HP 3456A voltage offset - Page 1
Forum section: electronics / repair

author: stevopedia
author_rank: Contributor
timestamp: October 08, 2024, 01:17:37 pm
content: Hello all,I've got an early (serial 2015A04414; has a fan) HP 3456A DVM that's been beguiling me for a while now. Ultimately the fault is that it displays a DC offset of more than 100 counts with the input shorted when cold, increasing to 400 counts or more as the instrument warms up. It passes all the self-tests and I've followed the procedure for input switching troubleshooting (8-C-26 in the op & service manual) at least twice now. I got to the final step of "replace Q102 and Q112" both times, and replacing them (once) with new PN4392s (by Linear Systems, sourced from Digikey) had no effect. The offset is real, in that it follows range adjustments and can be measured with another hi-Z DVM.Here's a list of all the tests, changes, or other work I've done thus far (all parts are on A20 unless otherwise specified):Replaced all the electrolytic caps in the inguard and outguard power suppliesAn error in the procedure under 8-B-10 in the OSM (steps c. and d. should be swapped) lead me to replace U405 with TLE2071 and Q408 with PN4392. As it happens these were unnecessary because autozero has always worked in this instrument, but as the comp changes haven't seemed to cause any problems they have been left in placeFollowing the corrected procedure under 8-B-10 shows the ADC is working perfectly (checked with Valhalla 2701C and HP 3458A at work)Found input amplifier oscillation; cured by replacing U307 with TLE2071 (fixed self-test failures on 4-6)GB2 has an offset of about 17 mV--OSM says to replace parts if offset exceeds 5 mV, but I doubt this isreallyan issueTried replacing Q120 with LSK489 and U105 with OP07 (C108 was removed) but this made no change. GB1 output offset is about 10 mV now. This was unfortunately done before I started taking meticulous notes, so I have no record of how it performed before the change, but it having made no difference is enough for me in this caseNoticed C701 (on the unregulated +33 V line) is bulged, but don't yet have a proper replacementJumpering across K103 at R09 & R110 has no effect on the displayed offsetAt this point I'm completely out of ideas. Admittedly I'm far from an expert in these instruments, no matter how thoroughly I've read (parts of) the OSM. What am I missing?!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2024, 01:52:54 pm
content: Is the offset a fixed 100/400 count, or a fixed voltage (e.g. 10 µV) ?Where is the offset measurable with another meter ?A point to check would be if the problem is really an offset voltage or maybe way too much bias current.One could trim the offset of the GB2 buffer via the resistors R320/R321. It is not ideal (may have more temperature drift) but easier than changing a dual JFET. I still doubt it would make a big difference for the overall offset.There is an offset adjustment via R150 ( resistor choosen to compensate an offset) - so maybe check how much offset this really is.

author: stevopedia
author_rank: Contributor
timestamp: October 08, 2024, 02:19:14 pm
content: The offset is a variable voltage. The numbers given are approximate.By memory, the offset appears somewhere between the switching input at R102 and the output at J19, and can be observed at all points downstream (TP303, TP401, etc).Bias current is a new one for me. How do I go about checking that?The OSM has a procedure for determining the correct value and placement of R150 as part of 8-F-18. The values it gives are for offsets of 2.0 μV or less. But the offsets I'm seeing are 100 counts or more on the 100 mV range, equating to 10.0 μV or more. The OSM simply says: "If the offset is >2 μV, calibrate and/or repair the instrument."

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2024, 03:12:35 pm
content: Ideally the meter should have a very low input current (e.g. < 20 pA or so) with zero input voltage and maybe a little more from the input resistance with high voltage. When dirty or with some bad parts the current can be higher and the input resistance lower.A simple test is comparing the reading with a short and some 10 M across the input (may be the meter internal divider and thus 10 M input resistance mode). A test also for other voltages can be done with a low absorbtion (e.g. C0G, PP or PS type capacitor of some 10 nF) at the input and abserving the rate of voltage drift. 10 pA of input current would than cause a drift of some 1 mV per second, that can be observed at the meter. With the capacitor one can start at different voltages and this way check the input current at different voltages, nor just near zero.It is normal to have some offset at TP303, TP401 and so on. The question is if the offset is also there in the auto zero mode.Is there also an offset in the 4 wire resistance mode ? There may be an extra zero compensation in software, that could make this test useless. However ideally there not be an extra constant.For some odd reason old LM339 comparators have a tendency to fail. So it may be worth checking of the gate signals for the input switching are all working, especially the gates of Q104 and Q102.

author: stevopedia
author_rank: Contributor
timestamp: October 08, 2024, 03:50:59 pm
content: Remember, I'm seeing this offset with the inputs shorted (my shorting "bar" in this case being some 2.5 mm desoldering braid, saturated with solder and just long enough to be securely clamped in the terminals). So while I don'tthinkinput current is the culprit, I'll try the procedure you described when I get home. (It's both clever and effective--I like it!)I should have said sooner that I can bring at least the 10 V range into cal, using the Valhalla 2701C and 3458A I mentioned in my first post.I'm aware of the offsets generated by the input amp and the correction for it that AZ provides. If you're asking whether the 3456A displays the same offset if AZ is on, the answer is yes.Now that you mention it, I haven't checkedanyof the ohms ranges yet, being so focused on DCV. I'll test them and see what happens.I also haven't actually checked the gate signals on the input switching FETs either--another good catch! I'll probe those too.Thanks again for the pointers!I'll report back with what I find. (That will be at least five hours from now!)

