Agilent 34401A where to look. - Page 1
Forum section: electronics / repair

author: sampler1
author_rank: Regular Contributor
timestamp: December 04, 2023, 11:12:47 pm
content: Hi all,I just purchased a broken Agilent 34401A from flee Bay that I will receive in a few weeks. The description says Passes self-test but will not adjust, fails to calibrate. No more detail than that.I guess 1st is Ill replace the Electrolytics and measure the power supply rails.What would you suggest after that? This is the 1st 34401A that I work on so I was hoping this might be a common problem someone has seen.Cheers all, and Happy Holidays !Sampler1

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 05, 2023, 12:39:57 am
content: Hi,That failure is not so common, I cannot remember to have heard/read of such a case. Maybe others know better.I recommend to check the capacitors 1st, not to blindly replace them. My unit is from 1992, and still doing fine.If you have the opportunity to check some of the ranges, that would reveal if it's necessary to calibrate the unit. It's very stable over decades.If you find gross errors, then maybe there's something fishy, or a calibration attempt has failed.Do you know something more about the pre-owner?If that's not an expert, maybe he simply was not accustomed with the calibration process, maybe he did not unlock the calibration mode, or forgot the (changed) pass code.In the other case, the storage memory might be defect. I don't know at the moment, if that's an EEPROM, have to investigate in schematics.Yep, there's U505-A, a 93C66. That's probably the calibration-EEPROM.Depending on the vintage of your unit, that might be a candidate.. In the past, I had some experience with this family, bit-flips, and fail of the internal charge pump, if I remember right.Frank

author: coromonadalix
author_rank: Super Contributor
timestamp: December 05, 2023, 02:24:37 am
content: had 3x of them, only one, i tought changing the caps would change a thing, done a before and after checkup at a cal facility, nada still in specs over many yearschanged an vfd in one of them, that was more problematic than the psu capsaint got problems dont touch it but an out of cal would imply over the spec values or simply out of the acceptable rangeyou'll have to do yourself a selftest on boot and recheck if any errors are shown after say an 1hr warm upthe psu caps could be the problem, the front back switch could be oxydized / simple as that, and the problem could dissapear, but if self tests passes, youll need to get precisions components, voltages sources etc ...some threads spoke of an input coto relay who's unobtanium .... and recently some sparks between the relay and gas tube discharge / input protection partsor if you know a cal facility, you can do a check without a calibration but it will cost $$$ to do soif bought on ebay you can go on you purchase and grab some photos to put here all the red plugs ??

author: sampler1
author_rank: Regular Contributor
timestamp: December 05, 2023, 05:20:18 am
content: Thanks Guys!When it is received, Ill run a self test and really see if there are errors. From there ill check the values and see why it would need calibration. The guy on Ebay looks like a surplus seller, so who knows. Ill get back to this post when i know the facts.Thanks for the great advise.Sampler1Happy Holidays.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 05, 2023, 08:26:48 am
content: I had a similar problem once and it turned out to be a bad U505-A EEPROM chip.Actually I had two failures with a bad U505-AOn the other side, I never had to change an electrolytic capacitor on a 34401A.The seem to last.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 05, 2023, 12:50:22 pm
content: Could be eeprom corruption as highvoltage wrote, could be oxydation, power supply caps, and could be nothing lollSince some surplus resellers have no time to test everything ?? you could snatch a good deal or not ...this eeprom keep the calibration if i recall

author: sampler1
author_rank: Regular Contributor
timestamp: December 17, 2023, 12:42:02 am
content: well guys, this one is going to blow you away.A real Christmas story !I purchased two Agilent 34401's as I said and in the discription both were supose to be defective. I installed the 1st one on my bench and could not find a dam thing wrong with it?? What I thought at 1st was the guy screwed up and sent me the wrong unit.I figured what luck !I then checked the second unit and OMG same thing. No issues at all ! I figured there had to be something so, I have a Fluke 515A portable calibrator and I ran a full test on both and Jingle bells ! Even the calibration is dead on.Thanks for all your suggestions, Now what the hell do I do to keep busy over the Holidays?Cheers,Merry Christmas.

author: floobydust
author_rank: Super Contributor
timestamp: December 17, 2023, 01:20:59 am
content: Let it run for an hour and see if polka dots show up on the display.

author: bdunham7
author_rank: Super Contributor
timestamp: December 17, 2023, 02:01:42 am
content: Quote from: sampler1 on December 17, 2023, 12:42:02 amWell, you could upgrade them.https://www.eevblog.com/forum/testgear/hp-agilent-34401a-hidden-menu/msg3228606/#msg3228606(read the whole thread, it is only 7 pages)

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2023, 09:21:41 am
content: With 2 supposedly bad units, this could have been a fault in the instruments used to test the meters. For the would not cal part a user fault. Starting the calibration is usually a bit tricky for a good reason.A thing to test, that is sometimes skipped on calibration is the input bias. E.g. look at the drift with a 10 nF low leakage (e.g. PS or PP) capacitor at the input in the 10 V high Z range, starting a a few different voltages (e.g. +-9 V and 0 V).

author: alm
author_rank: Super Contributor
timestamp: December 17, 2023, 09:48:22 am
content: Try running self test via front panel and GPIB. I have seen a unit where running self test via front panel would pass but via GPIB would hang the unit. I don't know what the fix was.