WTB [LV/EU]: HP 34401A transformer (9100-4972) - Page 1
Forum section: electronics / general

author: zii
author_rank: Contributor
timestamp: November 13, 2018, 01:40:49 pm
content: Hi,I would finally like to fix my HP 34401A DMM, which I bought as dead in January. I have confirmed that transformer is dead and hooked up all the necessary rails with separate small PCB-transformers to confirm that it would work with that change.But as separate transformers in the end they are just take inconveniently large amount of space as well as there is no transformer shield winding to reject high frequency noise, which I am a bit paranoid about.So I would much rather want to just buy a replacement.I see that on eBay You could get one for around $100 from US to be shipped to Europe (not including import tax). E.g., here:https://www.ebay.com/itm/283041271475Would anybody know where to get one at a more reasonable price in Latvia/Europe?Cheers,ZigmarsP.S. Also looking for the tinted front window, but that is not a priority at all.

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 13, 2018, 02:10:49 pm
content: Have you tried Keysight? The model should still be supported.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 14, 2018, 11:35:36 pm
content: Keysight have that transformer as obsolete, replaced by this part.Still available so you might be able to get it from your local keysight dealer as mentioned previously (order in)https://www.keysight.com/my/faces/partDetail.jspx?partNumber=34401-87901

author: zii
author_rank: Contributor
timestamp: November 28, 2018, 12:40:52 am
content: Thanks for the suggestions. I tried contacting local keysight dealer, but they asked 79€ + tax.Which I thought was a bit too high.Has anyone tried their local dealer in Germany? (I could forward from there to me for quite a reasonable number, I think).Otherwise, I will probably take this one currently on eBaywww.ebay.com/itm/173561930215Cheers,Zigmars

author: chekhov
author_rank: Regular Contributor
timestamp: November 28, 2018, 10:34:52 am
content: I'm just curious, why haven't you tried to rewind it ?If it is already dead (especially primary winding) - nothing may stop you from disassembling it (calc the windings, etc...), and I'm sure it is possible to find the required voltages.Or if half of primary is dead, probably the unit might be powered from 220->110V transformer.I understand that this may consume time, but transformer is quite small, not the beast from some linear power supply.

author: zii
author_rank: Contributor
timestamp: November 28, 2018, 11:14:35 am
content: Quote from: chekhov on November 28, 2018, 10:34:52 amWell, actually that was the first thing I wanted to try when I got the 34401A and found out that transformer was an issue, but I just could not pull out any of the transformer core "E" plates without applying (in my mind) a damaging force. I just spent too much time on it.Are there any tips for how to ease the plates and pull it apart more easily?Thanks for reminding of the option.Sent from my SM-G935F using Tapatalk

author: chekhov
author_rank: Regular Contributor
timestamp: November 28, 2018, 11:35:07 am
content: Unfortunately, the only way I know is some kind of heating, but not excessive (even boiling may work). There also a possibility to use the alcohol, but not sure about it.Which winding is broken (primary or one of secondaries) ?I'm asking because I don't know what's inside (under black sealing). it might be possible to change the winding without disassembling the whole transformer depending on its construction, so it is important to know.

author: zii
author_rank: Contributor
timestamp: November 28, 2018, 11:41:46 am
content: I do not remember anymore. I would have to take another look at it.What I do remember is that I changed the mains fuse and as I plugged in the mains power, it blew without making a sound.What kind of a resistance would You expect in the primary winding for this kind of transformer?CheersP.S. I guess this is starting to transform a bit into a repair topic...

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2018, 01:05:13 pm
content: Just a heads up on the price, that's pretty cheap for a transformer. I'd just buy it if you're sure the original is dead.If you rewind it, it will probably cost more than the new transformer. It's usually only worth rewinding stuff if you have (a) replacement formers (they break very easily), (b) suitable wire in stock already, (c) a lot of time, (d) lacquer, (e) suitable tape for it, (f) turn counter/winding machine, (g) vice and hammer to get the laminations out. It's a messy horrible time consuming job as well.You can easily end up with some piece of crap that buzzes a lot (been there)

author: TiN
author_rank: Super Contributor
timestamp: November 28, 2018, 02:52:42 pm
content: I agree, that's cheap (in relative terms) and these transformers often have extra shielding/guarding. Keithley transformers for their benchtop instruments often 300-600$+ a piece.

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2018, 03:12:18 pm
content: Good point with the shielding. If they have the copper tape around it that stuff is about €10 on its own.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 28, 2018, 03:15:29 pm
content: Quote from: zii on November 28, 2018, 12:40:52 amI have bought a few of these transformers from Keysight Germany directly over the years and usually they had them in stock.I never tried a 34401A transformer from ebay. Since the price difference is so small, I would recommend to get one from Keysight directly.

author: ogden
author_rank: Super Contributor
timestamp: November 28, 2018, 03:45:49 pm
content: Quote from: zii on November 28, 2018, 11:41:46 amIt's always worth to double-check that line voltage selection on the back of the instrument is correct, 230VAC instead of 100VAC or 120 VAC. Could be so that transformer is ok. Been there. Just saying

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2018, 04:25:57 pm
content: Also worth checking the fuse spec as well. I cocked up the other day by putting a quick blow in a scope with an SMPS. It blew every time due to the inrush. Thought I'd killed it. Then I RTFM and put a slow blow in. All good.

author: chekhov
author_rank: Regular Contributor
timestamp: November 29, 2018, 07:52:17 am
content: Quote from: zii on November 28, 2018, 11:41:46 amIt has ~120 Ohm across mains input for 220V set and ~60 Ohm for 120V.If I calculate this correctly, it effectively ~220 meters of 0.2mm wire, so yes, definitely cannot be rewinded without disassembling it.