author: stevopedia
author_rank: Contributor
timestamp: October 08, 2024, 11:10:17 pm
content: I'm back as promised, with results in hand (or, at least, what few results I have).The input current test with a capacitor was inconclusive. I don't think any of the film caps I have on hand are suitable (I believe they're all PET) and I couldn't find any C0G parts that were in the range we're after (all the ones I have are tens of pF or less). If PETisgood enough I'll do the test again, but I had a 100 nF cap and it drained rather faster than 1 mV/s.I checked all the switch FETs in the input switching that are controlled by logic (i.e. not the diode connected ones) and all the driving signals were OK.I put together a little harness to test 4-wire ohms (Volts inputs shorted, Ohms sense inputs shorted, volts low shorted to ohms sense low) and the display shows 0.00095-0.00105 Ω (it's a little noisy). Is that measurement what you meant by "an offset in 4-wire resistance mode"?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2024, 08:28:28 am
content: A resistance offset of 1 mohm would be some 1 µV with a 1 mA test current. So it looks like less offset.A polyester cap can be a bit tricky. They have quite some dielectric absorbtion (some 10 x more than PP) and one could be fooled from that.Getting more than 1 mV/s with a 100 nF capacitor is still a bit fast, even for a PET capacitor. The dielectric absorbtion effect would change with more waiting time (e.g. compare with 1 min and 10 min of soak time).What is the 10 M resistor case reading - that may be the easier, more direct way ?The resistors in the 3456 are still about balanced (100 K for zero and 4x27K for the input). So it would need some 1.2 nA of input current to get 10 µV of offset.How is the offset in the 100 V range compared to the 1 V range ? With the input divider the resistance matching should be a bit better, so input bias would have less of an effect.For a test one could also add some 10 K in parallel to R104 to intentionally get more imbalance and thus get more sensitive to input current. So if the offset gets significant larger it would point to a current problem. Going through the settings (4 W res, 100 V, 1 V with AF on/off) could also give a hint if one of the switching fets is the source: the one that is switched on usually no longer leaks as much.A possible test to find leaky semiconductors is by local heating. The leakage of most semiconductors goes up quite fast (about doubling for 10 K) and this often also applies to parts that are leaking more due to a defect. It does not need much heat to see a change, so a non touching hot resistor / soldering iron can be enough.

author: stevopedia
author_rank: Contributor
timestamp: October 09, 2024, 07:13:07 pm
content: I put in an order with Digikey for some nice polypropylene film caps. They should be here in a few days.Do I understand you correctly that you want me to measure a 10 megohm resistor with the 3456 and report what it says? I can do that, but I don't know how well calibrated the ohms ranges are.The offset follows DCV range adjustments as if it were a real input voltage: I see three digits in the 100 mV range, two in the 1 V range, and one in the 10 V range. To my memory there is no offset in the 100 V or 1000 V ranges--but I'll verify that after work.I like your ideas re: paralleling R104 and searching for faulty parts with heat. I'll try those too and report back.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2024, 09:23:00 pm
content: The idea with the 10 M resistor (could also be the internal divider) is to have this at the input with the meter in DC voltage mode. So 1 nA would result in some 10 mV which are easy to read with the meter.The sign of the offset could also help to check some leakage paths (e.g. the gate leakage would usually be negative).If the offset is due to leakage current it could be Q109 or the matiching between R104 and the 100 K from the divider that causes a low offset for the 100 V range.

author: stevopedia
author_rank: Contributor
timestamp: October 10, 2024, 04:11:56 am
content: Ohhh!!!I'm glad I asked for clarification regarding the resistor. I'd completely misunderstood you!With a 10 MΩ resistor across the Volts input terminals, the instrument shows an offset of about -2.78 mV and slowly increasing (moving away from 0). I also have some proper numbers with the inputs shorted after the 3456A had been powered on for 10 minutes or so. Values given are exactly as the display showed.100 mV range: -00.230 (the last digit was noisy)1 V range: -.00002210 V range: -0.00002100 V range: -00.00081000 V range: -000.001I tried connecting a 10 kΩ resistor across R104, but didn't see anything notable in the displayed values as I went through the various functions as you described. It could very well be that I simply didn't know what I was looking for or got some part of the procedure wrong.To test with heat, I used my hot air rework station. I set the temperature to 120 °C and turned down the airflow--enough to gently heat a targeted transistor but (hopefully) nothing more. That lead me to be suspicious of Q115, as it seemed to react the most to the air (the offset becoming more negative as Q115 was heated). I didn't remember if the portion of the input switching troubleshooting concerning the switch FETs (8-C-26 g. in the OSM) checked that part and I found that it did, so I decided to repeat the whole procedure in 8-C-26 g. Thatimmediatelyidentified Q104 as suspect: reading was -00.0245 mV at first and changed to +00.1440 mV when the gate was shorted to the Low terminal! I don't know how I missed that previously.Anyway, I replaced Q104 with a new PN4392, but this had no effect. In fact repeating the Q104 check given in the service manual has the same result: huge positive swing in the reported offset, with the manual prescribing replacement. I continued with the switch FET test procedure from there, but all the other parts tested good. I'll bet the original Q104 is also actually good, and the behavior when Q104 is turned on is a symptom of whatever the true cause is and not a bad Q104.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2024, 09:54:31 am
content: Some -2.8 mV at 10 M indicate an input current of 280 pA. So too high in the input current. The specs should be like < 20 pA.If the leakage is in the pos side part the effective resistance are some 104 K and 280 pA would result in some -28 µV offset.Balancing with R104 would only fully work if the leakage is more at the amplifier input or other side of Q103. So the input current could be the culprit for the offset.Reducing R104 would increase the imbalance in the resistance. With the extra switch Q103 the resistor balance may not be that important anyway. So not such a good idea.Due to the heating effect I would suspect Q115. For a test one could see if a voltage (e.g. 1.5 V battery) applied between the sense H and Voltage Lo terminal would effect the offset reading. If there is quite some effect this clearly points to a bad Q115. The test procedure for Q115 in the service manual should also work, but needs to open the case. For a first test one could check the function even with Q115 removed (obviously no 4 wire ohm then).The test procedure for Q104 looks a bit odd to me. Chances are it would show a change even if a different FET is leaky. It adds a 2nd path to ground reducing the resistance to about half and thus expect half the effect of input current.The 100 V range shows relatively little offset (-0.8 mV compared to -2.2 mV expected from 100 x the 1 V range offset). This make Q109 a little suspect - maybe worth retesting with some heat.

