HP 3456a odd wander - Page 1
Forum section: electronics / general

author: gmac34
author_rank: Regular Contributor
timestamp: October 03, 2023, 07:18:44 pm
content: I have an HP 3456a that I have been repairing (bad capacitors and some oxidation), and that is now “under observation” to see if it performs correctly.I do not have a calibrator so my checks are based on comparing the 3456 to a keithly 2000 using an HP 3245 as voltage source.Overall the calibration on the 3456 seems agreeing well with the 2000 (within 5 counts) but I have noticed an odd behaviour specifically in the top of the 10v range:Let’s say I start in the 1v range, feeding in 1v, the two meters agree very well (2-3 counts difference) and are stable compared to each other.If I then switch to the 10v range (keeping 1v in, but also true to up to 7-8v ) they also agree and are stableIf I then put in 9-10v they start very much aligned (less than 5 counts) but shortly after the 3456 starts drifting down and seems to settle to -20 counts from the 2000 within a couple of minutes.If I lower the input voltage for a while (even keeping them in the 10v range), and then back to 10v input the reading goes up a bit (let’s say within 10 counts), and then starts climbing down again.I would also add that even though the 2000 is not recently calibrated it is stable in it reedings so I would conclude that the 3456 is at fault and not the 2000It seems like something is warming up with the “higher” input voltage.I have been staring at the schematics and I’m not sure what could be the culprit.I cannot test with voltages toward the top of the 100V or 1000V range and I didn't notice anything similar in the 1v or 100mv ranges.I wish to collect some, all the participants are connected with GPIB so I can prepare some data logging to document the odd behaviour, so I'm open to suggestions on how to best set that up.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2023, 07:42:55 pm
content: It may be a good idea to do a similar test in the 1 V range, so near its upper range.If the effect does not happen in the 1V range this would indicate that the ADCs is not responsible.A thermal effect should no happen just near the end as if there is a threshold. Thermal effects should result in INL contributions that look like U² or U³ and thus already starting at lower votlages.One could also check the opposite polarity.For the 10 V range there should not be a gain stage before the ADC. So there are no other resistors involved to set the gain, except at the ADC.There is a slight chance to get a possible error from the input bias going up at a high voltage, e.g. from a JFET approaching it break-down voltage. 10 counts = 100 µV would be 1 nA with some 100 K resistance used for the protection. Such a current should be detectable.Added current could also happen in a more pulse form from AZ switching. This would become more obvious with shorter integration and than averaging.

author: gmac34
author_rank: Regular Contributor
timestamp: October 03, 2023, 08:05:20 pm
content: Hi Kleinstein, I cannot reproduce the effect in the 1v or 100mv ranges, they appear stable.I wish to setup some data collection to see exactly how this non linearity appears, and if the threshold that I seem to noticed is actually there.Another thing that would suggest that it might not be a thermal issue is that when I lower the voltage the reading is immediately stable.what do you mean by "AZ switching"?thanks

author: bdunham7
author_rank: Super Contributor
timestamp: October 03, 2023, 08:31:59 pm
content: Could you try it at 9, 10 and 11V and try to quantify the amount and rate of the settling?AZ = Auto Zero. You can try turning it off.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 03, 2023, 09:01:44 pm
content: I have repaired my 3456 last year. A question: Are the resistance measurements OK at different values? Especially at high values?Your problem sounds like a leaky fet.

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 12:35:46 pm
content: I made some tests:to read the plots you can see on the top the voltage measured by the keithley 2000 and on the bottom the difference in voltage between the 3456 and the 2000. the hp 3245a is used as voltage source, can only do up to 10v but it's quite stable. All the participants were warmed up for at least 1h1st test - 8V (10V range) for 20min:They start quite close but quickly the 3456 drops, the rate of change diminishes over time and they seem to stabilise after 15min (-50uV in the first 5 minutes and then -20uV in the following 10minutes)2nd test 1V ( 1V range)With a 1v input there is no drift, they stay within 5uV from each other (20min test)3rd test 5 to 10v (1v step on the 10v range) 10min per voltage stepthere is drift on all ranges, but it seems to jump up at every increment. even if the previous step had time to stabilise the following higher voltage takes a few minutes to drift down some more.4th test -10v,I cannot see any drift here5th test 10v on the 10v range, the 2000 measures tp 303, input of the ad converter, this should be 10v on full scale for each range. This one is measured with auto zero offvoltage on tp 303 drifts down with time but it's quite stable compared to the reading on the 3456, this would mean that the drift originates in the input amplifierthe resistance measure seems to work fine, but I can only test with 0.1% 25ppm/C resistors, so cannot really say if it's ok

