HP 34401A not measuring after storage, for a moment - Page 1
Forum section: electronics / testgear

author: m k
author_rank: Super Contributor
timestamp: July 04, 2024, 10:59:23 am
content: The machine was on its side few months, power side down.After turning it on it didn't measure DCV nor Ohms.Switching front/rear didn't change anything.Checking with other meter Ohms had no voltage.Trying DCV with a resistor dropped reading down.After a minute or so later everything was fine.Still is.What happened, a contact problem?

author: AVGresponding
author_rank: Super Contributor
timestamp: July 04, 2024, 04:15:22 pm
content: Sticky COTOs? Lazy coils?

author: Whitefoot
author_rank: Contributor
timestamp: July 04, 2024, 10:30:14 pm
content: Only a guess. Maybe the dielectric of an electrolytic capacitor needed some reforming. Leaving the meter on for a day or two would more thoroughly reform the cap.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 05, 2024, 09:32:49 am
content: Quote from: Whitefoot on July 04, 2024, 10:30:14 pmThat shouldn't affect measurements. If there was a DC rail problem or noise on the signal path I'd expect an error code.

author: m k
author_rank: Super Contributor
timestamp: July 05, 2024, 04:38:29 pm
content: This morning both meters, problem and checker, were 0.0 VDC.The house have a tendency to drop certain phase when grid blinks.Stove has a clock, it's fine, but it usually doesn't see the blinking.Now checker meter is in Ohms and the problem one is VDC with manual range.I've never experienced relays that are very intermittent.But reed types are replaced shortly and older styles operated somehow, even filed.After a second thought the storage is interrupted around a month ago.No problems then.Now temps are up and possibly moisture also.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 06, 2024, 08:52:31 am
content: I'd check inputs and outputs of K101 and 103. It isn't uncommon for relays to fail; I've a Solartron 7150 with a failed relay in the input conditioning (contacts welded, like that when I got it), which causes the 2V range to overrange. I do have some replacements, but procrastination keeps me from getting a round tuit.

author: factory
author_rank: Super Contributor
timestamp: July 06, 2024, 01:09:18 pm
content: Quote from: AVGresponding on July 05, 2024, 09:32:49 amDon't know what your experience has been like with these, but my older 34401A had some rails for the AC/frequency functions shorted by tants, no error codes are shown,unlessyou manually run the self test and that didn't tell me anything I didn't already know.David

author: m k
author_rank: Super Contributor
timestamp: July 06, 2024, 07:03:40 pm
content: Must try the self test.(Done, no problems)Before any inside checks I'd like to have a third situation.Generally problematic relays(or caps) are nothing new, I predict I have few around problematic Fluke 8505A modules.So the pair is moved to be aside and their functions swapped.No idea what phase of electricity is where around the house, but missing manual range will reveal the blinked grid.I remember reading that after longer storage session the machine must be on for several weeks before it's dried.But I guess that's for least significant digits.User guide states that high humidity affects only accuracy.So I guess the machine must be dripping before anything major should happen.User guide is also silent about the orientation of the machine.Maybe I'll put it on its side after few non problematic days.A side note about readings.The other machine is Fluke 8860A and its reading is fluctuating few ticks when 34401A is dead steady.But fluctuating is around a decade limit, so can be just bad luck compared to HP.

author: m k
author_rank: Super Contributor
timestamp: July 14, 2024, 05:50:59 pm
content: HP has now been on its side for several days.Grid also blinked one day.HP doing Ohms didn't notice, but Fluke was back to auto range.Nothing special after that.Mode change every now and then, and some tapping and rocking.No problems.So the decision, I'll put it away for few months, original place, original position.See you in a near future, stay tuned.

author: m k
author_rank: Super Contributor
timestamp: August 05, 2024, 04:24:22 pm
content: Quick test, no problems.One of those blue moon moments, it seems.

author: indeterminate
author_rank: Regular Contributor
timestamp: August 06, 2024, 10:31:50 pm
content: I think what is happening is that the relay contacts witch are maid from gold plated silver are suffering from a bit of age related corrosion.The gold plating has probably worn away and the exposed silver has developed a layer of silver sulfide on the contacts.giving a high resistance across the relaya bit of relay exercise wipes the contacts clean an all is well again

author: m k
author_rank: Super Contributor
timestamp: August 07, 2024, 08:32:31 am
content: The machine is a rack mounted early version from Japan, possibly been in same mode all its active duty.Now inside temps here are slowly lowering, but maybe those contacts are too far from ambient anything around these values.Alternative then would be galvanic corrosion, not very fast operation.Maybe the thing was a build up since a long ago.I think I can test how easily those contacts change their position, but I doubt it will be accurate enough to reveal anything conclusive.A rouge capacitor is another thing.Though visualization of anything can still be here and there.One example being a green legged axial electrolytic that was just fine, until after few weeks of its removing.

author: m k
author_rank: Super Contributor
timestamp: July 03, 2025, 06:52:14 pm
content: Next test, no problem, it seems.Can't be exactly sure, the other machine has lost its front/read switch function, and connection.External mechanics of the switch is broken, lock is nowhere, button arm's inner end is in two pieces and spring is loose.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 03, 2025, 07:12:26 pm
content: you have to check the switch for sure and lock it to front, in rare instance some people have removed the switch and put gold plated short pins from say long idc connectors etc ... or sometimes you have them on ebay at high prices ?the front / rear plastic lever is no longer available for theses

author: m k
author_rank: Super Contributor
timestamp: July 04, 2025, 04:43:35 pm
content: I've already ordered more switches from far away.Two types(~20pcs) of who knows what, but both seem to be constructed old way and very similarly compared to the original.Both have also that iron hook of lock mechanism and one has metallic lock washer as a spring stopper.Button arm is still one overall piece, so I thing I can construct a new surroundings on top of the inner end.U-shape epoxy layer, or two, should be enough.