author: stevopedia
author_rank: Contributor
timestamp: October 10, 2024, 01:50:02 pm
content: After reading your reply I took a closer look at the input switching schematic. I admit I half-glossed over it before, but something about what you said made it click for me--thank you!I'll try your idea to apply a voltage to the 4-wire sense high terminal as soon as I can.I should mention that in the course of my testing last night I found that there was a substantial voltage across the switching output resistor R103. The main DMM I've been using in the course of this diagnosis is a Keithley 196, which is rated to > 1 GΩ input impedance in the 3 V and 300 mV ranges. Using that meter with its low input connected to the 3456A's low Volts terminal, I found that the input switching side of R103 was at around +1 mV and the J19 side of R104 was at around +0.02 mV. That condition persisted even after disconnecting the input amplifier from J19. That would imply 100 nA flowing through R103 via some parallel path. That seems substantial to me now--I should have investigated that more closely. I'll do that too, when I can.I'm fairly certain Q104 is a major part in the autozero system, so in hindsight I completely agree that with your assessment that the manual's procedure for Q104 is flawed. If the two input nodes are for some reason not at ground even with the inputs shorted, then deliberately turning on Q104willforce a change in the measurement whose direction depends entirely on the polarity of the charge on the upper and lower input nodes (as the OSM refers to them). That's not the first problem I've found in the manual either, unfortunately.By the way, are discussing drain-to-source leakage or gate leakage with these FETs? Looking at the schematic now, it seems to me that shorting the gate of Q115 to the low volts terminal (which I did try, and per the manual it tested good) with the high reference terminal open would serve only to test Q115 for gate leakage or perhaps gate leakage of diode-connected Q117 since the fail condition is the displayed offset moving in a positive direction or to zero. Then again, since the high ref terminal is normally not connected, a Q115 that was leaky drain-to-source wouldn't cause any problems if Q108 and Q117 were both good.

author: stevopedia
author_rank: Contributor
timestamp: October 10, 2024, 10:02:02 pm
content: Applying +10 V to the high ratio ref/sense terminal with the Volts terminals shorted had no effect. Same with applying -10 V.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 11, 2024, 12:00:58 am
content: I know you already replaced Q 102I had a very similar problem where I could not get the 100 mV range to adjust high enough on my 3456.After switching multiple FETs and doing lots of things that did not help, I eventually replaced Q 102, the pre charge FET. Everything works fine now.I believe that that FET was the problem although I changed so many FETs that I cannot prove it. Anyway that was the last change I made and before that the 3456 would not adjust high enough on 100 mV range and after that replacement it now adjusts easily..I do not recall what the shorted value was I was using a newly calibrated volt source.I do think that I had problems on the other ranges, but I was able to adjust them out, so I was fixated with the 100 mV problem.The alleged troublesome FET tested OK when removed.As I remember this my repair went on for far too long. Good Luck!!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2024, 08:29:46 am
content: The main suspect is gate leakage. With open ohms sense inputs there should be little voltage on most of the other JFETs that may have drain - source leakage.As the 4W resistance case shows little offset (1 µV range) and with the strongest heat effect at Q115, I would still suspect Q115. This is even though the external voltage did not have an effect. The test for Q115 in the service manual should work better and should also work with the meter in AZ mode (less noise and drift). One could also repeat the offset test in the 4 Wire mode (100 ohm range) with an added resistor (1 M or 10 M) in series to the sense H input. This would measure the input current for the sense H input. If Q115 is the culprit the sense H input should have less input bias current than the main input.There is a chance that the leakage can be from more than 1 souce, as the 100 V range showed less (like 1/3) offset than expected, though in this case there is still a chance for some thermal EMF at the divider and trimmer.

author: FrodeM
author_rank: Contributor
timestamp: October 11, 2024, 12:38:35 pm
content: What is the reading at TP104?I had a 3456A with a front-end switching problem once, and when measuring TP104 with another known good 3456A, it mirrored theexactsame offset as my unit had. From that I was able to realize that the unit was measuring the gate bias and not the shorted input terminals. Turned out a previous owner had applied some sort of oil(!) to the connector between the two inboard PCBs, and in turn random switches were stuck.