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 04, 2023, 03:47:53 pm
content: Look at section 8 C 41 of the operating and service manual. The section deals with problems on certain ranges. It may help you to locate the problem. It could be a FET that is leaky. If you locate the FET that is in the 10 V range and short it out the range may work indicating the FET needs to be replaced. The FETs are just used as switches, not amps. The Resistors in the circuits are used for feedback and can also be the problem.I suspect the FET in the 10 and 1000 volt range. Please read the service manual at this section.Anyway start there.wally

author: Kleinstein
author_rank: Super Contributor
timestamp: October 04, 2023, 04:07:15 pm
content: I dont't think the measurement at TP really points to drift originating at the amplifier. Some drift of the amplifier in AZ mode is normal and would be suppressed in the AZ mode. The relatively stable reading more points to a problem with the switching part, like too slow settling at the AZ steps or maybe too much input current in AZ mode. It may help to measure the input current (e.g. 10 M input resistance handheld voltmeter in 200 mV to get a 20 nA current range. Another point could be comparing the 1 PLC and 10 PLC mode of the 3456 - so possibly more switching effect with 1 PLC.

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 05:52:03 pm
content: The measurement at the test point was with the auto zero disabled, if that makes any difference.I measured the input current using a uCurrent in series with the input of the 3456a and I see less than 3nA going in the meter with a 10v input.Regarding the FETs, I did go trough the manual and I tried to bypass q308, which switches the 10v range, but that did not make a difference.

author: bdunham7
author_rank: Super Contributor
timestamp: October 04, 2023, 05:58:19 pm
content: Quote from: gmac34 on October 04, 2023, 05:52:03 pmThat actually seems high. Can you measure it at 5V and then -10V?

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 07:01:43 pm
content: I think I am at the limit of what I can measure, I seem to get 2-3nA at 1 v and between 3-4nA for +5 -10 and +10I also tried +-50V and I get 5uA thenI'm zeroing the meter using the short position on the uCurrent

author: Kleinstein
author_rank: Super Contributor
timestamp: October 04, 2023, 07:04:58 pm
content: The µCurrent is not really good for measuring such small current. Just a 10 M input resistance Handheld DMM is more suitable 1 nA would be 10 mV at 10 M and thus easy to measure. Cable isolation can be tricky and also cause some leakage in the pA range.2-5 nA of input current could easily cause the observed error as internally there are 100 K resistors in the protection.

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 09:38:06 pm
content: I digged out my old fluke 8060 and with hat I can barely measure 0.5-0.2mv across it's 10Mohm input and this is coherent with the drop in the reading on the 3456a.I don't have anything that would be capable of measuring the input impedance of the 3456a but it is at least above the 100Mohm my 2000 can measure.

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 10:16:51 pm
content: The last suspect before going to sleep is q119 it is a N-channel Jfet connected as a diode clamp to 13v on the input (up to the 10v range, 100 and 1000v are routed to an attenuator), there is also a q118 clamping to -13v I think it would be worth a try to remove it and see if that could be the culprit.

author: bdunham7
author_rank: Super Contributor
timestamp: October 04, 2023, 10:35:25 pm
content: Wouldn't Q118 be more likely? If they are accessible, just popping them out might be easier than thinking about it...

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 10:52:07 pm
content: off course, it could only be the one connected to -13... I said it was late

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 04, 2023, 11:18:48 pm
content: I understand that you can only generate 10 V for test.It would be nice if you could generate 50 but...Does the unit drift at all when you use 10 volts in and you use the 100 volt manual scale using 6 digit resolution??

