Agilent 34401A Transformer - Page 1
Forum section: electronics / repair

author: adamslab
author_rank: Newbie
timestamp: September 06, 2023, 02:58:34 pm
content: I recently went to use my 34401A but it didn't power on. After some checking (fuses, voltages, etc.) I found that the transformer was not outputting anything. I've found a couple possible replacements on eBay but they are all from China and all appear to be knockoffs. I'm wondering if anyone has a good source for one of these transformers, or has a recommended seller on eBay or elsewhere?Thanks,Adam

author: coromonadalix
author_rank: Super Contributor
timestamp: September 07, 2023, 12:38:36 am
content: register at keysight and check if they are still available, ordered one years ago.... but not sure if they are still availableand sadly was more expensive ....

author: D Straney
author_rank: Frequent Contributor
timestamp: September 07, 2023, 02:24:40 am
content: I've got a transformer that I'm pretty sure was rescued from a broken Agilent "system multimeter" based on the 34401 years ago, that I haven't found a use for. See if it matches yours?Label on it says:9100-5608 REV BTRANEX14-7522232760 10 01Primary wires are brown, brown/black, green, green/black, red/black, yellow/black, orange/black, blackSecondary wires go to a big female connector meant to plug into a PCB header: green, blue, green, orange, orange, red, white, red

author: D Straney
author_rank: Frequent Contributor
timestamp: September 07, 2023, 02:39:49 am
content: Ok yep it's from the 34970, and according to the service manuals I saved it looks like it is the same transformer (9100-5608) in both. Now, I don't know for sure that the transformer's good: that could've been the problem with the 34970 for all I know.Either way though I'm happy to send it to you if you pay me back for shipping, or to do some simple checks first. The secondary resistances all look good, for red-white-red to be a center-tapped winding, orange-orange to be a 2nd winding, and green-blue-green to be a 3rd center-tapped winding. The primaries are much less obvious; I got continuity between the brown and brown/black, but no continuity between the green and green/black. But let me know what wires to look at and I can do some continuity checks on the primaries if you want.

author: jonpaul
author_rank: Super Contributor
timestamp: September 07, 2023, 02:49:14 am
content: Trsf seldom fail, unless set to 120V mains and 240V applied.1/ Check for mains V at primary?2 / remove suspect trsf , check pri/sec wdg resistances.j

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2023, 03:00:29 am
content: The transformer wiring looks like this according to the manual. The primary is a single winding with multiple taps for various voltages, so all of the primaries should have continuity with each other.

author: D Straney
author_rank: Frequent Contributor
timestamp: September 07, 2023, 03:22:47 am
content: Nice. Much better than the service manual I found. So it looks like the wiring on my salvaged transformer checks out good then - just so you know in case that really is the problem.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2023, 03:35:45 am
content: Not only that, it looks like if the primary did open up and there's nothing to the BLK/RED wire, if there is continuity from BLK to BLK/YLW he could do a bit of swapping and use the previously unused top side of the winding.

author: adamslab
author_rank: Newbie
timestamp: September 07, 2023, 01:16:34 pm
content: Ok, I checked the primary and secondary windings. On the primary there is continuity between BLK, YELLOW, GREEN but noting between RED and BLK, YELLOW, GRN which indicates that the neutral primary connection is faulty (?). Secondary continuity checks our OK. I also tried swapping one of the 220v taps as @bdunham7 suggested with the same result (no voltage output on the secondary), which makes me think more that it's the faulty neutral on the primary.@D Straney - I'll take you up on the offer for the transformer you have. Let me know how you want to work out the logistics to get you paid for shipping, etc.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2023, 02:45:00 pm
content: Quote from: adamslab on September 07, 2023, 01:16:34 pmIf you have continuity from BLK to BLK/YLW, I'd disconnect and insulate all of the other taps and just connect those two to 120V. Your meter should light up and then you'll be able to test the rest of it. Obviously the generous offer of a spare transformer is your best long-term fix.

author: adamslab
author_rank: Newbie
timestamp: September 07, 2023, 03:07:16 pm
content: Oh, that was a good idea and it brought the meter back to life!Agree that long term I'd prefer to swap the transformer, if the offer is still valid.Thanks for the help!

author: D Straney
author_rank: Frequent Contributor
timestamp: September 07, 2023, 04:41:36 pm
content: Great, glad it's working for now. PM sent.

author: Gyro
author_rank: Super Contributor
timestamp: September 07, 2023, 04:46:42 pm
content: Is there any sign of an internal thermal fuse on the transformer? If so, that might be in the common leadout. If that's open, worst case it might indicate an excessive current draw on the secondary. Best case it might be possible to dig it out and have a working spare.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2023, 05:31:10 pm
content: Quote from: Gyro on September 07, 2023, 04:46:42 pmIf it helps, at power-up with no connections, mine has a draw of 100mA @ 120V, 10W/12VA with a PF of 0.83.