How To:  HP/Agilent 34401A Display - Page 1
Forum section: electronics / repair

author: Docholiday
author_rank: Regular Contributor
timestamp: May 23, 2016, 01:24:50 am
content: I have a HP/Agilent 34401A and after buying a new VFD display from Keysight I am ready to swap out the old with the new VFD display. I know how to get the case open but any further I am afraid of breaking the plastic with prying and pulling. Cant' be this hard to do!Searched for a video how to on the display - but no luck.Anybody got some tips & techniques or perhqaps a video? I have the service manual but all it shows is a drawing and that does not help me at the least.Thank you,Nicholas

author: Macbeth
author_rank: Super Contributor
timestamp: May 23, 2016, 01:50:24 am
content: Ok, I could forgive you if you were asking about the latest smartphone and the methods involved of taking them apart including micro torx drivers and digging in through the sides of the screen with super fine teflon podgers, etc. But an old solid screwed together bench meter? Seriously there are no surprises, just use common sense man!(Someone will now tell me that HP booby trapped their 34401A's? I can't see it!)

author: Docholiday
author_rank: Regular Contributor
timestamp: May 23, 2016, 02:11:37 am
content: Quote from: Macbeth on May 23, 2016, 01:50:24 amMust be that english humor? No offense taken...

author: Andy Watson
author_rank: Super Contributor
timestamp: May 24, 2016, 09:21:16 pm
content: Do you have the correct service manual? The one that I have (downloaded directly from HP/Agilent I believe) shows how to remove the front panel and display PCB on page 9-4.

author: Docholiday
author_rank: Regular Contributor
timestamp: May 24, 2016, 09:31:04 pm
content: Quote from: Andy Watson on May 24, 2016, 09:21:16 pmI have that page (drawing) as you said. But how is MP6 removed so I can pry on the front panel?Also, it seems one has to apply a lot of pressure to that front panel to clear.Thanks,Nicholas

author: Andy Watson
author_rank: Super Contributor
timestamp: May 24, 2016, 10:20:57 pm
content: The hook part of MP6 needs to be pried out of the (red) terminal block to allow it to pivol upwards and release the front panel.Have a look at the rear terminal block - it uses the same system to hold the block against the metal work - but it is a bit more accessible.Quote from: Docholiday on May 24, 2016, 09:31:04 pmYes it does seem like a lot of pressure - note that the metal sides can be pressed inwards slightly too.

author: Docholiday
author_rank: Regular Contributor
timestamp: May 24, 2016, 11:14:26 pm
content: Quote from: Andy Watson on May 24, 2016, 10:20:57 pmThank you so much for the hand holding and the drawing you did. Getting MP6 out was a slight challenge but getting out the front panel was difficult. Onward in the disassembly of the front panel. May ask you for help on this too.Thank You,Nicholas

author: Docholiday
author_rank: Regular Contributor
timestamp: May 25, 2016, 02:10:10 am
content: Quote from: Docholiday on May 24, 2016, 11:14:26 pmAll done! Thank you again Andy its all done.All thats left is a rear bezel which I will order tomorrow.

author: bitseeker
author_rank: Super Contributor
timestamp: May 25, 2016, 02:13:40 am
content: Glad to hear it all worked out, Nicholas.It is a disconcerting process getting one of these apart, especially the first time trying it. I was nervous too, doubly so considering the age of the device. Hard to believe they expected the use of so much force on plastic components. HP power supplies of the era are much friendlier.

author: Docholiday
author_rank: Regular Contributor
timestamp: May 25, 2016, 02:19:17 am
content: Quote from: bitseeker on May 25, 2016, 02:13:40 amThis was my first time taking any HP/Agilent equipment apart. I was sure the plastic would crack or a piece break off in applying that much force, but it didn't.Again Thank you for all your help....