author: gmac34
author_rank: Regular Contributor
timestamp: October 04, 2023, 11:27:35 pm
content: I can generate more with a power supply but there will be lot more noise, I didn’t notice any drift in the 100v range while I was feeding around 95v from a bunch of power supplies in series but the last few digits where jumping around a lot. I also didn’t see any drift putting 10v in the 100v range. A better test would be the 1000v range as it uses the same gain configuration as the 10v but there is no way I can make anything close to that.Q118 was not the culprit. Next I will check the two FETs that are in series with the input 116 and 103.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 07:39:00 am
content: Quote from: gmac34 on October 04, 2023, 09:38:06 pm0.2-0.5 mV corresponds to 20-50 pA, which is about the usualy spec limit for a high end DMM. Unless the current goes considerably higher on warm up this should not be the problem.The low leakage current implies a high input impedance in the >100 Gohm range.If looking for leakage from semiconductors a quite effective technique is local heating of the suspect parts, as the semiconductor leakage (usually also the one due to defects) goes up the temperature.Possible culprits could also be the LM339 comparators used to control the JFET switches. There are reports of these chips failing in different old meters. If there is leakage to the negative side some JFETs may not turn on fully. For the 10 V range this would be pin 13 of U102 that should not be much lower than the input in the non AZ mode.A point to check is if the measurements are done with the analog fitler enabled or not. The filter may have leakage current to load down the signal in some cases.

author: gmac34
author_rank: Regular Contributor
timestamp: October 05, 2023, 08:07:06 am
content: 13 of U102 measures just a bit under 10v and should be ok, now I managed to trace 10v to the end of the input switching, I can measure a stable 10v after r103then it goes into q310-11 U307 and U308, and from there it is incorrect (measured on both side of q308).The analog filter was always disabled and doesn't seem to add any problem if enabled.I also made another run with plc of 1 with similar drift

author: gmac34
author_rank: Regular Contributor
timestamp: October 05, 2023, 11:05:13 am
content: I tried to make a test with 100v but it's inconclusive, if you squint there might be a drift as well but it's quite small and with such an unstable source there can be other factors

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 11:44:09 am
content: At the amplifier the 30 V supply could be an issue (e.g. ripple or too low). Another possible point at the amplifier could be too much current for the JFET amplifier, e.g. visible as too much voltage at R308

author: gmac34
author_rank: Regular Contributor
timestamp: October 05, 2023, 12:16:20 pm
content: 30v supply is stable and measures 29.8V did not find a tolerance on the SM.for r308 I get -18v on one side and -15 on the other

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 12:41:35 pm
content: The 30 should be well good enough. Similar the current through R308 looks like the expected value and thus not too much voltage headroom lost.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 05, 2023, 12:49:48 pm
content: I think you are very close.Just to be sure you said the signal is low on both sides of Q 308 ? Not both sides of U308?HA1 is the signal that turns on the 10 volt and 1000 Volt ranges. HA are on P 155 of manualIt turns Q 308 ON. thru U 304.Make sure HA 1 is stable and should be 5 V pin 7 of U304. It should be present in 10 Volt and 1000 Volt ranges.If this is OK then the problem is either in U 304b or Q 308 P 188Kleinstein Mentioned that a LM 399 can be the culprit, He was referring to a different LM 399, Hopefully he is correct. U 304 is a LM 399, if it is leaky the voltage to the gate of Q 308 will change causing your problem.This is the Unity Amplifier and the other amps are not involved. This fits with your observation the the other ranges are OK,

author: gmac34
author_rank: Regular Contributor
timestamp: October 05, 2023, 01:29:17 pm
content: both sides of Q308 measure the same slightly low 10v if you look at the picture, I measure very close to 10 in the blue (up to pin 3 of Q310) then I get 9.99985 or similar on the red.I tried before to bypass Q308 with no difference, on the gate of 308 i got either -18 (on the other ranges) or almost the same voltage as on the source/drain, so I think it should be correctly switched not sure how to check if it's leaky apart from replacing it, but HA 1 is there, very close to 5 when it should be as well as the other HA10 and HA100

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 01:39:33 pm
content: The JFET amplifier part may have a significant offset (like some 10 mV, though ideally the trimmer would set it to a lower value) so a slight difference between the blue and res side can be just that offset.The software can normally handle the offset, especially in the AZ mode.Q308 not turning on all the way would more like cause additional gain. So I doubt that this would be the culprit.A unity gain amplifier like in the 10 V range rarely has a problem with a little to low gain. If at all this would be more on the input side, like running out of headroom - but this does not seem to be the problem.A degraded U302 resistor array that does no longer like a high (e.g. > 20 V) voltage could be an issue to cause nonlinearity / drift on the amplifier.For a test one could check if the reading (e.g. some 8 V in 10 V range) changes (goes down) when U302 is heated. A working U302 should not show much of an effect.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 05, 2023, 02:05:06 pm
content: If U304 is leaky it will cause both the drain and the source voltages to sag on Q 308. Which is what is described. Jumping it out will not help. Perhaps removing it, then jumping it or just lifting the gate lead from the board might show us something.Apparently Q310 / 311 is working on the other ranges

