Agilent 34401A Ohms does not Zero with a short across terminals -FIXED !!!! - Page 1
Forum section: electronics / testgear

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 12:50:44 am
content: Agilent 34401A Ohms does not Zero with a short across terminals. A short reads about 10 ohms. All resistance readings are 10 ohms high.If I do a NULL it reads zero, but the resistance drifts or bounces. Turn off and back on the NULL is no longer.This one came back from calibration as not good. I swapped the main PCB with another one I had with a dimmer display (wanted to keep same external SN). That is working and is back off for calibration again. I put the bad board in the one with the dimmer display and it has the same problem. Ohms does not zero with a short.All other functions seems to be fine.I'm looking at Q203->Q210. All the PNPs seem to check okay in-circuit. The manual points to a failed Q202 or Q211 (I can't figure out what that one is doing other than used for gate leakage current).Q202 is a MMBF5461 that I can not find anywhere.Is this a common failure, and of so, what JFET is a good substitute. If that would not fix it, I don't know what else to look at.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2023, 01:02:46 am
content: I'm not sure why you are looking at the ohms current source for this fault without checking the test current in ohms mode. If you select the 2W 100R range manually you should see ~1mA and about 6.8V open circuit. If you get that, the entire ohms current is not your problem. If you're reading 10R in the 100R range with a short, that indicates that there is about 10mV on the sense circuit.Try testing with a 4W short in 4W mode (no need to get fancy, test lead will do) and see if you get the same result. Also, check the 100mV range and see what you have with the input shorted. Unfortunately, you may have an issue with U101.

author: Martin72
author_rank: Super Contributor
timestamp: July 26, 2023, 01:11:21 am
content: Quote

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 11:40:08 am
content: "I'm not sure why you are looking at the ohms current source for this fault"Cause I don't know what I am doing trying to trouble-shoot this problem. That is why I am here. 8-)" without checking the test current in ohms mode. If you select the 2W 100R range manually you should see ~1mA and about 6.8V open circuit. "in 100R manual range, I am seeing .997ma and 6.85volts. This is what you say to expect."If you're reading 10R in the 100R range with a short, that indicates that there is about 10mV on the sense circuit. "Where do I start looking for that? C102 (on HiSense) has 31.2mv on it (no resistance load).The reference voltages are all good.Does the HiSense go to AMP+ in to the input amp on the 3rd page of the schematic (same page with the resistance current source)?

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 12:05:23 pm
content: P.S. 4-wire mode works perfect.https://www.utsource.net/sch/1SK6-0001.html$25

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 12:14:58 pm
content: More info that may help:As measured from the negative input to U101-5: the resistance value that is floating around matches the voltage reading, in mv, at U101-5ex: input shorted, meter reads 4.6 ohms, voltage at U101-5 reads .0046volts.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2023, 12:21:23 pm
content: If the 4 wire mode works perfect, the problem is likely with the front-rear switch or maybe a realy that is in the path for 2 wire ohm. A first try / check would be repeated (e.g. 10-20 x) switching and look if the reading with a short gets lower. I would be as little as dirty contacts that can clean from use.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2023, 01:44:08 pm
content: Quote from: daveyk on July 26, 2023, 11:40:08 amNo, look at the previous schematic, 9-9 Function Switching. You'll see that the HI_SENSE and LO_SENSE are both switched by U101. It measures both and then calculates the difference, or IOW the autozero uses the LO-SENSE rather than AGND. So the input (sense) circuit for 4W is completely different than the 2W. There's also a function switching chart on p.95 of the service manual, Ch. 5 (Theory of Operation) that explains this.QuoteIf you short the inputs in the 100mVDC range, what do you get? If it is acceptably close to zero, then you may just have some resistance somewhere in the circuit that shouldn't be there. The Front/Rear switch and K101 are probably your main suspects at that point.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 26, 2023, 01:56:32 pm
content: Yes, at the very first, please check the front / rear switch!Put a short on the rear INPUT jacks as well, select 2W Ohm, and switch to REAR.If there's a significant difference, or if the reading changes strongly when you actuate the front/rear switch, then latter is the culprit.Cleaning / Repair has been described already, somewhere here.Next candidate would be Relay K101, whether pins 3/4 are lo Ohm.Meanwhile, please hands - off the transistors and the current source!Frank

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 02:19:39 pm
content: I think I have it.If I short out the contacts of K101, the problem goes away and it measures resistance correctly in two wire mode.Thank you kindly for your assistance. Now to find another relay or see if I could do some surgery (doubtful).

