Agilent 34401A Strange Display Behavior - Page 1
Forum section: electronics / testgear

author: BertSP
author_rank: Contributor
timestamp: October 06, 2017, 12:21:14 am
content: I recently purchased a used 34401A. When I received it, I tested it with my trusted Chinese quad reference. It was OK. I sent the unit to a Calibration lab and when it returned the display segments were apparently shorting. The result is that a 5 displayed as an 8, a 3 displayed as a one legged 9 etc. So in fact, the unit is doing a good job of calculating the voltages but the display has to be interpreted due to the extra segments being on at the wrong place for the wrong digits.Has anyone had this problem, is there a scenario whereby this unit had a display replacement and there is some shorting going on, on the PCB. Reading this forum, some mention shorting some of the pins, would that create my problem? The display is real bright and all tests pass (if you can read the letters...).I have not had the courage to take the front off yet as it does not seem to be that obvious and there are some threads on this issue.Any thoughts

author: BertSP
author_rank: Contributor
timestamp: October 06, 2017, 11:39:33 pm
content: I successfully took the front assembly off (no breakage) and all appears OK with the VFD. Playing around with the significant digits settings, the malformed numbers stay malformed as they shift position. This would indicate that whatever chip transcribes data into symbols is corrupt? Would that mean that I am due to replace the front board? Providing that is where the data transcription occurs?Bummer. The unit is still calibrated but hard to read certain numbers.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 07, 2017, 10:06:23 am
content: Quote from: BertSP on October 06, 2017, 12:21:14 amWhy did you not return it to the calibration lab. Obviously something went wrong there.I have repaired many 34401A instruments but never have come across this problem.I would start by checking, if all solder spots are ok from the display to the PCB.

author: BertSP
author_rank: Contributor
timestamp: October 07, 2017, 02:14:18 pm
content: The board looks fine. It seems to me that the decoder chip is corrupt since the same letters/numbers occur and when I shift position they shift also, indicating to me that its the encoding of the number/letter that is corrupt. I think that I will have to find a parts unit and change the display board where the chip is. Thanks. The cal lab experiment was a disaster, so it will not go back there.

author: Dachpappe
author_rank: Contributor
timestamp: October 07, 2017, 02:44:42 pm
content: Quote from: BertSP on October 07, 2017, 02:14:18 pmThat's sad to hear... What else happened except they may have f*cked up your display?I also want to give my 34401A a new cal, but now I might think twice before giving it away.Edit: typo fixed

author: BertSP
author_rank: Contributor
timestamp: October 07, 2017, 03:27:19 pm
content: Well, actually it was a teaching moment. It turned out to be a cal sticker (or not) service. No adjustment. So I sent this 34401a and a 3457a and got a broken 34401a stickered ( barely in specs) and a failed 3457a. Mailing cost alone was 57$ one way plus the cal sticker service. For a hobbyist, not a good day. Could have done better myself using my 335A like Dave's calibration video(eev). Bummer. So now I am looking for front panel assembly replacement or parts unit.