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 03:10:00 pm
content: The signal through Q308 is the feedback signal. So If U304b would be leaky and pull it down the amplifier would give a larger output signal to get the same FB signal. So this would be the opposite effect.The point that is different with the amplifier seeing +10 V at the input is that the Q310/Q311 see a relatively positive voltage level, so less voltage over Q311 and more over U302.Q311 should still have plenty of headroom with some 26 V at the drain side and ideally some 10-14 V one the soure side.U302 sees Its highest voltage (some 25 V) with the 10 V at the input. There are a few reports of this type of resistor array failing in the HP3457.

author: gmac34
author_rank: Regular Contributor
timestamp: October 05, 2023, 03:26:50 pm
content: I disconnected the gate from Q 308, and then bypassed it, so it seems to be a lot more stable, if it is drifting, it is doing it very slowly, it has gone down 7counts under the 2000 in 40 min, before it was doing it in seconds.If I heat up U302 the reading goes down maybe 10 15 counts and then goes back up.but just to be sure, you call it resistor array but the SM only mentions it being a transistor packageare we talking of the same part?"U302, a transistor package withtwo PNP’s and two NPN’s connected to sense two cur-rent mirrors (-12 V Ref, to ground and -18 V toground) provide a stable current source for these Fet’sto assure this high CMR."

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 05, 2023, 03:29:44 pm
content: There is your problem. It is either the FET you removed or U 304 which is a quad comparator, LM 399 one of the comparators is not good and is leaking. It is tthe b comparator.https://www.ti.com/product/LM339-N

author: gmac34
author_rank: Regular Contributor
timestamp: October 05, 2023, 04:09:00 pm
content: I see, well I will order some 2N4338 and some LM339 and see what happens.The manual notes that the LM339 is selected... hopefully a modern one is good enough

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 05, 2023, 04:36:42 pm
content: I used J310 in my repair, I think they are a lot cheaper. My source does not have any more.Don't order any cheap FETs from China, Last time I did tht I ended up with a bag of NPN transistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 05:41:52 pm
content: U302 is of cause a transistor array. If it is bad and the current changes quite a bit at high voltage (e.g. from self heating) this can cause the observed drift.Some change in the current (more current with higher voltage and higher temperature) is normal. So the question is still a little if it is from a bad U302 and too much change in the current or something bad with the JFETs (ideally the fets would not react much to a change in the current). Both could be possible.How much the amplifier reacts to a change in the current depends on the offset setting (the offset correction is proportional to the current). So one could try to set the offset trimmer (R334) to about the center to trim for no effect of a change in the current / temperature of U302. If the amplifier offset / error still depends much on the amplifier current for all trimmer settings, this would point to a problem with the JFETs, especially Q311.For the LM339 I don't see a large need to select. If at all it would be for output leakage and the ability withstand a relatively high voltage at the output. I don't see a need for extra good performance.The J310 fets are usually only specified for 25 V, so they may not be such a good choice here, with up to some 30 V with 12 V input voltage. The more obvious one would be the J201.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 05, 2023, 05:56:45 pm
content: Kleinstin is correct, the 310 is a little lwer rated in voltage, but I used it anyway, because that is what I had,J201 are even cheaper though.Please use a socket in installing your new IC. Makes life easier and you can switch ICs easily for testing etc.

author: gmac34
author_rank: Regular Contributor
timestamp: October 06, 2023, 07:39:33 am
content: I have made a few tests with the gate of q308 disconnected and bodged 100k restore between the gate and the tp303 side of q308 (just to keep q308 onand not have to use a long jumper).What I noticed today is that:The drift is lower than before, but it is still there.Measuring a low voltage for a while resets the drift, and then switching to 10 will show a slow downward trend. (maybe cooking U302 had something to do with the new rate of change?What I now see more clearly is that 1plc seems to drift faster than 10plc as well as settle lower, just switching between 10 and 1 will make the reading jump 5 counts.if I turn off the AZ the drift is much more visible (around 10v, it is quite stable with just 1/2v)

