it’s something new to me just as the 3458A is.I understand that the display should be kept OFF while the meter is ON for prolonged period to prevent wear and tear of the display. For this I’ve programmed one of the keys with “DISP MSG” and that does the trick for me but I guess this is old news.TiN has some interesting views on oven temperature; lower it and get lower drift. When doing that I understand the reference voltage seem to raise when die temperature is lowered. Does this say something about oven temp when voltage at my reference A9 is 7,20741? Could it mean that there IS a lower oven temperature in my meter?CAL? 2,1 (A9 voltage) = 7,20748898; since this doesn’t change with time, I suppose it is the reference voltage when it was calibrated. It matches closely to what I measured on A9 while instrument was open.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2020, 08:09:16 pm
content: The interesting parameter is CAL78 , and one needs to ran ACAL (at least part of it) to get an updated value.After no being used for longer time (e.g. a few weeks), it may need a week or two to stable again (likely removing humidity from some parts).For the reference oven the absolute reference voltage has only very little correlation with the actual temperature. I know of no easy way to directly read the oven temperature. The heater voltage may be a crude starting point, but the heater resistance also varies.The variation in heater voltage with an external temperature change could in theory give a hint.Reducing the oven temperature is a 2 sided thing and also includes disadvantages:The voltage calibration is lost.The TC may change and get larger (the refs are likely screened for low TC at the given temperature).For the first some 12 months there can be more drift, as the reference is settling to a new target.The old ref is likely well aged to the set temperature and thus relatively stable by now.

author: franklin
author_rank: Contributor
timestamp: April 04, 2020, 06:59:45 am
content: About oven temp; I’m not going to change that but was just interested in a possible correlation between temp and reference voltage. I understand there is a correlation but I guess nothing like that I have a lower oven temp due to a slightly higher reference voltage.I got interested in that CAL? 78; what does it represent? All this, like CAL? 78, CAL? 59, CAL? 2,1 etc., where can I find this kind of information? Can’t say I’ve seen that in any of the manuals or maybe I’ve overlooked it. Is there somewhere to find info that’s not covered by the manuals?

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 04, 2020, 09:33:10 am
content: Quote from: franklin on April 03, 2020, 06:47:11 pmHello Franklin,you are checking theshort term driftof your 3458A, following SN18.The relevant parameter for you is CAL? 72, that is the DCV gain 10V auto calibration constant, see table of cal constants in the calibration manual, pages 65 - 71.Do an ACAL DCV once a day and afterwards read this constant CAL? 72, and the internal temperature TEMP?.This constant should not change too much over one week if the A/D chip U180 is ok, AND if the inner temperature is always the same.Latter aspect is missing in the SN18A.Please provide a table of these values after one week, or so.Thelong-term driftdescribed in SN18A, of the LTZ1000A, you can not measure, as you are probably lacking a stable and precise external 10V reference.The external CAL constants 1 and 2, that are the 40kOhm and 7V reference values, are fixed and are set only during an external calibration, therefore can not change by an ACAL, see calibration manual. So your idea about that constant is correct.The LTZ1000A reference is between 7.0 to 7.5V, typically 7.2V, see datasheet.Therefore you can not judge from the value itself, on which temperature the oven is running.The circuit w/o heating has a T.C. of about +55ppm/°C, a reduction of the oven temperature would thendecreasethis reference voltage by about 12mV, i.e. to about 7.19586 V.You could as well not determine a modification of the oven temperature from that.HP, agilent and Keysight always delivered references with 15k over 1k, i.e. 95°C oven temperature, so a modification would have to be done by the former owner of your instrument. Simply look at the reference board, if an additional resistor in parallel to the 15k is assembled, like in my unit, that big 100k. In the picture you made of your instrument, there seems not to be any modification.If you would do this modification, your instrument would loose its DCV calibration.PS: unfortunately, our colleague Kleinstein yesterday posted mostly incorrect statements, please ignore all about this.Frank

