HP 3465A DMM repair - Page 1
Forum section: electronics / testgear

author: bd139
author_rank: Super Contributor
timestamp: May 24, 2022, 09:07:25 pm
content: Quick repair on this one. Obtained from a hamfest. Unloved and uninterested. 4.5 digit manual range bench multimeter from HP. This one circa 1979. Felt sorry for it so here it is.Of course when I got it, it was broken. Drifting on all ranges and a severe offset towards one polarity on every measurement. Checked the power supply and reference voltages and all good. So I spent some time staring at the wrong version of the service manual. Of course there are a couple of revisions of this DMM and the components are in different places. Eventually found the correct one, which was the really badly scanned one from Keysight. Ugh.First diagnostics concentrated on the auto zeroing circuit. This was eliminated after grounding the input FET and getting sensible zeroing. The front end was buzzed out and impedances checked. All good. Hmm. Eventually I was poking around the Hi-Z parts of the input chain and one of the input clamping diodes fell to bits!As you can see the leg has disintegrated. When this is out of circuit it behaves properly! The diode does look like a diode out of circuit but clearly has some leakage problems going on.This is an HP 1901-0586 which is completely undocumented and expensive to get hold of. The service manual specifies it as a general purpose diode 30V 25mA, so pretty much anything will do here apparently. Well that's not good enough for this as it's very high impedance IMHO and it was clearly affected by leakage of some sort. I'm not paying for an original diode so after doing some research and simulations of the clamp in LTspice, the B-E diode in a 2N3904 is sufficient for the job so I will snip the collector off one and solder it in, then carefully clean it and re-test.That I will do tomorrow.Calibration is slightly off which is probably my fault for turning the wrong pot inside when I was checking out the amp balance.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 24, 2022, 10:10:42 pm
content: Quote from: bd139 on May 24, 2022, 09:07:25 pmFortunate accidentQuote from: bd139 on May 24, 2022, 09:07:25 pmNice way to replace a hard-to-find low-leakage diode. I read about it, never got to try it myself. Low leakage PN4117s connected as diodes (S and D tied together as the cathode, G as the anode) are cited to work better, but if 2N3904 is good enough, why not. Let us know how it behaves. I have a handheld DMM drifting on DCV only, was planning to work on it. Now I'm inspired

author: bd139
author_rank: Super Contributor
timestamp: May 24, 2022, 10:29:01 pm
content: I actually looked at JFETs, but not the that one. That specifies 1pA leakage but apparently you get around 4-5pA on a 2n3904 B-E junction which is probably fine for this job. The auto-zero will kill any small persistent offsets off from leakage. The problem I had was it was dragging it heavily down. I probably don't have the time or energy to work out leakage here so it'll be a cut and try approach.And I just discovered I have no 2N3904s around so this will be a few days off. And on to the HP 427A sitting here. That uses current injection into a FET to kill offsets as well. More fun which will probably be broken!

author: MrYakimovYA
author_rank: Regular Contributor
timestamp: May 25, 2022, 12:22:51 am
content: Nice old equipment! I do like it! Some time ago I bought an old GDM-8145 for $25. It is completly working! Sorry for offtop) Manual range switching is fantastic. Sometimes it is more convinient than auto ranging or pushing button "Manual" on autoranging devices.

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2022, 01:07:17 am
content: Would think you should get lower leakage and higher breakdown from the B-C junction rather than the B-E junction of the 2N3904.Best,

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 25, 2022, 02:28:33 am
content: Quote from: bd139 on May 24, 2022, 10:29:01 pmQuote from: mawyatt on May 25, 2022, 01:07:17 amAccording to this plot in the Art of Electronics, the leakage current of the B-E junction of 2N3904 is <100fA (up to ~80mV). Not bad at all... It's also smaller than the leakage current of the B-C junction at the same forward voltage. The breakdown voltage is indeed 10x higher for B-C, per the data sheet (60V vs. 6V).(leakage forward current vs. applied voltage)

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 06:50:56 am
content: Good find with that reference. Didn’t even think to look in the bible for this one

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 06:52:28 am
content: Quote from: MrYakimovYA on May 25, 2022, 12:22:51 amIndeed. I generally tend to prefer manual range meters myself as well.

author: BU508A
author_rank: Super Contributor
timestamp: May 25, 2022, 12:07:11 pm
content: Quote from: bd139 on May 24, 2022, 09:07:25 pmA proper replacement for the input protection diodes would be the PAD1. But they are not cheap, I can tell you!Mouseris asking ca. 15 Euro per diode.Attached datasheet from Siliconix.