author: gmac34
author_rank: Regular Contributor
timestamp: October 06, 2023, 07:48:33 am
content: with the az off and 1plc I can also see more clearly that warming up U302 (150 deg form hot hair station) makes the meter drop 20-30 counts.shall I order a 1858-0054 as well? this repair is getting quite pricy...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2023, 09:17:40 am
content: AFAIK the 1858-0054 is an old standard transistor array CA3096E. This may now be a bit hard to get. It is used as 2 current mirrors and I see not need to have coupling between the NPN and PNP pair. It would more make sense to have them separate so the PNP pair stays constant temperature at least. The NPNs are seeing the variable power, though no really that much (e.g. 200 µA*10 V = 2 mW).It sould be possible to replace the 1858-0054 with 2 separate pairs. With the rather large emittter resistors even single transistors may work.The AZ mode should be able to compensate for the slow heating effect, especially the 1 PLC mode. A thermal effect should be larger in the 10 PLC mode.The weak point with simple current mirrors is more the early effect and similar instant effects.Having more drift / nonlinearty with 1 PLC could be a settling effect with the amplifier. So maybe a cracked capacitor (C303,C304) that got humidity inside.It may be worth looking at TP303 for ringing or slow settling.

author: gmac34
author_rank: Regular Contributor
timestamp: October 06, 2023, 09:56:11 am
content: I see some CA3096AE on eBay from Europe that should be original NOS, at least from the picture... I'll see what I get..I don't see ringing on tp303, the rise is not particularly sharp but I cannot see 36us having an effect on a 30ms pulsep.s. sorry for the terrible screenshot, I cannot find my box of floppy disks

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2023, 11:34:49 am
content: The transition looks perfectly fine. If at all the short drop some 200 µs later may be an issue.I don't know the details, but there is usually some waiting time (e.g. a few 100 µs) after swiching and even that extra drop may be still in the dead time.

author: gmac34
author_rank: Regular Contributor
timestamp: October 06, 2023, 06:20:10 pm
content: I thought to check the waveform in the 1 and 0.1 ranges, to see if the glitch was still there, and surprisingly they look much worse, the second glitch becomes much larger.I remember reading that the AZ is done in three stages, with a pre-charge phase between the two main parts, so that might be the glitch between the two phases.I see the same glitch probing r103, before the input amplifier.I'm not sure how the nicest waveform would be on the failing range, so I would rather think that this is not related to the AZ.

author: tridac
author_rank: Regular Contributor
timestamp: October 06, 2023, 06:31:19 pm
content: Something to do with the input range switching ?. Quick look at the schematic, looks like either a 1:1 or 100:1 range switching, so perhaps something adrift there, an fet switch gone leaky, or similar. Great meter when right and very impressive cal stability over the long term...

author: gmac34
author_rank: Regular Contributor
timestamp: October 06, 2023, 07:16:25 pm
content: Earlier have tried to check the switching FETs. I lifted one side, the source I think (the one away form u200) of q309,307 and 306 to take them out, but I didn’t see a change.I didn’t check q304 q303 and q302

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2023, 07:37:19 pm
content: The precharge phase makes absolute sense to explain the 2 steps. With the 1 V and 100 mV range there is additional gain and thus a similar (possibly smaller due to lower voltage) disturbance just amplified.I don't see Q306/307/309 as possible culprits as 2 of them are stiff off in the 100 mV and 1 V range, with a FB signal that is way more sensitive. So the error would be even stronger in the 1 V or 100 mV ranges.An odd error could come from a lost wiper contact the the R147 / R148 trimmers for the charge injection. This could cause a little delayed signal that could have less effect on the 100 mV/1 V range if there is additional waiting time for these ranges. At least this would be easy to measure in circuitas the wiper part is only capacitive coupled.To check if the drift is not related to input leakage, one could repeat the test with a series resistor or maybe 1 M: a leakage related effect would get about 10 x larger.

author: gmac34
author_rank: Regular Contributor
timestamp: October 07, 2023, 07:26:56 am
content: I didn't see any difference with a 1M in series with the input.I have now ordered a bunch of sockets, LM339 and CA3096AE. will see if those make any difference.I imagine after swapping the CA3096AE I will have to go trough the input amplifier adjustment procedure before doing more tests

