Agilent L4411A (headless version of 34410A and 34411A) troubleshooting - Page 1
Forum section: electronics / testgear

author: zrq
author_rank: Frequent Contributor
timestamp: May 16, 2023, 12:11:17 pm
content: Does anyone have information on the power supply module in Agilent L4411A?I got one from ebay when in the US for a conference, but when I returned to Switzerland, its power supply got unhappy. When it works, it can keep running for a few hours without problem. But it appears if I leave it on for a time more than 30min, then it will not start in the next turn on. When it's dead, the 12V output of the SMPS module is 0V, and it somehow recovers automatically after disconnecting the power cord for a few minutes. It doesn't look like there is a short in the main board, as the same can happen even when the meter is not turned on and left plugged in for a while.The power supply part of L4411A seems to be quite different from the human interfaced cousins (34410A and 34411A). Instead of a transformer and linear regulators to power everything, a basic 12V 2A OEM SMPS (Phihong PSA25L-120-R) is used to obtain unregulated DC, then switchers and an additional transformer are used to power the floating part. Finding a 12V 2A SMPS sounds not too hard, but the original seems to be discontinued. Anyone can suggest a replacement? Is there any risk of using power supply modules in adapter form factor? Common mode voltage?

author: TheSteve
author_rank: Unknown
timestamp: May 16, 2023, 02:19:50 pm
content: I'd start by swapping all of the electrolytic capacitors in it - chances are that will fix it.

author: zrq
author_rank: Frequent Contributor
timestamp: May 16, 2023, 02:31:11 pm
content: Yeah that can be an option, but I'm not sure if this is going to be cheaper than swap out the entire module. Or have anyone seen this failure mode before can point out where in particular should I look around for failing caps, so I don't need to change all.The meter is actually less than 10 years old and it would be a bit surprising that capacitors already start failing this soon.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 16, 2023, 04:39:15 pm
content: There is 34410a schematics floating around (attached here) who is maybe 85 -90% of the same technology, there is none for the L4411A and none for the 34411ayou have a thread here with picture wich i contributedhttps://www.eevblog.com/forum/testgear/agilent-l441a-multimeter-teardown/The main psu is based on a MSC Lx ic .... you can find the spec sheets, it work with the red colored xformer put on the frame to give +18 -18 and a 5vdc to the analog partand you have the other smps module under the flat cable ....And pls dont rush to change parts to change parts ...... if not send me the meter ill check it loll, i had one of theses not too long ago and now im on a 34410a, i needed buttons lollFully controllable with the old agilent dmm software, and some R&S visa drivers ( better than Keysight )There is TestController who could control it, but you need to define some coding to properly use it.It would not be a problem to change this smps with another of the same specs, just take one who can go up to 240 - 265 Vac .... like some meanwell, traco, avoid china craptraco TPI 50-112A-J .... and they dont cost a fortune, mmmm sure slightly expensive over china crap lollsorry i did not want to insult you in any ways by the way ...

author: zrq
author_rank: Frequent Contributor
timestamp: May 16, 2023, 07:25:23 pm
content: It's an interesting design to use a class D power amp chip (LX1722CDB) to drive transformers for the in-guard power transformer. Maybe Agilent was considering making DC power an option, so took such a detour in the power supply? Or just because the 34410/1A transformer is too big to fit in 1U height?It appears the PSU is getting quite hot when the meter refuse to turn on, maybe it overheats and fall to protection, either because of itself or something is wrong on the main board. Going to try powering the main board with a lab PSU and see if it drains too much current.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 17, 2023, 12:56:38 am
content: yeah myself i was surprised to see this technology used to create some smps supply loll

author: zrq
author_rank: Frequent Contributor
timestamp: May 19, 2023, 07:07:36 pm
content: Tried to replace the power supply with a Meanwell EPS-65S-12, it should have worked fine after modifing the connector from 5 pin to 4 pin. However, I'm too stupid to properly fix the open frame PSU to the chassis and something on the AC side shorted to the earth by the screw hole, a big boom and RIP the Meanwell. Luckily the meter itself is intact.I'll revert to the original PSU before I can justify my next digikey order.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 20, 2023, 01:38:52 am
content: mmm you played hard in this one lolll