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 12:25:15 pm
content: That’s what I paid for the entire meter2N3904 it is

author: Vince
author_rank: Super Contributor
timestamp: May 25, 2022, 12:28:41 pm
content: BD, this thread is lacking in the annotated schematic department... this thread needs more illustration please !

author: MrYakimovYA
author_rank: Regular Contributor
timestamp: May 25, 2022, 12:39:31 pm
content: Quote from: Vince on May 25, 2022, 12:28:41 pmThere are plenty of circuits diagrams, theory, figures, descriptions ():http://hparchive.com/Manuals/HP-3465A-Manual-SNprefix-1546.pdf

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 12:51:10 pm
content: If only it was that easy. The revision of the DMM I have is not covered by that manual and the component placement and front end are slightly different.The only copy is the Keysight one and that's awful quality scan:

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2022, 12:52:04 pm
content: Quote from: Caliaxy on May 25, 2022, 02:28:33 amInteresting, would not expect the B-E junction to be lower, even in forward direction!! However, suspect the protection diode in the meter is used in reverse direction and the B-C junction should have lower leakage in reverse as well as higher breakdown.How is the protection diode used in the meter? This will determine which PN junctions in the 2N3904 are best for the intended meter use.Best,

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 12:53:21 pm
content: Purely coincidence but I just posted the front end and clamp in my previous postCR9 is the faulty diode.

author: xrunner
author_rank: Super Contributor
timestamp: May 25, 2022, 12:53:27 pm
content: Quote from: Vince on May 25, 2022, 12:28:41 pmDid you check that option when you paid for your subscription?

author: Vince
author_rank: Super Contributor
timestamp: May 25, 2022, 01:08:10 pm
content: Quote from: xrunner on May 25, 2022, 12:53:27 pmPretty sure I did !

author: Vince
author_rank: Super Contributor
timestamp: May 25, 2022, 01:23:09 pm
content: Quote from: MrYakimovYA on May 25, 2022, 12:39:31 pmYes I know what a SM is thanks... but the point of a repair thread is to explain and show people what there is to show to understand and see what's going on. If it's just about a one liner " I replaced a diode have a good day", well might as well not create the thread to begin with

author: Vince
author_rank: Super Contributor
timestamp: May 25, 2022, 01:27:23 pm
content: Quote from: bd139 on May 25, 2022, 12:51:10 pmWow crap scan indeed, can't even see which one is CR9 !Looks like the one on the up side of the input path, going to the negative 7V rail.... OK... so you were wrong, your offset repair DOES pertain to my old Rochar Nixie DVM, as this is general purpose stuff and my meter has something similar a s well

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 01:48:59 pm
content: Quote from: Vince on May 25, 2022, 01:23:09 pmTo be fair I did say "one of the input clamping diodes" which is pretty easy to locate if you have any interest in the front end of the DMMs.Sometimes it's better to leave breadcrumbs and a journey because the people who don't follow the breadcrumbs as a rule aren't going to be satisfied with the destination either

author: bdunham7
author_rank: Super Contributor
timestamp: May 25, 2022, 03:07:56 pm
content: Quote from: bd139 on May 24, 2022, 09:07:25 pmThe original HP part appears to be a "low-leakage PIN diode" according to some descriptions, but it also might be a JFET in disguise. I believe someone else has already pointed out an appropriate replacement from InterFET. In this case, I think it is just the low reverse leakage that matters, not much else. However, looking at the circuit you posted, I think a reverse-biased 2N3904 BE junction won't quite cut it as the circuit as shown could have as much as 8 volts reverse bias across one diode before the other one clamps. Having the reverse-biased BE junction go avalanche might actually sort of work in an unintended way, but I'm not sure it is a good long-term plan.The CB junction might be a better bet, otherwise the PN4393 JFET from Central Semiconductor is something I've used in this sort of application with excellent results. At 4 volts they would likely leak <1pA. And you should check to make sure that the resistors and zeners in the biasing circuit for these clamps are all working properly as well.