author: stevopedia
author_rank: Contributor
timestamp: October 11, 2024, 05:56:38 pm
content: Thanks for your inputs, everyone.FrodeM:With the input shorted, TP104 is steady at about -17.5 mV (yes, I know that's technically out of spec, but I don't think that's my issue). It does not move with the 3456A's displayed offset.Wallace:Thanks for the reminder to adjust the charge balancing circuit--I've probably done enough under the hood to warrant that. This particular instrument has been beguiling me for aboutten yearsnow, on and off. I started thinking of it as "the basket case" a while ago for a reason!Since I couldn't shake the feeling of going in circles chasing my tail in this troubleshooting, I've done some sanity checks on my 3456A using the HP 3458A and Valhalla 2701C I'm lucky enough to have access to at work. I've verified that the ADC and input amplifier are both working extremely well, so the problem absolutelymustlie within the input switching. I also verified with the 3458A that there is in factnodifference in voltage across R103, so I'm not dealing with stray current paths--the low-range DCV input impedance on the Keithley 196 is simply low enough to have a significant impact on the circuit.I also tried recalibrating the DC V ranges just to see what happened. I was able to bring all the ranges into cal at full scale, but they all read substantially low (some 20 counts with a 6-digit display) at 10% scale--for example, with 1 V input on the 10 V range the instrument showed about 0.99980 V. Since I'm quite certain the ADC and input amp are both good, this means my apparent linearity error is actually an offset arising within the input switching that is proportional to the voltage at the high Volts input. That would align nicely with a leaky gate somewhere, I think. And I just realized that gate leakage implies we're looking for FETs that are off in the DC V ranges. Now I understand even better why Q115 is particularly suspect.I tried a test for 4-wire ohms: I shorted the Volts input terminals together and to the Ohms Sense Low terminal, and connected the Valhalla 2701C across the Sense terminals. I found that the offset was still present in that configuration, if I was reading the results correctly. So given a FET with a leaky gate must be turned off, IthinkI can rule out Q115. I have ideas for more things to try, but I do have to get some Real Work™ done today!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2024, 07:16:54 pm
content: A FET with a leaky gate can still turn off OK. There can be separate leakage from the gate to source and drain (no need to be symmetric).I point that hints to a bad Q115 is that the 4 wire ohm case showed very little offset. 0.8 mohm correspond to 0.8 µV and thus way less than some 20 µV seen in the DCV case.In 4 Wire mode the input path switches between sense H and sense L. So Q115 is still off half the time, so an external meter could still detect leakage current at sense H, though not necessary the same as to the meter input side.Current through R103 would be the input current of the amplifier. Even if there would be some input current there this would see a resistance of some 100 K in all paths and thus a rather constant offset for both readings in the AZ cycle and thus less (like 1/10) offset from the same current.The likely culprit is gate leakage at one of the FETs (Q115,Q109,Q112,Q114,Q120,Q110, Q118) that than causes a voltage drop at the 4x27 K for the protection or R106 or the 100 K from the divider. For the other reading in the AZ cycle Q103 is off, and thus the current not reaching R104 for the zero reading. Leakage from Q104 would be off when the FET is on. So it could also cause and offset voltage and measurable input current.Gate leakage from Q103 would cause a positive offset as the zero reading would be negative.

author: stevopedia
author_rank: Contributor
timestamp: October 11, 2024, 11:00:48 pm
content: I found one of the culprits!Earlier today I found that turning on the analog filter caused the input offset to drop substantially--it seemed to have been the primary source of offset when the instrument was fully warmed up. So putting that together my realizations I replaced Q114 with a new PN4392. And--it worked! The offset with the filter off is now the same as with the filter on! I'm going to let it warm up again and see how the offset changes then. I've got four new 4392s left--hopefully that will be enough.I hadn't considered that Q115 was actually switched off during half a 4-wire ohms measurement cycle. In that case, it definitely could still be a problem.

author: stevopedia
author_rank: Contributor
timestamp: October 13, 2024, 04:43:57 pm
content: I tried replacing Q115 earlier today with another PN4392 and it didn't seem to have any effect... offset is still sitting at around -60 mV after warmup, same as before replacing Q115.

author: stevopedia
author_rank: Contributor
timestamp: October 14, 2024, 01:46:27 pm
content: Since the offset is (as best I can tell) greatest in the low DC Volts ranges, I decided to try changing the high DC Volts switch Q109. This doesn't seem to have had much of an effect; I'll know better when the instrument is properly warmed up. I've got two PN4392s left... may be time to order more.Given that the main suspect here is gate leakage, it occurs to me that the diode-connected FETs Q106, Q108, and Q118 could equally be at fault here. I'll try probing them with some gentle hot air when I have the time.Speaking of, why did HP use diode-connected JFETs here? Is their leakage performance that much better than a simple silicon switching diode like 1N4148?Edit:I answered my own question: It is. If you want picoampere-level reverse leakage, it's a diode-connected JFET (including the PAD/JPAD series) or nothing. TIL.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2024, 02:23:09 pm
content: The diode connected JFETs have pretty low leakage. They may have been cheaper than speical low leakage diode. Today there is the BAV199 as a cheap low leakage diode with usually bettter performance (less leakage, higher current rating) than a 2N4392. In some meters a JFET for the same type is also used to compensate for the gate leakage - in this case having the same type as the switches can help. It does not seem to be case here, as the diode connected FETs are used in pairs, with also a FET towards the negative side.For an intial test one could leave some of the FET switches out and this way be sure to not get a little leakage from multiple sources.As the excess input current is from the negative side it would be the diode connected FETs towards the negative side, that could be an issue.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 14, 2024, 03:52:59 pm
content: I think HP used this sort of FET Diode configuration in other HP gear that I have fixed. The FETs in the switching section are all the same type. Makes things easier in a lot of ways.In my 3456 repair :I usedThompson 2N4117 FETSas clamping diodes. I was able to get them on ebay.They have four leads, one for the case, which I left flying.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2024, 10:42:13 pm
content: The 2N4117 (and 4118) are rather small JFETs and the maximum gate current is small and the series resistance is a bit on the high side. They may not provide enough protection. One could use these higher resistance FETs for the switches in some places. With anyway some 100 K from the protection another 3-5 K are not that bad.The HP meter may use JFETs selected for low leakage - the normal spec limit for the PN4392 don't look like sufficient (may depend on the manufacturer).