author: franklin
author_rank: Contributor
timestamp: April 15, 2020, 06:31:24 pm
content: Hi Dr FrankThere is no additional resistor in parallel to the 15k on the reference board on my A9 and I didn’t expect that. About the modification; I’m NOT going to modify A9, I’m happy with the present condition. Besides, I under¬stand that having it done results in an expensive calibration which I want to avoid.Here is the result of the measurement you suggested; seems OK to me. They’re not 35,7 ⁰C all of them but 35,8 is close enough for me. It’s hard to keep a stable temperature in an ordinary house.Date / Time DMM Amb. RH CAL? 72 Deviation(dd-mm-yyyy/time) (⁰C) (⁰C) (%) (CAL const.) (PPM)29-03-2020/2045 35,7 1,00125178 Ref31-03-2020/2045 35,8 23,4 1,00125174 -0,0401-04-2020/1010 35,7 23,3 1,00125179 0,0102-04-2020/1335 35,7 22,6 38 1,00125187 0,0904-04-2020/0845 35,8 23,2 33 1,00125174 -0,0405-04-2020/1545 35,7 23,3 34 1,00125176 -0,0207-04-2020/1215 35,7 24,0 36 1,00125177 -0,0108-04-2020/1555 35,7 22,8 38 1,00125184 0,0609-04-2020/1835 35,7 23,6 36 1,00125173 -0,0510-04-2020/1920 35,7 23,1 38 1,00125173 -0,0511-04-2020/0830 35,7 23,8 37 1,00125188 0,1011-04-2020/0905 35,8 23,5 37 1,00125176 0,0211-04-2020/2135 35,8 23,2 37 1,00125181 0,0312-04-2020/1130 35,8 23,0 39 1,00125194 0,1612-04-2020/1730 35,4 23,1 37 1,00125182 0,0412-04-2020/1845 35,6 23,1 36 1,00125191 0,1313-04-2020/1720 35,7 23,5 30 1,00125182 0,0414-04-2020/1630 35,8 23,7 34 1,00125179 0,0115-04-2020/1045 35,8 23,2 35 1,00125186 0,08When doing this test, I was running ACAL DCV a lot of times and more than what the list indicates. Something interesting happened and that was the error message which turned up every 5th to every 25th run. Made me wonder if it’s coincidental or maybe a latent error somewhere. The error message is always:- ERRSTR 204, “HARDWARE FAILURE - - DC VOS DAC CONVERGENCE:61- ERRSTR 210, “HARDWARE FAILURE - - CALIBRATION REQUIRED- ERRSTR 101, “CALIBRATION ERRORNext run and there’s no error. What’s your view on this.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2020, 07:02:31 pm
content: The CAL?72 values look very good. So there is likely no problem with U180.The message “HARDWARE FAILURE - - DC VOS DAC CONVERGENCE:61" suggest that there is some iterative method adjusting an offset. The parts may be boarder line here so that sometimes it does not find a good setting or get a residual error close to the edge. So there may be something like an offset that is a little high or DAC problem.The service manual may give a few more details which DAC / offset is in question.

author: TiN
author_rank: Super Contributor
timestamp: April 15, 2020, 10:35:13 pm
content: Most likely that error related to elantec parts on A2 AC board, but my guesstimate may be wrong.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 15, 2020, 10:52:24 pm
content: Hello Franklin,happy to hear again from you.. good messages.. U180 also to my opinion is fine.This sporadic failure, I agree to Illya, maybe caused by the ELANTEC comparator chip.As long as this does not occur too often, simply don't bother.Try keeping the instrument as cool as possible (it's probably a heat problem).Please keep the fan cleancool, so that the inner temperature does not rise further (I got it at the same level)Maybe you would like to become a volt-nuts, also?Stay healthy!Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2020, 12:18:21 pm
content: For the cooling fan, one may want to check / clean the air filter, if not already done.

author: franklin
author_rank: Contributor
timestamp: April 16, 2020, 01:13:00 pm
content: Hi KleinsteinChecked the cooling fan and filter when I got the DMM; no dust at all just as if it was never used.

