HP34401A multimeter repair - Page 1
Forum section: electronics / repair

author: Synthtech
author_rank: Regular Contributor
timestamp: May 20, 2020, 09:33:07 am
content: I have a failure in one of my 34401As, it's coming up with a 621 error. Nothing is burned inside, the tantalums all check out OK, I can see the correct voltage drop across the four zener diodes in the AC circuit and the ceramics on the power rails aren't shorted. I am going to remove the relay (K102) and test it out of circuit, can I ask, has anyone come across this issue before? I am looking for a replacement relay just in case but I can't find any Omron relay with the correct spec as stated in the service manual. It's a G6AK-274P unit with a 5V coil which is easy to find but the manual also states a contact rating of 220V and 3A DC and I cannot find that rated version anywhere.Alternately can anyone suggest a competent tech in Australia with experience in the 34401A that I could send it to?Thanks!

author: Zucca
author_rank: Unknown
timestamp: May 20, 2020, 09:57:33 am
content: This should be in the repair section.Anyway please search there:https://www.eevblog.com/forum/repair/agilent-34401a-k101-relay-replacement/msg3044812/#msg3044812https://www.eevblog.com/forum/repair/hp-34401a-current-fail/msg2662425/#msg2662425https://www.eevblog.com/forum/repair/recovery-of-the-relay-k101(0490-1914)-in-hpagilent-34401a/msg2459241/#msg2459241

author: Synthtech
author_rank: Regular Contributor
timestamp: May 20, 2020, 10:40:53 am
content: My apologies, it is indeed posted in the wrong category, it should be in the repair section. It looks like Keysight has the relay available as a spare part, it may not be worth it though, test equipment repair isn't in my area of specialisation and I doubt that there are any techs in Australia that work on them so I will retire this unit for spares or scrap.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 20, 2020, 11:38:43 am
content: You can use the move topic function to send it to the repair board .It seems a shame to scrap it unless it was already shagged out !.Perhaps now is a good learning opportunity for repairing your own gear.

author: Synthtech
author_rank: Regular Contributor
timestamp: May 20, 2020, 08:10:19 pm
content: I just moved this topic to “repair” as I had initially posted it in the wrong section. To elaborate, I run a repair centre but my niche is not test equipment and I have no experience servicing precision multimeters. I have confirmed the obvious things such as checking the zeners and tantalum caps in the meter’s AC circuitry against the schematic in the service manual and so I am reaching out to enquire as to whether anyone might have dealt with this failure mode in the 34401A and have any insights as to what else might cause this failure. ‘AC RMS full scale failed”. I can see that a relay contact fault might be a possible candidate but I would like to know if there are any common failure points in this AC section of the circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2020, 08:39:04 pm
content: To narrow down the error one could follow the signal path: send a test signal to the DMM and check the AC signal down the path. Much of the AC circuit is just amplification with a settable gain. To early generation CMOS switches do sometimes fail, though not sure if this also applies to the 34401.The relays are in the AC input path, but they may not be in the path for the self test.AC current would also be different contacts.It the meter still giving reasonable readings in both AC and DC mode ?

author: aqibi2000
author_rank: Regular Contributor
timestamp: May 20, 2020, 09:58:16 pm
content: I’d happily pay for this if you consider it to be scrap

author: Synthtech
author_rank: Regular Contributor
timestamp: May 20, 2020, 11:17:06 pm
content: Quote from: Kleinstein on May 20, 2020, 08:39:04 pmI checked it over this morning against my other 34401A with DC voltage, current and resistance standards and it is dead accurate with those parameters, DC volts are perfect, 2W and 4W resistance is spot on as is DC current. I didn't make AC measurements as I assume that they will not work, I will test it with a signal generator this afternoon. I can live with it as is and just not use it for AC work but the problem there is that in that state I can't send it to a cal lab for the 2 year calibration cycle that I keep in my workshop.I did wonder about the CMOS switches in the AC circuit, I replace through-hole 4051s, 4052s and 4053s by the bucket load in the 30 year old plus equipment that I work on on a daily basis.