author: stevopedia
author_rank: Contributor
timestamp: October 15, 2024, 03:23:12 am
content: There are a tiny handful of switch FETs in the 3456A that have generic JEDEC part numbers given in the "replaceable parts" table. Theoverwhelmingmajority of them have a custom HP part number, which I agree refers to some kind of special selection... but I haven't seen any record of what it is they're selecting for. Gate leakage is probably a good guess.Thanks for pointing out the BAV199--those look like nice little diodes! It's too bad you'd have to get the lead stretcher out (i.e. solder leads onto a SOT-23) to use them in this application.As an experiment I tried lifting the gates of Q106, Q108, and Q118. That didn't seem to have any effect, so I reconnected the gates again... and found I had modestly increased the offset somehow. Flux residue? Heating from two solder operations altering the components? No idea, but I'll try thoroughly cleaning the input switching area with isopropyl alcohol later.I tried readjusting the charge balancing circuit (R147 and R148) but this seemed to have almost no effect.During that last adjustment I found that with autozero and the analog filter enabled and an integration period of 100 cycles, the displayed offset changed from about -3.0 μV or whatever to about +35 μV. Thisonlyoccurred with all three factors; changing any one of them brought the displayed offset back to sanity. I saw a bunch of high-frequency spikes on the input amplifier's output (TP303) that correlated perfectly in time with the ADC's slope changes, which were only present with the analog filter enabled. Ithinkthis might be because the TLE2071 I used in U307 is too fast/performant. I'll try swapping it with an OP07 just to see what happens.All that aside, I'll think aloud here for a moment. As has been established previously, gate leakage is the primary failure path we suspect. That implies we're looking for FETs that are 1.) off at least part of the time during a measurement, especially low-range DC Volts; 2.) have their gates pulled to -18 V when off; and 3.) are connected directly to one of the two main measurement nodes. By my reckoning, that makes my list of suspects the parts below, listed in no particular order:Q109(changed; no improvement)Q101 (don't remember if checked or changed)Q114(changed; greatly reduced offset)Q116, DC V low-range switch (doubtful; offset in high DC V ranges is similar to that of low ranges)Q115(changed; no improvement)Q105, 4-wire ohms low switch (unchecked)Q102(changed; no improvement)Q103(changed; no improvement)Q110, AC V switch (unchecked)Q112, self-test #7 switch (unchecked)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2024, 08:15:24 am
content: The leakage from flux residue usually does not change that much with temperature (may even go down from a humidity effect). It is still possibly to have leakagef from residue. AFAIK some of the critical nodes use extra teflon isolation posts, not just the PCB diretly.Some spike during the AZ switching is normal (specially with non zero voltage measured), as the compensation circuit for the switching charge can't be perfect with nonlinear capacitance of the FETs.Q105 should not be less of an issue (at least not cause a negative offset) as it is connected also for the low side reading and thus with compensation of the input resistance.U307 is originally a LF351. There should be better matches than an OP07, that could well be too slow. So more like TL071, AD711, LF411 or the LF351 as in the original. I don't see how a fast OP-amp there should cause much porblems. It is more that too slow an amplifier could cause instability / ringing. A faster amplifier may need better decoupling (e.g. maybe add an extra capacitor directly on top of the chip). However the +30 V supply may not like capacitive loading - so there could be an issue in the design. The TLE2071 has the oddity that the supply current goes up a low when saturated in one direction - this could be an issue more than the higher speed.Maybe also check if the +5 V is clean near the amplifier - it is used as a auxiliary reference and interference there could end up at the output.

author: stevopedia
author_rank: Contributor
timestamp: October 15, 2024, 01:07:24 pm
content: Yes, most of the sensitive nodes are on Teflon insulators.It isn't the autozero itself that's causing the spikes, it's the ADC switching between the S+4 and S-4 slopes during run-up (which happens very frequently with a 0 V input). I'll grab some scope captures when I can. Those spike are much more apparent with the input amp in the x10 and especially x100 gain configs.I'll freely admit that my understanding of things like circuit poles and zeroes aren't nearly as well-developed as I'd like. Why would a slower part oscillate where a faster one wouldn't? All things being equal I'd figure more gain at frequency would tend to reduce stability. Would a slower part necessarily have less phase margin at frequency?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2024, 04:37:53 pm
content: U307 works with local feedback for a gain of ~25 and thus quite some gain. The main loop for the amplifier has separate compensation (set by C303 and R324-R327 depending on the gain). A slow amplifier for U307 would additional phase shift and could destabilize to amplifier. The LF351 is already not that fast for the rather high gain.A problem with a fast amplifier for U307 could be that the +30 V is just from an OP-amp output and no good decoupling (and also not easy to add) at this supply.It is rather unusual to get effects from the S+4 / S-4 switching at the amplifier. Normally the amplifier should not see much, as the signal goes to the virtual ground at the integrator and only to tiny residual signal there (may be a few 10 mV) and some 100 K to the amplfier. So interference would more be via the supplies. Especially the +5 V may effect the amplifier output - so maybe check decoupling caps at the +5 V (at the ADC, maybe near the amplifier). An effect from the +5 V should still be about the same for all the gain settings - so likely not the culprit.Having some signal from the ref. switching at the amplifier or supplies may lead to nonlinear errors.

author: stevopedia
author_rank: Contributor
timestamp: October 15, 2024, 07:03:16 pm
content: I tried replacing Q110 with a new PN4392 and... it had no effect.I think Q101 can be ruled out because there is no change in the displayed offset when the analog filter is on or off. Q112 was changed previously, so unless the new part is unacceptably leaky it can be ruled out. That leaves Q105 and Q116. I haven't checked them yet.I tried some more investigation with warm air and everything I heated resulted in the offset very quickly moving in the positive direction--in fact it went comfortably positive (5-10 μV) before slowly falling back to -5 μV after heat was removed.@Kleinstein, your reply reminded me to get the scope captures--I'll do that now.