author: Maxlor
author_rank: Frequent Contributor
timestamp: January 29, 2017, 09:59:13 pm
content: I've also swapped out the display on my 34401A because it was dim and uneven. I figured people that are considering doing this might be interested in seeing the results, so I made some pictures. The old display is on top, still attached to the front panel PCB, the new display is at the bottom.I got the display from china, ebay I think. It's not an OEM part, but it matches the original. It was no trouble to replace it, except I didn't notice that the metal frame around the display made contact to one of those bent tabs holding the glass, which cause a short. After I fixed that, everything worked.The pics are intentionally a bit dark, because I had to set the exposure settings initially, and wanted to leave enough headroom for the new, brighter display not to be blown out. In real life, the old display was quite hard to read under direct illumination, the new one's fine in that situation.

author: JohnPi
author_rank: Regular Contributor
timestamp: September 16, 2017, 04:23:14 am
content: I was able to remove my MP6 clip; I can also pry off the right-hand side clip over the two studs on the case frame and rotate that side of the display off a bit. However the left side gets hung up on the on/off switch extender (MP3) -- how does that come off ? I can't just pry the left-hand side of the front panel off the left side of the case frame (the part that holds the transformer) -- there are 2 studs there also and not as much play to pry the plastic over them (I did remove the screw there).Any clearer guidance ?

author: bitseeker
author_rank: Super Contributor
timestamp: September 16, 2017, 05:09:09 am
content: If you haven't already, remove the power switch extension rod. It should pull off from the switch and then can be removed.

author: JohnPi
author_rank: Regular Contributor
timestamp: September 17, 2017, 12:49:26 am
content: Thanks -- that did the trick.For anyone else, here are some additional details that might help:Don't forget the torx screw holding the left-hand side of the front onto the frameYou can remove the power switch extension from the actual switch by separating them slightly and inserting a small screwdriver in the gap and using it to lever them apart. The extension can be removed (and replaced) without taking the front panel off.When you remove the frond panel, the PCB (and attached VFD) slide off it to the right. Around the centre of the PCB there is a small tongue-shaped cutout in the PCB which you must lift to allow the PCB to slide out.When replacing the actual VFD itself, you are unlikely able to free all ~ 40 leads with solder wick or a vacuum -- so just cut them and unsolder one by one. Each shield half has only 3 solder points and can be removed with the aid of solder wick.It is tricky to get all leads of the new display in place -- since they are longer than needed, it might help to cut them with (slightly) decreasing lengths in each group -- that way you can align one by one. When you get it all in place, bend one lead of each group 90 deg, just to prevent the whole thing from falling out as you then replace the metal shields.The replacement VFDs have wider leads coming out from the glass and these might short to the shield. It might be possible to bend the shield out slightly at those points, but I just put some Kapton tape on the sides of the VFD which insulated them.

author: bitseeker
author_rank: Super Contributor
timestamp: September 17, 2017, 03:28:19 am
content: Thanks for the additional notes, John.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 17, 2017, 11:03:29 am
content: By now I have repaired many 34401A instruments and replaced several displays.But I remember the first time I took a 34401A apart, the front did not want to come off at all.There is some good advise in this thread for anyone starting to repair a 34401A and following this at home.Just one additional hint:If I need to take the front part off a 34401A, the first thing after removing the cover is to remove the power ON switchpush rod. As long as everything is in place, the long push rod is easily removed. Once the front is loose, the rod gets stuck and can even break when you try to remove it on older meters.(This is even more important on the 34410A and 34411A)The good thing: Almost all parts are available from Keysight to repair a 34401A

author: iMo
author_rank: Super Contributor
timestamp: July 16, 2019, 08:25:51 pm
content: I've got a 34401a off ebay and the color of the display is green.Frankly, all pictures I've seen so far show blue one.Could it be there are different variants of the display used over the time?Or the front window has been replaced with greenish one?Mine is HP one, red posts, REV 10-5-2.PS: A year back I sat a half day long in front of a 34401a and I am pretty sure it was blue.. Or not?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 16, 2019, 09:02:36 pm
content: Its not a pure blue, its kinda blue silghtly greenjade i thinkNot a pure green, at least never saw green in any 34401a meter, if its a clone / copy of an original, could beI've bought 2 vfd's clones on aliexpress and they have the same hp agilent color segments / characters ??

author: iMo
author_rank: Super Contributor
timestamp: July 16, 2019, 11:26:45 pm
content: ..exactly.. JADE.. that is that color.. thanks!