HP 34401A loose wobbly reference U403 - LM399 - Page 1
Forum section: electronics / testgear

author: Neo2199
author_rank: Contributor
timestamp: January 12, 2022, 11:43:12 pm
content: I have recently scored classic HP 34401A that was for parts on ebay. By shear luck, looks like there is nothing wrong with it, but I am not sure about the voltage reference. This is my first real fancy piece of measurement equipment which has a nice voltage reference. What is scaring the hell out of me is how it wobbles. I know the TO-46 package is surrounded by what appears to be ceramic so there might be some tolerance, but this is literally "flapping in the breeze". Is this normal? Do all of these do that? I was trying to search the interweb, but no luck.

author: alm
author_rank: Super Contributor
timestamp: January 12, 2022, 11:56:10 pm
content: The cap on the LM199/LM299/LM399, which is for thermal insulation, has no rigid attachment to the package, so it is normal that this cap can move around a fair bit. It just surrounds the package. If the reference itself would actually be loose, the meter wouldn't be able to give any stable reading and would probably give a power on self-test. So if the readings are even slightly sensible, you can be sure the reference is solidly connected to the socket.Both pictures look normal to me.

author: TheSteve
author_rank: Unknown
timestamp: January 13, 2022, 12:50:40 am
content: Yep, it is normal, continue to let it flap.

author: Neo2199
author_rank: Contributor
timestamp: January 13, 2022, 01:08:38 am
content: THXFells much better.I will continue with accuracy checks.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 13, 2022, 02:57:37 am
content: BUT it is not normal to see the lm399 on a machined socket, it must have been tampered or changed or tested in a way ?? it may affect the calibration ... it smell fishy for me

author: Cerebus
author_rank: Super Contributor
timestamp: January 13, 2022, 03:08:39 am
content: Quote from: coromonadalix on January 13, 2022, 02:57:37 amNo, that IS normal for the 34401A. Here's the underside of the PCB under the reference on another 34401A. Those are clearly sockets, not LM339 leads.Also, another "Don't worry, the insulating cap on the LM399 flaps around quite a lot, this is normal".

author: coromonadalix
author_rank: Super Contributor
timestamp: January 13, 2022, 03:21:56 am
content: I wont open my meters, since they have a calibration seal, when not calibrated they did not have theses sockets under the lm399 i can vouch for that on the 3x 34401a i have ...Unless this is not a socket, but somekind of stress relief ''guide tubes'' ?? did not have that either, slightly longer leads going to the pcb ?? and they were never been tampered

author: TheSteve
author_rank: Unknown
timestamp: January 13, 2022, 03:56:16 am
content: Every 34401A I have opened has the LM399 socketed - they don't go through reflow.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 13, 2022, 11:40:20 am
content: Okay, maybe my memory is bad ... if so i'm sorry lollBack to the main topic, yes its an encapsulation to create an isolation ''room'' for the precision zenner. it can wobble a bit