author: stevopedia
author_rank: Contributor
timestamp: October 15, 2024, 07:47:22 pm
content: Here are the captures. They were all made with the Volts input shorted, and the instrument in the 100 mV DC range with autozero on. The only difference is that some were made with the analog filter off and some were made with the filter on, as the file names indicate.To reiterate, the only time this appears to affect the measurement is when the 3456A is set to 100 cycles integrationandautozero is onandthe analog filter is on. Change any of those and the problem goes away.And this is all secondary to the offset anyway

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2024, 09:49:17 pm
content: There is quite some high frequency background at TP303. Also for the case with the filter off it does not look like a clean signal, but more like something oscillating. An oscillation can also cause an offset. At least it is not a good sign to have that much background.A point worth noting is that the background changes between the normal input reading and the zero reading in the AZ cycle. This makes me think of some unwanted coupling to the input / switching part or filter circuit and not so much the supply to the amplifier.

author: stevopedia
author_rank: Contributor
timestamp: October 15, 2024, 11:01:42 pm
content: I had my scope in peak detect mode as I normally do, which has the side effect of accentuating noise. But your point stands: the noise is greater than I'd like. What's most interesting to me is that in the filter off case the waveform at TP303 is identical for both the input measurement and the zero measurement, and for some reason turning on the analog filter, which shouldremovethis kind of noise, instead adds it in.In hindsight, I think I may have made an error when I replaced Q120 and U105 (gate bias amplifier #1) quite a while ago. I made the same mistake I almost made with U307: substituted a slower part (OP07 again) with the thought that this would improve stability and performance. I think I'm going to replace Q120 with an LSK389D (it is currently an LSK489, which has a transconductance about an order of magnitude lower than LSK389) and the op amp with a TLE2071.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 16, 2024, 06:37:06 am
content: For Q120 it is not about low noise, but reasonable low low offset and low input current. Especially a positive offset ( > 20 mV) could lead to addition input current. One could consider replacing Q120 and U105 with a low input bias MOSFET - possibly just a selected TL071H / TL031 or OPA145. So only a low bias OP-amp as a buffer (these were rare when the 3456 was designed and Thus the dual JFET plus OP-amp as the standard solution at that time).For U105 a slow OP-amp could be OK, as here the OP-amp speed sets the overall BW of the amplifier. The FETs just work as source followers and thus little effect of the transconductance on the BW. The limited slew rate of the OP07 may be more an issue than the GBW - not so much with the shorted input, but possibly with a higher voltage.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 16, 2024, 12:21:17 pm
content: I fixed an episode of buzzing on the amp. I think this is what you are talking about..... I do not know if this will help but here is a rendition of my notes from earlier this year:Had buzzing on the square wave output from amplifier. tried increasing the mica cap and this helped but noticed more square wave distortion on low range-----putting my finger on Q303 stopped buzzing, replaced Q 303 with 2N4117This fixed the buzzing but did not help the offset. I was still not able to adjust out the 100mV range to zero, it remained 120 uV low.I know my notes are not really complete but maybe check Q 303.that was not helping.EDIT: I also had trouble with a noisy 33 Volt Supply It was fixed by replacing caps.

author: stevopedia
author_rank: Contributor
timestamp: October 16, 2024, 03:37:15 pm
content: I GOT THE OTHER ONE!!Removing Q105 rewarded me with an input offset voltage of0.0 μV!!!I replaced it with my very last PN4392, and with the instrument cold/just powered on I had an offset of about -2.0 μV, which is very similar to that for the 3456A that's on my bench at work and which also has a current cal. I'm letting mine warm up now to see if the offset settles out to zero like it does on the working instrument.(Edited to correct the part changed)

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 16, 2024, 03:53:19 pm
content: I thought you had already replaced Q 110???

author: stevopedia
author_rank: Contributor
timestamp: October 16, 2024, 03:56:41 pm
content: I did. In my excitement I got it wrong--the one I actually did just now was Q105.Also, the new part produces the same offset as the old one. Still, this is a powerful clue, and if nothing else I'm fine with sacrificing 4-wire ohms for an otherwise working instrument.Edit:I can't even saythatmuch. The offset quickly increases (decreases?) to about -5.0 μV over the course of a minute or so after power-on even with Q105 removed. So much for that.

author: stevopedia
author_rank: Contributor
timestamp: October 16, 2024, 06:25:05 pm
content: So I replaced the PN4392 as Q105, and I think my pessimism over that not working was a bit excessive: the offset is definitely reduced with the new Q105 in place vs. either the old part or no part at all.I think the noise on TP303 (input amplifier output to ADC) is responsible for a lot of what I'm seeing now, both the offset and the noise in the measurement; the least significant (100 nV in the 100 mV range) digit is quite unstable/noisy even with the input shorted and generally always has been. I checked all the power jumpers by the ADC section (JMPR403 and JMPR503-507) and the +12 V and -12 V rails had the same noise that I saw on TP303. That certainly won't help, especially given the -12 V rail sets up the constant-current tail on the input amplifier's input JFETs Q310 and Q311.Looking at the schematics (mine has the LM399-style reference of schematic 5B), there's shockingly little in the way of filtering on those rails. I count exactly one cap on -12 Va, a 0.1 μF wet tantalum (C401) which won't do much to reduce high-frequency noise. -12 Vb has a 2.2 μF wet tantalum, C404. But the +12 V reference hasno filtering at all!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 16, 2024, 09:45:25 pm
content: Not much decoupling on some of the voltages is not good. However the problem is that some voltages, like the +12 V,-12VB or the +30 V (at the amplifier) are directly driven from OP-amps and thus can not have much capacitive load. So there is no easy way to improve on that situation. Part if the signal on the supply is likely normal and that way by design. Instead of a normal decoupling capacitor one may have to include a series resistance to get at least some dampening.At leat the +12 V part seens very little load and should not see transients - so here capacitance is not needed.The problem is more the -12 V.I agree that the "noise" / high frequency background on the amplier signal can be a problem. It may be enough to get some rectification at the JFETs and additional bias this way. If all supplies and the signal seem to have the same noise, maybe check the ground that is used for the scope. It could be just a bad ground point.Not having reference filtering is a common point in many DMM designs. The LM399 ref. has quite some white noise that can effect the ADC. This part is missed in other DMMs too. Adding some ref. filtering could be possible, e.g. at the input of U501 and between pin 2 and 6 of U505. It could be worth the relatively moderate effort.