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 02:20:19 pm
content: If I short out the contacts of K101, the problem goes away and it measures resistance correctly in two wire mode.Thank you.

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 02:49:13 pm
content: After some research on the innerwebs, I ordered a Koto 3501­-05­-511 from Mouser to replace K101. It was the closest I could find that Ithinkthe pin pattern will match. We'll see. It is only rated at 200v, where as the one in the service manual shows 500volts.I can find a "pull" on ebay, but what's the odds it is good?

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2023, 03:17:36 pm
content: Quote from: daveyk on July 26, 2023, 02:49:13 pmThere is some risk with a used relay, but I'm not sure a lower voltage replacement is a good idea either. You can try few alternatives:-contacting Coto (p/n 3200-0121) and Keysight (p/n 0490-1914) directly-contacting the NSN resellers (this might be expensive) with NSN 5945-01-396-3010-wait for one to show up on eBay

author: AVGresponding
author_rank: Super Contributor
timestamp: July 26, 2023, 04:04:10 pm
content: Quote from: daveyk on July 26, 2023, 02:49:13 pmYou'd be better off finding one that matches coil current and voltage, and contact ratings. You can always make an adapter board, if you can't source a new COTO one.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 26, 2023, 04:49:59 pm
content: Quote from: daveyk on July 26, 2023, 02:49:13 pmIf I remember correctly, we had this exact same discussion recently, with the same error of K101.The HV version of the relays was unobtainium, no chance asking COTO, and the other guy asked, if the degradation of the voltage rating would be OK.My opinion: For personal / non professional use, that's fine, if you don't use the DMM at higher voltages.Don't sell it to other people, then it's also fine.Frank

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 05:18:38 pm
content: "My opinion: For personal / non professional use, that's fine, if you don't use the DMM at higher voltages.Don't sell it to other people, then it's also fine."I do not read voltages about 16 volts with the 34401a. I basically use it as a A/D converter, HOWEVER, it does get certified once per year.I wonder how much voltage does show up ACROSS the contacts? There is a lot of resistance, in the HIGH input network. In DC mode, it looses its ground reference with K102 open. In DC mode it looks like about 10Meg Ohm to ground. Alas we are dealing with voltage and not current. I don't know how much that 10MOHM resistance discourages arcing.Someone else had a suggestion. I am going to look for a reed relay that can handle some voltage and make an adaptor PCB.

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 05:29:45 pm
content: "You'd be better off finding one that matches coil current and voltage, and contact ratings. You can always make an adapter board, if you can't source a new COTO one."That's a good idea. Here is one possibility:https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3DContact resistance is rate at 150mOhm. I think the original had a rating of .200 ohms max. So, it sounds like it should work with an adaptor pcb. Will an adaptor PCB introduce any issues? (noise/oscillations)?

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 05:32:19 pm
content: "If I remember correctly, we had this exact same discussion recently, with the same error of K101."Dr. Frank, what do you think about this one with an adaptor board?https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3D

author: alm
author_rank: Super Contributor
timestamp: July 26, 2023, 05:35:23 pm
content: Quote from: daveyk on July 26, 2023, 05:18:38 pmIs this certification limited to 16V? Normally a cal lab will test all the ranges up to their max value, e. g. 1000 VDC and 750V AC up to 100 kHz. I think you should at least clearly mark it for its limited input voltage on the front panel and notify the cal lab when you send it in if your argument is "I only use it up to 16V".