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2023, 07:39:37 am
content: I don't think changing the CA3096 would make that much of a difference. The current is mainly set by the resistors and the current is anyway somewhat temperature dependent ( 2 mV/K relative to some 11.5 V and thus some +200 ppm/K).

author: gmac34
author_rank: Regular Contributor
timestamp: October 07, 2023, 07:44:01 am
content: I see, then I didn't understand what you meant by degraded resistor array, did you mean the U200 resistor package? or maybe the supporting resistors around U302?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2023, 08:56:21 am
content: Quote from: gmac34 on October 07, 2023, 07:44:01 amThe possible problem could be with the transistors in U302. Calling it ressitor array was a mistake from my side. I don't think the supporting resistors around U302 should be that critical as there is the TC anyway and in addition the resistors don't see a change in the current when the input voltage changes.It is somewhat hard to find mechanisms to cause drift in the gain only when the voltage is large positive (e.g. > 5V). A buffer with gain 1 is normally very stable and it is a bit hard to get a gain < 1 for some voltages. A starting point is to look at what changes with a high positive voltage at the amplifier input (the output side gets the same votlage in the 1 V range). The possible points I see:1) a higher voltage at the U302 current source. The test with a warm U302 shows that there is some effect of the current on the output. A relevant failure mode would be if the current goes up more than normal at higher voltage.2) less voltage at Q311 , though still no that low with a drain voltage of some 26 V. I don't see a good reason why a lower voltage would have much effect, except for the normal thermal effect.An issue may be a totally failing Q311 (e.g. D-S short). It may be worth checking the voltages at Q311 and Q310 (e.g. with a 0 V input) : a non working Q311 may cause a linearity problem.Normally the drain to source voltage at Q310 should be relatively fixed at some 2-5 V.3) the AZ switching in the 10 V range seems to reach the slow rate limit of the amplifier as seen in the scope trace. This may not happen with the lower voltages, as the switching of the compensation allows for a higher slow rate with gain. With the relatively long waiting times (some 200 µs for the precharge phase and likely at least that long after the 2nd switching) I don't see a settling issue.4) JFET gate leakage in the gain setting part would be higher at a high voltage, but this would mean more gain there - so an error in the opposite direction. In addition here it is the voltage at the output and thus also effecting the 1 V and 100 mV range and even more there.

author: gmac34
author_rank: Regular Contributor
timestamp: October 07, 2023, 11:39:09 am
content: I have tried to make the 8-F-19 DCV adjustment, it seemed maybe necessary as the residual input offset was a bit above 0.9uV.But I cannot get past step p as I cannot seem to affect the reading with r147.r147 and 148 are ok, I checked them. maybe something else is wrong in that part of the circuit

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2023, 12:22:48 pm
content: R147 and R148 are for the gate charge compensation on switching, especially the AZ switching part. So they effect how much of a switching current spike is seen at the input, not so much the offset.With the input filter enabled they would effect the input bias current and this way indirectly the offset.The most direct effect would be seen the the transients at the amplifier output like in this post:https://www.eevblog.com/forum/metrology/hp-3456a-odd-wander/msg5098257/#msg5098257The setting is a bit crude anyway as the input voltage also changes things.To have an effect the meter should be in AZ mode.If they show now effect at all - maybe check U108 (LM339) pin 1 if it generated the 0 / -18 V signal as expected. With a 10 V input the effect would be even larger than with zero input - so may this could be the culprit.The offset adjustment via the extra resistor R150 is just an added overall offset. This would be a part to worry about at the very end.

author: gmac34
author_rank: Regular Contributor
timestamp: October 08, 2023, 07:50:42 am
content: I understood that that would be something to re-do at the end, but I was wandering if that could clean up the glitches in the AZ switching. the last part of that procedure at least. Anyway, what I thought was odd is that, if I understand that procedure correctly, R147 is supposed to adjust for +-1 counts offset starting from a <9 counts offset, so it should have a "+-9 counts effect", and I didn't manage to really see that. R148 is to be adjusted with the filter inserted, and then the one has some effect.It also seems to be a struggle to do the padding part, because my meter seems very sensitive to temperature change, I didn't see a mention of a service cover with holes for the trimpots, so assuming that one has to remove the cover to adjust those things, my offset goes from -7 (stabilised with all covers on) to maybe +20, almost instantly after removing the covers, and with the covers off it doesn't really manage to get back down to -7.U108 is generating 0/-18 correctly (with shorted input, the 0 portion is otherwise following the input voltage)one last thing that I have noticed, I checked the gain of Q305 U305, the manual tells to inject 5v to pin 6 of Q305 and specifies 5mv as the maximum difference between that and tp304, I got 17mv, but that doesn't seem like it could be the problem.One last thing, that I didn't mention in this tread, initially this meter seemed to have a problem with U200, the 100v and 1000v were reading high, so I got another U200.In the end just swapping them in and out a few times solved the issue, I guess there was some tarnish on one of the golden fingers, which I now cleaned with some deoxit. Now the meter has it's original U200 but I tested the other one and got the same effect on 10v