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 03:25:26 pm
content: Good points there. Actually I just noticed there was an error in the LTspice simulation I did of the clamp. Just running my test cases through it again now...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2022, 03:51:45 pm
content: There are also afforable low leakage diodes BAV199 available in a SMD case. Not tested for super low current, but typical leakage current < 10 pA.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 25, 2022, 05:05:26 pm
content: Quote from: bdunham7 on May 25, 2022, 03:07:56 pmTotally agree, the B-C junction would be the appropriate one here (for 2N3904), as both diodesarecan get reverse biased at a voltage higher than the reverse breakdown voltage of the B-E junction...

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2022, 05:07:56 pm
content: After the introduction of silicon planar transistors, you should always assume that the B-E reverse breakdown voltage is much lower than the B-C breakdown voltage, even for high-voltageVCEOdevices.

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 05:28:55 pm
content: The voltage is clamped at +/- 5V by the opposing circuit so it will never hit the 6v breakdown voltage, thus the B-E junction is fine.(checked with LTspice)Edit: this is actually wrong. I am clearly drunk driving LTspice at the moment. There is 9v across the junctionJFET it is!

author: Zoli
author_rank: Frequent Contributor
timestamp: May 25, 2022, 05:47:38 pm
content: Quote from: bd139 on May 25, 2022, 05:28:55 pmSorry to ask, but CR10/11 isn't 3.9V Zener? Better check in reality, because I wouldn't trust the schematics...

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 05:48:45 pm
content: CR10/11 are 3.92v zeners. CR8/9 are low leakage diodes. So the zeners set the clamp constraint plus one diode drop across the low leakage diodes.

author: Zoli
author_rank: Frequent Contributor
timestamp: May 25, 2022, 05:57:55 pm
content: Quote from: bd139 on May 25, 2022, 05:48:45 pmOK so (3.9+0.6)*2=9V; now I understand.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 25, 2022, 06:30:52 pm
content: Quote from: bd139 on May 25, 2022, 05:48:45 pmDat scan... The one in MrYakimovYA's link is much better. What's the expected voltage range in the node pointed by the arrow "See Note 5" during normal operation? What happens if you apply there, say, +3V?

author: bd139
author_rank: Super Contributor
timestamp: May 25, 2022, 06:48:03 pm
content: It’s also a different revision of the DMM. I think that part of the front end is the same though.Range from my LTspice sim applying +/-1000V to the left of the 100k resistor is +/-5v at the clamp point.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 25, 2022, 07:40:52 pm
content: Quote from: bd139 on May 25, 2022, 06:48:03 pmRight, but for applied voltages that result in more than 2.1V (6-3.9V) and less than (3.9V + 2N3904 B-E drop) at the clamping point (which might or not might not be within the intended operating range, I don't know), the other B-E junction will clamp (or try to...) by breaking down due to reverse bias >6V. This might work fine for a while, but it's not ideal (and would make the B-C junction a safer bet). I believe this is the main point bdunham7 was making above (Reply #20).

author: MrYakimovYA
author_rank: Regular Contributor
timestamp: May 26, 2022, 01:44:20 am
content: Quote from: Kleinstein on May 25, 2022, 03:51:45 pmI looked over a couple of datasheets. It's strange for me but there aren't any detailed information of the leakage current. Either a typical value (3 pA) or max. value (80 nA at reverse voltage 80 V). It's very strange inspite of BAV199 is specified as low-leakage diode set... So I guess they must be sorted and tested before installation in equipment...