author: franklin
author_rank: Contributor
timestamp: April 22, 2020, 04:53:56 pm
content: Hi.Still running the stability test here.Now, when reading the Operating Instructions for Agilent 34137A Test Lead Set (which accompanied the DMM I got), there is a statement which is:This Test Lead Set is designed for use only with the Agilent 3458A Multimeter.Do not use with other Agilent meters.Does anybody know the reason for not using these test leads with a different DMM?The ELANTEC comparator chip, I can’t seem to locate that on the A2 schematic. Am I overlooking something here?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 22, 2020, 05:04:22 pm
content: There are a few of the Elantec comparator on the A3 board and there is likely also one on the AC board.For the test leads the limitation to the 3458 is odd. It may be because they have a simple banana plug without a shroud. The modern DMM usually support the newer safety plugs and should use them if possible. However there are also still some older meters around with old style terminals.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 22, 2020, 06:14:10 pm
content: Quote from: franklin on April 22, 2020, 04:53:56 pmHi Franklin,Meanwhile, you should have had collected enough data to calculate drift and T.C., and to decide how well your 3458A is doing. Any hints or data that you might want to publish here?Your Test Lead Set looks very similar to the 34138 set which is delivered together with recent instruments like the 34465A, which has recessed jacks, see pictures, like probably any other handheld DMM also.As the lugs of your set are also isolated but have to fit over 'normal' banana test jacks (similar to the Pomona CuTe ones), either set does not fit mechanically these other instruments.A photo showing the mechanical setup of your lugs would be highly appreciated.There are even two ELANTEC chips on the AC/DC board. I marked the comparator EL2018 with that red arrow, it's in the right upper part of the schematic.You've overlooked this chip, because the designator is written over other items .Frank

author: franklin
author_rank: Contributor
timestamp: April 26, 2020, 11:06:12 am
content: Hi FrankSpring has come so there has been some assistance (and delay) in gardening on my part. Yes, I will publish data but I do have a couple of questions concerning these measurements. In the start data seemed to be fluctuating a little around the starting point (centre) but this centre seems to be drifting very slowly away but still within limit. I feel I can assume that this is to be expected or am I wrong?About the test leads; as far as I can see they are suited for any kind of multimeter since the connectors have retractable shrouds. Nice set but I like the probes from Probemaster in USA better. I have enclosed pictures of the probe set.About the ELANTEC chip; is replacing it something to think about if it proves to be a problem or will that ruin the calibration?

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 26, 2020, 11:18:00 am
content: Hello Franklin,to understand and judge your drifting data, I need to see them. No chance for me to imagine anything this way. Your data from 15th of April already were fine.These retractable shrouds may get stuck in the jacks of other DMMs, or may not retract far enough, so that the contacts can not stick deep enough into the jacks.The ELANTEC chips are not available any more, the ones offered on the common marketplaces are mostly or very probably fake ones.Their exchange would not affect the AC calibration, as this chip is not inside the core AC circuitry.The Volt and Ohm calibration of course would not be affected at all.Frank

author: franklin
author_rank: Contributor
timestamp: April 26, 2020, 01:53:04 pm
content: Here are data for a 28 days period. I’m guessing short time drift of about 0,2 ppm/day is not unusual although it seems excessive to me (with little experience).Date / Time DMM Amb. RH CAL? 72 Deviation(dd-mm-yyyy/time) (⁰C) (⁰C) (%) (CAL const.) (PPM)29-03-2020/2045 A 35,7 1,00125178 Ref31-03-2020/2045 35,8 23,4 1,00125174 -0,0401-04-2020/1010 35,7 23,3 1,00125179 0,0102-04-2020/1335 35,7 22,6 38 1,00125187 0,0904-04-2020/0845 35,8 23,2 33 1,00125174 -0,0405-04-2020/1545 35,7 23,3 34 1,00125176 -0,0207-04-2020/1215 35,7 24,0 36 1,00125177 -0,0108-04-2020/1555 35,7 22,8 38 1,00125184 0,0609-04-2020/1835 35,7 23,6 36 1,00125173 -0,0510-04-2020/1920 35,7 23,1 38 1,00125173 -0,0511-04-2020/0830 35,7 23,8 37 1,00125188 0,1011-04-2020/0905 35,8 23,5 37 1,00125176 0,0211-04-2020/2135 35,8 23,2 37 1,00125181 0,0312-04-2020/1130 35,8 23,0 39 1,00125194 0,1612-04-2020/1730 35,4 23,1 37 1,00125182 0,0412-04-2020/1845 35,6 23,1 36 1,00125191 0,1313-04-2020/1720 35,7 23,5 30 1,00125182 0,0414-04-2020/1630 35,8 23,7 34 1,00125179 0,0115-04-2020/1625 35,7 23,4 35 1,00125181 0,0316-04-2020/0935 35,7 23,5 33 1,00125180 0,0216-04-2020/1330 35,6 23,4 31 1,00125171 -0,0717-04-2020/1710 35,7 23,0 31 1,00125162 -0,1618-04-2020/0935 35,5 23,4 33 1,00125171 -0,0719-04-2020/0655 34,6 22,3 35 1,00125168 -0,1019-04-2020/2120 35,0 23,1 34 1,00125161 -0,1720-04-2020/1245 37,6 26,0 33 1,00125168 -0,1021-04-2020/2035 37,4 25,5 33 1,00125157 -0,2122-04-2020/1805 35,0 23,7 34 1,00125178 0,0023-04-2020/1310 35,7 23,2 35 1,00125176 -0,0223-04-2020/2005 35,7 23,1 37 1,00125169 -0,0924-04-2020/0945 35,7 23,6 37 1,00125166 -0,1225-04-2020/1440 35,7 23,4 38 1,00125168 -0,1026-04-2020/1115 B 35,5 22,5 36 1,00125163 -0,15Drift (A-B)*1E+6 / A*DA B A D=28 days(1,00125178-1,00125163)*1E+6 / 1,00125178*28 = 0,00535