author: stevopedia
author_rank: Contributor
timestamp: October 17, 2024, 01:02:38 pm
content: For the moment I'm focusing on the slope-switching noise/hash I'm seeing on the +/- 12 V references--it doesn't show up on any of the other power rails. Of these, -12 Va is probably the most important, since the others are generated from it, as you noted. I think the noise is coupling in there and U504 and U505 are faithfully duplicating that noise on their outputs.Yesterday I did a little further investigation. I found that the switching hash was present on pins 2 (- input) and 6 (output) of U501, but not at pin 3 (+ input). I connected a 470 pF ceramic cap directly across U501 pins 2 and 7 (ground, conveniently) on the bottom of the board and this quieted U501's output beautifully. The noise was still present on the collector and emitter of Q501. (It was much stronger at the collector--I think Q501 is acting as a common-base amplifier for it in this case. Not especially important, but I thought it was an interesting observation.)U402 looks like it could be the source of the switching noise. I'll try putting a 470 pF cap across its power inputs (not coincidentally -12 Va and ground) and see what happens.

author: stevopedia
author_rank: Contributor
timestamp: October 17, 2024, 03:43:16 pm
content: It didn't work. Nothing changed.I amsoclose to giving up on this thing again. I don't know why I'm suddenly so demoralized--perhaps because I thought I'd finally cracked it after ten years--but the fact remains.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2024, 07:02:08 pm
content: The collector of Q501 should not have much AC signal and there is already a capacitor to ground (C506). One could try adding extra capacitance there. Different from the amplifier output there is no problem adding capacitance there. With not enough capacitance there the transistor can not drive the -12 V fast / strong enough as it should. Maybe a problem with the ground connection ?There is already a 100 nF capacitor (C401 on the ADC page) at the -12 Va. So 470 pF more does not make a big change.From the scope pictures it looks a bit like ringing as the interference signal. Some RC combination e.g. 22 or 39 ohms and 100 nF could help in dampening the oscillation more than added capacitance.Also just a little more load (e.g. 5 K to GND) to the -12Va could help, as the output resistance of the transistor gets lower with more current.The loading of the reference switches is a principle tricky part for this switch configuration. So don't expect the -12 V to be perfect, there is some good enough. After all the HP3456 normally works OK and is usually considered a good meter for it's time, depite the weakness with the -12 V ref. / supply.The question is only how much AC part is normal acceptable for the -12 Va.

author: stevopedia
author_rank: Contributor
timestamp: October 18, 2024, 01:33:31 pm
content: Quote from: Kleinstein on October 17, 2024, 07:02:08 pmYou're absolutely right, of course. I've been letting myself get carried away. I think it's time to take a deep breath and a step back. I also have the luxury of access to a working 3456A I can take reference measurements from--a capability I haven't taken advantage of.

author: stevopedia
author_rank: Contributor
timestamp: October 23, 2024, 12:46:40 am
content: I took the opportunity today to measure the gate leakage current of all the switching JFETs I've recently extracted from the 3456A with a Keithley 485 picoammeter. For most of the FETs it was too low to measure (less than 100 fA!). But for the one that was on the analog filter circuit, it measured about 50 pA with 18 V reverse bias on the gate-channel junction. On a whim I replaced Q102, Q103, and Q104 with some of the verified-good original 1855-0298s, but as expected this made no difference.With the Volts inputs shorted, the meter's offset is about 6.5 μV. I tried shorting the lower input node (Q102, Q103, Q104, Q105) to the low Volts terminal with a probe lead and the displayed offset went to zero, as expected. But when I shorted the upper input node (Q109, Q116, Q115 etc) to the low volts terminal, the offset shot all the way to -1.1mV, a full three orders of magnitude higher! I don't know what to make of that, though it would seem to me that if the instrument was working perfectly then shorting the upper input node to ground should also result in a 0 V reading... though I'll test that on a known-good unit tomorrow.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 23, 2024, 12:57:54 pm
content: Soemthing like 50 pA at 18 V is about what is expected from a somewhat bad JFET. Less than 100 fA is surprisingly good for the good ones.Getting the zero readign with the lower input not shorted is no surprise, as both reading of the AZ cycle would see the same voltage and thus zero difference. There much be something badly wrong to get a different result.Connecting the upper input node (Q109,Q115,Q116,...) to the input should not make that much difference. It could make a difference for some self test / gain check, that tries to read the internal -12 V reference. To protect Q112 one may want some series resistance (e.g. 10K) and not a hard link to the input terminals. A wrong ref. reading would result in totally off readings - maybe the factor 1000 to high, though it should trigger an error message too.The test with the upper terminal ground could still show a small offset, e.g. from amplifier input bias acting on R104 and the offset from R149/R150 that would still be active for the zero reading.A point to check could be if the offset changes between 1 PLC and 10 PLC setting (100 mV range). A difference can come from addition input current from the switching / compensation ciruit around R147/R148 / Q102.Too much offset of the Q120/ U105 combination could still be an issue here, though I would expect only a small error. If needed one could adjust R126 or R127 to trim the offset. This may be easier than exchanging the dual FET.