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 05:55:00 pm
content: "Is this certification limited to 16V? Normally a cal lab will test all the ranges up to their max value, e. g. 1000 VDC and 750V AC up to 100 kHz. I think you should at least clearly mark it for its limited input voltage on the front panel and notify the cal lab when you send it in if your argument is "I only use it up to 16V"."Where does it get its 1000volt input now? That relay may have two reed elements in series, but the relay, in the service manual, is only rated at 500v?I am also trying a 500v reed relay with an adapter pcb. We'll see how that goes. Again the relay is only rated at 500volts, the same as the HP/Agilent but 1000volts in to the meter.If anything, I would have the lab limit their voltage cal to 200volts DC.Dave

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 26, 2023, 07:07:04 pm
content: Quote from: daveyk on July 26, 2023, 05:32:19 pm.. looks fine ... if some time in the future my own, 30 y/o HP34401A will show this error.. I'll remember this

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 07:09:02 pm
content: ".. looks fine ..."Thank you. It should be delivered Friday. I will be anxious to install it and test. 8-)

author: yo0
author_rank: Regular Contributor
timestamp: July 27, 2023, 05:06:49 am
content: same problem in the past, still working finehttps://www.eevblog.com/forum/repair/agilent-34401a-2w-drift-and-high-readings-when-power-up/msg1270277/#msg1270277best regardsPio

author: HighVoltage
author_rank: Super Contributor
timestamp: July 27, 2023, 08:12:14 am
content: Years ago, I was able to buy the original relay from Keysight.Who knows, maybe it is still available, even if it is not listed anymore.I would ask Keysight.Coto will not sell it, because it was made for Keysight / Agilent.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 27, 2023, 04:05:43 pm
content: Quote from: daveyk on July 26, 2023, 05:29:45 pmTo some degree, it's a case of "suck it and see". If you keep the traces and leads as short as possible, it'll reduce the chances, at least.

author: daveyk
author_rank: Frequent Contributor
timestamp: July 28, 2023, 07:14:13 pm
content: ".. looks fine ... if some time in the future my own, 30 y/o HP34401A will show this error.."New 500volt relay installed and working perfectly.

author: daveyk
author_rank: Frequent Contributor
timestamp: July 28, 2023, 07:19:44 pm
content: "To some degree, it's a case of "suck it and see". If you keep the traces and leads as short as possible, it'll reduce the chances, at least."It's not like we are dealing with a high bandwidth instrument.I replaced the relay with a new 500volt relay that arrives a few hours ago and it is working great again. I'm shipping it off for calibration in Monday.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 29, 2023, 08:02:23 pm
content: True, but with the test current at the higher resistance ranges being only 500nA, it wouldn't take much to add extra noise in there.Don't forget to let us know how the calibration goes!

author: daveyk
author_rank: Frequent Contributor
timestamp: August 02, 2023, 04:05:29 pm
content: Will do, they just received it today and they can be rather slow. I had sent one of the Siglent SDG2122X gens with it. They know I wanted them to hold on to that maybe a month so as to better stagger the gens. I hope they don't think that about the 34401A - lol, only because I am anxious to see if it passes.

author: daveyk
author_rank: Frequent Contributor
timestamp: August 16, 2023, 04:43:38 pm
content: https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3D.. looks fine ... if some time in the future my own, 30 y/o HP34401A will show this error.. I'll remember this----------------------------------------BTW, the meter passed ISO17025 calibration perfectly. That relay from Mouser worked great with a small adaptor PCB made from perf board.

author: daveyk
author_rank: Frequent Contributor
timestamp: August 16, 2023, 04:44:32 pm
content: Quote from: AVGresponding on July 29, 2023, 08:02:23 pmNo problems passing an ISO17025 Calibration. Worked great with that Mouser relay.

author: daveyk
author_rank: Frequent Contributor
timestamp: September 05, 2024, 03:14:39 pm
content: I have another 34401A doing the same thing, plus failing "AC" certification (which I never use these meters for).I just ordered another BT05-2A66 from Mouser to fix this one. I do not know if that will have anything to do with passing "AC" or not, but if it passes everything else with the ISO17025 certification, I can live with a limited calibration on this one.If the "AC" is not the relay, and it was just a little out of spec, cannot someone re-calibrated the A/C measeurments with software?Dave

author: std
author_rank: Contributor
timestamp: January 07, 2025, 03:57:01 pm
content: Quote from: daveyk on July 28, 2023, 07:19:44 pm500V?Looking BT05-2A66 datasheet:Switching voltage: 180VBreakdown voltage: 200V.