HP 34401A ERROR 604 - Page 1
Forum section: electronics / repair

author: sonicyang
author_rank: Newbie
timestamp: October 03, 2015, 09:03:01 am
content: Hi,I recently scored a HP 34401A Multi-meter off the web with a non-working status.It Has following IssuesIt's reporting Error 604.DC measurement is extremely inaccurate.Ohm measurement is non-working, but do reports ~ 0ohm if I short the test leads.Conductive Test is Working as excepted.As I tear it down, measuring the DC supplys, +-18V +- 15V and 5V, are all goodand Voltage refs, 7.5V, 5V, +-10V, are also good.IC, Numbering U420, U105, U106, U312, U305, and voltage regulator are all hot as about 40~45 degrees Celsius, as other chips are 33~36 only.Any way to repair this thing or to do more detailed testing?Any suggestions welcomed.

author: Psi
author_rank: Super Contributor
timestamp: October 03, 2015, 10:56:24 am
content: This may helphttps://www.eevblog.com/forum/repair/please-help-to-fix-my-agilent-34401a/Checking U201 and U102 looks like a good place to start

author: HighVoltage
author_rank: Super Contributor
timestamp: October 03, 2015, 11:34:30 am
content: I also had a 34401A with the Error 604 a while back.This instrument had big problems in the OHMS reading and it turned out to be U201 that fixed it.

author: sonicyang
author_rank: Newbie
timestamp: October 03, 2015, 11:59:24 am
content: Thanks for pointing me out, I will have them replaced and test again.BTW, there is a strange phenomenon while the test leads are open in Ohm mode.The Realys are clicking randomly, about 2 switch per second. I hope replacing the Op amps will have this problem fixed.And, I measured U102's resistance.Everything is on spot exceptPin15 - Pin21 should be 100K measured 69KPin29 - Pin30 should be 30K measured 20KPin15 - Pin13 should be 100K measured 82KOf course, those are measured on board, so I don't think it's a huge problem.

author: sonicyang
author_rank: Newbie
timestamp: October 03, 2015, 02:18:31 pm
content: And, sadly the DC measurement isn't working for range larger than 1V.Reading are zeros, no matter what i put up for measurement.But Tests 609 610 are passing which means 10V and 1V range are good, and test 611 failed, which are not consistent to the using experience above.I would like to ask someone also have this unit to tell me, will this guy keep making clicking sound running test 609 610 611?Mine is doing so, I am not sure it is what it should be.

author: TiN
author_rank: Super Contributor
timestamp: October 03, 2015, 02:48:29 pm
content: Taiwan? If you in Taipei, we can meet up and do some repairs togetherIm working on repair of HP 3458A now.

author: sonicyang
author_rank: Newbie
timestamp: October 03, 2015, 05:33:49 pm
content: Yeah, I am in Taiwan, but Tainan.I often travel to Taipei, maybe sometime later i can bring this unit to your place?May you p.m. me which part of Taipei your in? Thanks

author: TiN
author_rank: Super Contributor
timestamp: October 03, 2015, 05:59:01 pm
content: PM sent

author: TiN
author_rank: Super Contributor
timestamp: January 18, 2016, 04:19:31 am
content: Hello everyone,sonicyangvisited me yesterday with his sick meter.Repair and debug:PSU issues and failures:This did not fix it, so we tested front-end:ADIN signal was present and correct, but reading were still garbled/useless..Current source for ohms was alright. I focused on U102 resistor hybrid and wasted a hour or two on checking it.Reference voltages were fine and dandy, and resistors in network OK too (had to desolder chip to confirm that).8 hours after...I traced control to A/D from digital side, and quickly found that integrating switch U411 is controlled by 74HC74 Dual DFF.And tada....CalibrationNow it's measuring stuff correctly, let's cal it.All pass with flying colors.Bad parts:Final test result:Quick and dirtyworklog and repair article.Now meter is in sonic's hands, hope he will keep and use a good tool in his student projects. I learned a bit or two about 34401A too (never mess with this one before).

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 18, 2016, 04:56:33 am
content: Thanks for your usual detailed description of the repair. Too bad 3458A isn't as straightforward. 1 page for 34401A -vs- 20+ pages for 3458A.

author: sonicyang
author_rank: Newbie
timestamp: January 18, 2016, 06:53:18 am
content: Nice detailed log. I really appreciate TiN for the repair.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 18, 2016, 07:59:22 am
content: Thank you for your great repair information and report.I had many bad 34401A meters but never seen this part broken.

author: lukaq
author_rank: Regular Contributor
timestamp: January 18, 2016, 12:02:47 pm
content: Great, nice to see another one working

author: HighVoltage
author_rank: Super Contributor
timestamp: January 18, 2016, 12:58:08 pm
content: Hello TiNI read in your great report:"Higher voltages calibration. I used HP 3245 with x10 option 001 as 100VDC source, and Ultravolt DC-DC as 1000VDC source for this task:"1. What is your Ultravolt DC-DC?2. And how do you calibrate AC voltage and current, if at all?

author: TiN
author_rank: Super Contributor
timestamp: January 18, 2016, 01:06:25 pm
content: It's low noise DC-DC module, with adjustable output, made by Ultravolt. I got some for my calibrator neverending-project, and covered some details inhereandhere.EDC MV106 was used for reference adjustment signal (module takes 0V as 0V and 5V input as maximum, bit over 1000V) and DC-DC input power was taken from K2400.This 34401A had old Rev.1 firmware, so it does not have AC calibration steps. So I did not calibrate AC, only checked if readings matched K2001 using 3245A as source.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 18, 2016, 05:11:40 pm
content: Thank you TiN, that was very helpful information.

author: TiN
author_rank: Super Contributor
timestamp: January 23, 2016, 07:32:23 pm
content: vid linkJust a quick video on calibration and testing, which I recorded before but got ahold of converting just now. 3x normal speed to skip the boring stuff..

author: radioFlash
author_rank: Regular Contributor
timestamp: January 31, 2016, 12:24:49 am
content: Very nice job troubleshooting, thanks for the informative repair log!

author: Muxr
author_rank: Super Contributor
timestamp: January 31, 2016, 03:55:49 am
content: So well documented, nice job TiN! Great pics as well.

author: bitseeker
author_rank: Super Contributor
timestamp: January 31, 2016, 04:24:54 am
content: Outstanding job, as always, TiN!This is the first time I've seen so many problems in one 34401A.

author: Spectrum
author_rank: Regular Contributor
timestamp: January 31, 2016, 02:42:46 pm
content: Really nice job TiN, thanks for the great report

author: Samogon
author_rank: Frequent Contributor
timestamp: October 07, 2019, 03:37:32 am
content: In my case error 604 was caused by blown U106 op27 opamp in the input amplifier.Go figure.