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2020, 05:22:38 pm
content: The short time variations can be just noise, temperature, magnetic field or humidity effects and maybe some hysteresis. For this reason it usually takes longer to establish a long time drift value. The 0.005 ppm/day look good. So I would say no need to worry so far.

author: TiN
author_rank: Super Contributor
timestamp: April 26, 2020, 06:41:37 pm
content: Yup, good healthy ADC.With precision instrument like 3458A I would consider anything changed under the hook as automatic need of recalibration. But if your precision/traceability requirements are not high, ACAL ALL after replacement would factor in changed comparators, since AC board is part of internal calibration procedure anyway.

author: franklin
author_rank: Contributor
timestamp: May 13, 2020, 04:25:01 pm
content: Quote from: TiN on April 26, 2020, 06:41:37 pmI’d like to keep the 3458A up to peak both in precision and traceability but I’m not quite aware of what it takes in terms of effort, professional calibration or what. Professional calibration I would say is out of the question (there goes the traceability) for an amateur like me; the reason is the cost which is about 1600 USD. So, aside from that, what would you suggest I do?

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2020, 07:02:54 pm
content: Quote from: franklin on May 13, 2020, 04:25:01 pmHello franklin,please excuse me, that I answer this question, on behalf of Illya (TiN).Me, like many other so called 'volt-nuts' (metrology amateurs) around the Globe are in the very same situation, i.e. being owner of a 3458A, needing import of Volt and Ohm in their labs, but not needing and not able to afford official calibration or traceability.Therefore, there are several Cal-Clubs, which exchange 10V and 10kOhm references, which allow to check the 3458A, and even might allow to adjust it to a good enoughestimateof uncertainty.I recommend that you 'simply' try to contact other volt-nuts in your country, or in Europe, whether an exchange of standards is possible, and which (estimated) uncertainty is available.The 3458A has the big advantage, that all analogue functions are auto-calibrated from only two references, apart from the RF adjustments, which you probably might not need.If you would already own a DIY LTZ1000 and 10kOhm reference, the exchange would be easy, for the cost of these references, and for the postage costs.Frank

author: BU508A
author_rank: Super Contributor
timestamp: May 13, 2020, 07:45:13 pm
content: Hello franklin,if it's doable for you, there is another option what you can do:The Maker Faire in Hannover. ThePTBis one of the exhibitors there and they are coming with calibrators. Sadly, for 2020 the Maker Faire in Hannover is cancelled because of Covid-19. But 2021 can be an opportunity for you to compare your 3458A with some gear of the PTB. And the best thing is: it is free of charge as far as I know.And even if you don't want to travel with your 3458A perhaps you can assemble a 10kOhm reference resistor and a 10V reference as Frank has already suggested.This is, of course, no official calibration, put perhaps a good starting point.PTB @ Maker Faire 2019:https://maker-faire.de/maker/ptb-natuerlich-genau/Some impressions:Here are some further explanations (in German) from the German mikrokontroller.net forum:https://www.mikrocontroller.net/topic/398160

author: franklin
author_rank: Contributor
timestamp: May 14, 2020, 02:17:08 pm
content: Hello Dr Frank and BU508A.Thank you very much for the info and advice. I will try to locate somebody who share this interest. And the 10 V reference and 10 k resistor, I have neither so there is a challenge. About the Hannover fair; I visited back in 1970 but today this would be a little difficult.

author: KingSolomon
author_rank: Contributor
timestamp: May 14, 2020, 08:10:10 pm
content: I remember that Marco Reps did a video several years ago where he brought his LTZ1000A reference to check it against their 3458a.