HP 34401A Reset Loop - Page 1
Forum section: electronics / repair

author: FeedMyZappetite
author_rank: Newbie
timestamp: May 24, 2025, 06:34:04 pm
content: Hey, I recently bought a broken HP 34401A DMM off of ebay, it turns on and displays the GPIB address then resets and continually turns on and off in this same sequence.After messing around with it a bit I found that if I quickly hit DCV -> Shift -> Hold it will stay on and I can navigate to the settings. I can change most of the settings and they seem to stick when it resets but navigating to the GPIB-address it freezes on the PARAMETERS screen. Leaving the settings screen causes it to go back to its reset loop.Feeling confident that the issue is somewhere on the Ground-Reference Circuit I began checking voltages against the reference schematics but so far haven't seen anything obvious. The transformer does seems to be a bit low and is reading 4.6V but this seems close enough to 5V to not the cause of the problem.So has anyone know of what may be causing this? I can tell the reset is caused by the reset control circuit but I'm not sure what conditions will initiate the reset control?Service Manual with Schematics:https://www.ecb.torontomu.ca/guides/instrument-manuals/Agilent-HP_34401A_Service_Guide.pdf

author: Kean
author_rank: Unknown
timestamp: May 24, 2025, 08:46:39 pm
content: First, that link is to Edition 6 of the service guide from 2005. There is later versions, such as Edition 9 from 2014 which I am looking at.Should be available on Keysight website and others - but I can provide if you would like it.In both editions on page 106 "Power Supplies", it states:QuoteSo, when you say:QuoteWhich "transformer" output are you referring to, and what exactly is measuring 4.6V?There are two 5V rails in the power supplies schematic on page 9-15:- the output of U751 called "+5V_0" earth referenced power supply - going to sheet 7 (page 9-14)- the output of U553 mentioned above called "VCC" for the floating logic power supply going to the other sheets.U553 also outputs XPONRST which resets the main mcu U500 and ASIC U501 on sheet 6 (page 9-13)

author: FeedMyZappetite
author_rank: Newbie
timestamp: May 24, 2025, 10:44:44 pm
content: I got that newer manual, thanks.I found that note about U553 little bit ago and have been going through that and the components near it.I measured the pins of U553 and it is diffidently the component sending the reset signal so everything I was saying about the ground circuit system is wrong.The voltages I was measuring was for the ground circuit power from the transformer. The voltages going to the floating circuit are roughly what they should be but the wires feeding U553 are reading about 4.9V but I'm getting the correct voltage out of the diode bridge. The filter capacitor C556 seems to be the cheapest component to replace that controls the voltage to U553 so I'm going to replace that first then the transformer if that doesn't fix the problem.

author: Kean
author_rank: Unknown
timestamp: May 24, 2025, 11:19:00 pm
content: The transformers should rarely die, unless abused - e.g. 220/240V applied when set to 100/120V.I agree that C556 would be the first thing to check/change.Are you able to measure the voltage across C556 with a different DMM? I'd expect at least 7V DC going into U553, possibly up to 10V.Maybe check both DC and AC voltages? If the cap is bad, you'll see high AC ripple.Do you have a LCR meter or capacitor tester? If C556 is bad, there is a good chance some of the other electrolytic capacitors have also gone bad with age, especially the other large ones like C551, C553, C752. It might be worth replacing them all together.In another thread, there is mention of C558 being bad and causing a reset signal issue - although I think different symptoms, not looping.https://www.eevblog.com/forum/repair/help-me-repair-agilent-34401a/