author: MrYakimovYA
author_rank: Regular Contributor
timestamp: May 26, 2022, 01:58:51 am
content: Quote from: bd139 on May 25, 2022, 06:48:03 pmI just forget about a lot of revisions, sorry

author: Robert763
author_rank: Super Contributor
timestamp: May 26, 2022, 07:23:22 am
content: Quote from: Vince on May 25, 2022, 01:27:23 pmTo be pedantic these HP (and a lot of US military document digital archive documents) are not bad scans, they are scans of bad microfilm / microfiche "originals". These were typically done to comply with archive legal requirements by third party companies who didn't really care about quality.

author: xrunner
author_rank: Super Contributor
timestamp: May 26, 2022, 11:42:29 am
content: Looks like the adjustment procedure (Cal) is pretty easy. Power supply uses jumpers to adjust the 10 V reference. Says you need 1k & 10 M 1% resistors, and an AC low voltage source "standard" up to 20 kHz.Of course you can compare its readings to another reliable DMM too if you have one - I think you do ...

author: bd139
author_rank: Super Contributor
timestamp: May 26, 2022, 12:07:05 pm
content: Yes it's fairly easy. Actually I checked the 10V cal against the 34401A and it's showing 10.00012 which is surprisingly good. Either that or both are surprisingly wrong in the same direction. Probably should get the 34401A calibrated as last cal was with HP in 1999.AC voltage standard is going to be an AWG with a comparative measurement. The resistors I have and didn't manage to throw away in the grand cock up. A few years back I bought a full set of Dale 0.1% resistors from 10 ohms to 10M in magnitude steps just for this scenario

author: BU508A
author_rank: Super Contributor
timestamp: May 26, 2022, 12:11:10 pm
content: Quote from: bd139 on May 26, 2022, 12:07:05 pmAnd you didn't throw them away by accident?

author: bd139
author_rank: Super Contributor
timestamp: May 26, 2022, 12:17:57 pm
content: No they were in the "really important stuff" boxI needed more of those boxes

author: xrunner
author_rank: Super Contributor
timestamp: May 26, 2022, 12:26:52 pm
content: Quote from: bd139 on May 26, 2022, 12:17:57 pmYea that's the key - label all important boxes with "really important stuff" so you don't lose them. Trouble is I forgot where my labeller is!

author: AVGresponding
author_rank: Super Contributor
timestamp: May 26, 2022, 12:56:24 pm
content: Quote from: xrunner on May 26, 2022, 12:26:52 pmObviously you should have put it in a box labelled "really important stuff"...

author: bd139
author_rank: Super Contributor
timestamp: May 26, 2022, 06:57:15 pm
content: Ordered a 2N4117A JFET for £3.50 from Langrex. Will install and validate again.

author: tggzzz
author_rank: Super Contributor
timestamp: May 26, 2022, 07:10:38 pm
content: Quote from: bd139 on May 26, 2022, 12:07:05 pmCheat: with a 0.1Hz signal, use the 34401 to measure min and max DC voltages. Easier with a 34410, though.Then presume the AWG will have the same peak output voltage at 400Hz.QuoteI'm just about to buy some 100/1k/10k 0.1% resistors, to replace the o/c wirewound (silk covered enamelled wire on a PCB bobbin!) resistors in a Muirhead decade resistance box.

author: bd139
author_rank: Super Contributor
timestamp: May 30, 2022, 09:25:56 pm
content: Ok 3465A is now fixed. 2N4117A subbed in as a diode connected FET as suggested by both HPAK and EEVblog forum members. Works perfectly, clamp works too.Did DC and ohms cal. I don't have any test cables () to do the AC cal yet.Thanks all for suggestions and support

author: MrYakimovYA
author_rank: Regular Contributor
timestamp: May 31, 2022, 01:00:05 am
content: Quote from: bd139 on May 30, 2022, 09:25:56 pmOh, my congratulations! You've done a great work! I'm glad you have this nice DMM in working state now!