author: stevopedia
author_rank: Contributor
timestamp: October 24, 2024, 03:14:53 pm
content: To be clear, my experiments with directly connecting the input nodes to the low Volts input with the Volts inputs also shorted together were only temporary, using a regular "pokey" multimeter probe, just to see what happened. The meter never does a check of the -12 V reference (via Q112) unless it's in the self-test mode, and I had it in the normal DC V mode. I'd have to repeat it to be sure, but I don't think AZ made a difference.The effect of shorting the high node to the Volts input terminal is to bypass the section from R103 through Q116. What doesn't make sense to me is why that should result in the meter's displayed offset beingalmost a thousand times greaterthan without the short, and AZ having no effect.GB1 (Q120/U105) is working fine; offset is about -6 mV.Another thing I noticed: with the Volts inputs shorted, I connected one end of a meter probe to the low Volts terminal and touched the other to GND at TP101, which should have literally no effect (the schematic shows the terminal shorted to ground). The displayed offset went from about 7.0 uV to about 4.0 uV and returned to 7.0 uV when the probe was removed. I think that indicates at least part of what I'm dealing with is that I need to implement the one service note HP put out for the older 3456As that involves drilling out the rivets originally used for grounding the outguard case parts and replacing them with screws. The purist/preservationist in me is sad that I can't (easily) get the prescribed Pozidriv hardware, but such is life.

author: stevopedia
author_rank: Contributor
timestamp: December 09, 2024, 03:16:08 pm
content: It's been a while since I last posted an update here. I've tried a bunch of things, and now I know a lot of things that don't affect the offset issue!Implemented the service note where the grounding rivets are drilled out and replaced with screws: No effect.Replaced Q101: No effect.Replaced some of my replacement FETs with verified-good original parts: No effect.Carefully and scrupulously cleaned the top and bottom of the A20 board in the vicinity of the input switching circuit, separately using 99% isopropyl alcohol, acetone, and soapy (Triton X-100, a nonionic surfactant, in the form of Photo-Flo) water: No effect, or maybe a few single-digit counts on the 100 mV range (tenths of a microvolt).I was messing around with it last night, probing with my oscilloscope and another DMM, when I noticed that the offset had dropped to -1.5 μV or so, or well less than half its normal value of about -5.0 μV, and I have no idea why. I also found that turning on my desk lamp, which as an LED bulb and is connected to the same outlet (an isolation transformer) as the 3456A, consistently reduced the magnitude of the displayed offset by about 1.0 μV (10 counts on the display). I turned it off for the night, and this morning I turned it back on and the offset was back to its usual value, though I was able to reproduce the lamp's effects again. I wonder if I'm dealing with common-mode noise on the power input generating this offset somehow. If so, that raises the question of why the autozero doesn't cancel it out--that's the whole purpose of having an AZ system!I dunno, man. This thing isstubborn.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2024, 04:20:31 pm
content: An EMI effect is a real possibility. EMI (e.g. from a noisy LED lamp or SMPS) can cause some extra offset, that the AZ mode is not allways able to correct. The AZ mode is not made to correct EMI effects.When the 3456 was build there were a low less SMPS and cell phones around and no Wifi. I don't really think it is common mode noise, as the power supply is still via a classic mains transformer and not a separate DCDC converter. A CM filter at the mains side could still have an effect (could be part of the line filter already).It can still be tricky to improve the EMI suppression, maybe some ferrites at the input side. In many other aspects at least the circuit diagram looks good - could still be a layout thing for the really high frequencies. For a quick check one could test click on ferrites with the mains cable or a the analog inputs.

author: stevopedia
author_rank: Contributor
timestamp: December 09, 2024, 10:44:03 pm
content: I agree. I don't doubt that's at least a significant portion of what I'm seeing, especially with respect to noise. After all, I live within line of sight of an FM broadcast transmitter tower!It gets weirder, though. I left the 3456A on during the day with its volts inputs shorted and the outer upper cover sitting on top of the instrument. When I got home from work the display read about -7.0 μV. I took the cover off and set it aside. A few minutes later, the display was bouncing around +/- 0.2 μV! I rested the top cover back on the DVM and the indicated offset slowly began to rise again.With the cover on, I tried to reproduce the effect my desk lamp had, and--nothing! I knew I saw what I'd seen then, so I took the upper cover back off and bingo--instantly reproducible. I realized that I had the desk lamp pointed directly at the instrument so I tried blocking the light, and the effect was the same as if I'd turned the light off and with immediate effect. So either I'm seeing something thermal that's ridiculously sensitive, or there's something photosensitive in the input switching area.As it happens, C104 on my instrument appears to be some kind of ceramic capacitor in a transparent glass or epoxy housing (not sure which; haven't touched it to find out). The parts list says it's just a 470 pF, 100 V DC capacitor, but for some reason HP saw fit to use a different component here instead of the identically-rated polypropylene caps specified for A20C101-103. Of course the part number and mfg code point to an HP custom part (probably a custom spec) so it's impossible to know for sure what the difference is, but it's worth at least trying a replacement.Edit:I just tried moving my desk lamp right up to C104, and I could make the offset go as strongly positive as it went negative! I wonder if these caps were always light-sensitive, or they became more sensitive with time.Second edit:It's not C104, but there'sdefinitelysomething either directly photosensitive or something phenomenally sensitive to temperature on that board somewhere.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2024, 11:10:16 pm
content: Some parts are indeed light sensitive. This was however less an issue in the old time with THT parts, but more with small SMD parts like TSSOP and similar with not much plastics. In the old days it were diode (and transistors) in a glass case. For some odd reason they still make low leakage diodes in a glass case.Thermal EMF can produce an offset of a few µV with some parts.For the FM band it can make a difference of the cover just rests on the meter or it is screwed down. For me RF effects are still a bit unpredictable.