Agilent U3401A - Page 1
Forum section: electronics / repair

author: perrylv
author_rank: Newbie
timestamp: November 28, 2017, 12:40:58 am
content: Hello,Does anyone have a suggestion where I could find a schematic for my auction site acquisition of an Agilent U3401A? Seems fine except it doesn't measure current with the amps range input. Fuses are fine and current passes through but doesn't read...Thanks in advance.

author: Samogon
author_rank: Frequent Contributor
timestamp: November 28, 2017, 03:53:29 am
content: Did you chech shunt resistors?

author: bitseeker
author_rank: Super Contributor
timestamp: November 28, 2017, 04:50:23 am
content: Welcome to the forum, Perry.The service manual for this DMM makes no mention of schematics, so it's unlikely you'll be able to get official ones. It's pretty rare for modern test equipment to have publicly available schematics. If schematics are critical for your repair adventures, it's best to acquire them or at least confirm their availability before purchasing the associated equipment.As Samogon mentioned, first check the shunt resistor(s). If the shunt(s) have the correct resistance, then follow their sense traces to check the circuitry that measures those resistor(s).

author: perrylv
author_rank: Newbie
timestamp: November 28, 2017, 05:55:46 am
content: Thanks for the replies. Yes I checked the shunt resistors 2nd thing. Something must be amiss with the way it measures the voltage across the shunt. mA input works as it should.

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2017, 11:03:53 am
content: My GW GDM-8341, albeit not Keysight, won't auto-range into the 12A range I suspect because it's difficult to select the input based on the range automatically between the 12A and 500mA ranges. You have to manually set the range for that one. Can you try manually ranging and see if it reads on that?Edit: Actually it says in the U3401 Manualhttps://literature.cdn.keysight.com/litweb/pdf/U3401-90001.pdfthat you need to select the range manually after selecting ACI or DCI. See section headed "Measuring AC (RMS) or DC Current up to 10 A"I very much doubt any meter in this class and age is broken.Incidentally when I got my GW I thought it was broken as well.

author: bitseeker
author_rank: Super Contributor
timestamp: November 28, 2017, 10:39:47 pm
content: That makes sense, now that you mention it, bd139. RTFM, eh?

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2017, 10:40:47 pm
content: Only RTFM after you've spent an hour taking it to bits to wonder why it doesn't work (yes I did that too)

author: bitseeker
author_rank: Super Contributor
timestamp: November 29, 2017, 01:39:12 am
content: Hehe

author: perrylv
author_rank: Newbie
timestamp: November 29, 2017, 06:56:15 am
content: Wow, RTFM indeed! Working correctly now. It may have been a bit more than that however.The seller advertised this meter as broken, specifically that it wouldn't measure amperage on the high range. This being a new meter to me I believed it (as did other bidders apparently) allowing me to purchase it for under $100 US. My very cursory review seemed to confirm the issue and I didn't even attempt to manually change the range. Hours of probing and hand drawing a schematic I could find nothing wrong.Tonight, after reading your posts, at first the high range didn't work. I continued to step through the ranges, up and down (internals relays clicking) and alas, it started to read!I am a very happy new owner of this fine instrument. Thank you all for your help.

author: bd139
author_rank: Super Contributor
timestamp: November 29, 2017, 07:38:00 am
content: You got a bargain there!Worth watching out for other things on eBay with similar problem descriptions. I got a nice Philips oscilloscope once for equiv $10 because it wouldn’t sweep. The controls in the picture were set in X-Y mode

author: bitseeker
author_rank: Super Contributor
timestamp: November 30, 2017, 02:22:39 am
content: Maybe the relay was a little stuck; just needed some exercise.

author: perrylv
author_rank: Newbie
timestamp: November 30, 2017, 11:13:25 pm
content: I believe it was likely stuck for some reason. Thanks again for all the advice. I am very pleased!

author: TP126779
author_rank: Newbie
timestamp: August 18, 2021, 08:35:52 am
content: Thank you!!!RTFMThe same thing happened to me, although in addition the mA measurement was cutting out, depending on the orientation of the instrument.Its connection is made internally with a molex connector to the board, where the crimped terminal was slipping out of the cable connector depending on the orientation.It was pushed in the wrong way around