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2023, 09:08:16 am
content: The 1000 / 100 V ranges can be quite senstive to dirt on the PCB, like a tiny bit of resistance in parallel to the 9.9 M part or U200 can cause an error there.It is not a surprize that the offset is somewhat effected by temperature / opening the case. Chances are much of the offset (low µV range) is from thermal EMF and it does not take very much for some resistors and connectors.15 mV offset from the main amplifier by it self may not be that bad, but it indicates some imbalance in the JFETs that is not supposed to be there originally. This could also explain the relative large effect of heating U302. Asymmetry in the FETs could also cause higher than normal dirft in the the non AZ mode. So it could really make sense to check the voltages between Q310 and Q311, if there is something wrong with Q311. If these voltages are OK one could try getting a bit better symmetry and maybe less drift in the non AZ mode by adding some resistance in parallel to one of the 40 K drain resistors (part of U200).

author: gmac34
author_rank: Regular Contributor
timestamp: October 08, 2023, 11:25:49 am
content: Haven't had time to take it apart again but I noticed another piece of weirdness.If I apply and then remove a voltage above 7-8v the meter starts increasing the voltage very quickly until it arrives to 12v then it changes range, drops a bit and starts again.I know it is normal to have odd voltages given the large input independence but this seems odd.Also, the voltage raises faster with 1plc than with 10plc and the effect seems to happen only with the AZ enableda video of that here:https://youtu.be/dfuo4ITzSJA

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2023, 11:40:22 am
content: The wandering voltage with an open input is normal with the high impedance. On some units it drift off scale and with others not. It depends on the individal input bias that is normal one specfied to the within some +-20 pA or +-50 pA depending on the model. One can actually use the rate of drift as a way to measure the input current, usually with a small (e.g. 1 nF to swamp the internal capacitance that is not that well known), low loss (e.g. PS or PP type) cap at the input. 1 pA than corresponds to some 1 mV per second.Part of the input current comes from input switching as some added charge pulses and lower PLC means more pulses. The input current specs may only apply to 10 PLC - the 1 PLC limit may be higher (or no extra specs).Chances are that R147 / R148 for the charge compensation, may effect the amount of extra current in the AZ mode.As there are some 100 K of series resistance for protection, the input bias current can also cause some voltage error. So additional current above 7-8 V may somewhat explain some error/drift in this range. It still need quite some current (1 nA for 100 µV).

author: gmac34
author_rank: Regular Contributor
timestamp: October 08, 2023, 12:00:31 pm
content: With 1plc, it goes from 8 to 11v in 33s so about 90mv per second.With a 1nf styroflex capacitor, that should be low Leakage

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2023, 12:13:33 pm
content: There is another 470 pF inside the meter. 90 mV per second and some 1.5 nF total capacitance would be some 135 pA of current, causing the meter to read high by some 13 µV.So there is some effect, but the opposite sign of what the starting problem is. So the current from charge injection is not good, but not causing the drift.

author: gmac34
author_rank: Regular Contributor
timestamp: October 14, 2023, 08:32:56 am
content: I had the parts cannon loaded with a bunch of things to attempt to replace, but it seems the first one was the lucky one. A new CA3096AE in U302 seems to have solved the issue,I also tried to go trough the 8-F-19 DCV adjustment, as you should when replacing parts in the input amplifier, and I still cannot get a good hold of r147. still the residual offset I get at this point is quite ok, -0.6uV.The meter has been on for more than a day now, seems quite stable and holds 10v within 2 counts of the keithlyhaven't made a long data-log but I cannot see traces of the switching drift that was there before.Thanks you very much for the help, I wouldn't have managed to pinpoint the problem this quickly otherwise.