Agilent 34401A VFD replacement - Page 1
Forum section: electronics / testgear

author: Veteran68
author_rank: Frequent Contributor
timestamp: November 29, 2022, 04:10:11 am
content: 2-3 years back I purchased a very good condition 34401A from a fellow eevBlog Forum member, and it's been a great meter for my hobby use. I have over a dozen handheld meters ranging from a Fluke 87V and Dave's BM786 on the higher end to Aneng and Voltcraft meters on the low end, but I always wanted a quality high-digit bench meter and this 34401A fit the bill nicely.While we were in between houses, I had it packed up and in (climate controlled) storage for nearly a year. After getting the basement of my new house remodeled into an office/lab/makerspace I got it setup but hadn't used it for some time. Recently I've been working on a laptop repair and noticed that the VFD is flaky in places. Some spurious extra segments are appearing and disappearing at random. See photo I just took showing the extra segment on the "V" making it look like a "K". They've also come and gone from different areas of the display. Sometimes, I think after it's been powered up for a long time and warmed up, they disappear and everything looks normal.Is this as simple a repair as picking up a $50 VFD off of eBay? Or is there more to it? I've seen comments online that indicate old vs new models have different video driver boards that aren't compatible. Does this look like a driver board issue vs just a VFD issue? Looking for input from anyone who has seen a similar issue and undergone a repair.Thanks!David

author: daisizhou
author_rank: Super Contributor
timestamp: November 29, 2022, 06:15:51 am
content: Compatible displays are made of LED,It is recommended that you first disassemble the host for inspection before determining the maintenance plan

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 29, 2022, 06:39:49 am
content: I ran across the following mod for this meter. It replaces the VFD with OLED display. Uses an STM32 microcontroller. THe HP uses SPI for the display I read in the article. He shares the kicad files, gerbers and firmware source.https://hackaday.com/2018/08/29/faded-beauty-dmm-gets-an-oled-makeover/I have no clue how good the mod is.

author: Hydron
author_rank: Super Contributor
timestamp: November 29, 2022, 10:03:52 am
content: If you're lucky it's just the VFD driver (scope it to check I guess), and you'll be able to get one off ebay or something. Schematics are available in the service manual.IIRC the units with black LO connectors use the newer display design with separate driver and micro-controller, so you've avoided the ones with an un-repairable display. The VFD itself looks nice and bright (better than mine at least!).

author: HighVoltage
author_rank: Super Contributor
timestamp: November 29, 2022, 10:09:26 am
content: The original front panel PCB with VFD is still available from Keysight.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 29, 2022, 11:18:47 am
content: But they will cost around 200$ usd if it's still available like @Highvoltage wroteI was lucky to buy VFD clones from Aliexpress, they worked really well, they where 35$ usd at their launch loll now they have doubled in price.You need the right equipement to remove it, many vfd pins, and if you're not good in desoldering, you may bust pcb padsWith the black plugs you have a latest version of drivers ic's in it, but normally the serial number will tell what you have in it ... and in the service manuals or search here i posted in the past the serial number ranges who got the older or the newest driver ic'sBut some checks have to be made in the supply sections for the electrolytics capacitors and tantalums ... and theses too require very good soldering skillsAnd for the dissasembly if you go that wayIf you ever remove the front panel from the main pcb, removing the front case panel is tricky, you have to remove the black special latchs on the inputs connectors on the front and back/rearThe front panel too is complicated ......the white connector who hold the front pcb flat cable is tricky, you must be very careful, (normally you push on it), stay pushed and gently pull on the flat cable to remove it while pushing on the white connector it act like a latch, and maybe if it was never removed, the wire are getting like glued by pression over timeONCE AGAIN BE CAREFULI would say looking at it i would not touch the vfd, even with some slight garbage in the display, i've had way worse then that

author: wasedadoc
author_rank: Super Contributor
timestamp: November 29, 2022, 01:03:36 pm
content: I'd suspect the problem is elsewhere, not in the VFD itself.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 29, 2022, 01:31:25 pm
content: I as well assume, that the VFD driver IC is having a problem, maybe caused by humidity, as I read your description.The replacement of the VFD itself probably won't fix the problem at all, so maybe you live with that effect, if it does not lead to mis-readings.If that extra segment vanishes after a very short time, maybe the driver IC has some beginning problems with spikes on its supply, or the supply voltage is faulty, i.e. a burnt diode, or a faulty capacitor. I'd suggest to check some repair descriptions here in the forums first.Btw.: I also started my private metrology lab with a 34401A, but that's 30 years ago, and my instrument still works fine and stable.Frank

author: Hydron
author_rank: Super Contributor
timestamp: November 29, 2022, 01:58:27 pm
content: Before going any further, try and get the revision number from the system menu. This will confirm which display PCB version it's running. If it's rev 1 (v. unlikely) then I think the KS replacement assembly is the only option, though I'm no expert on this as my meter is running a rev 2 board.If you do end up replacing the whole board with the Keysight assembly rather than just trying to get at the driver IC (SN75518FN thanks to the following post for the correction of package type, probably under the VFD itself) or find that a PSU or something is the cause, then I'd be interested in the old board for potential VFD salvage. Been tossing up whether to risk getting an ebay VFD and trying to replace my faded one, but there don't seem to be any sources with known-good-quality VFDs (sellers previously recommended here aren't selling any more). I've got a US mailing address btw.Otherwise, good luck with repairs (and if you really want to get adventurous with some EEPROM hacking while it's open, see here:https://www.eevblog.com/forum/testgear/hp-agilent-34401a-hidden-menu/)

author: pqass
author_rank: Super Contributor
timestamp: November 29, 2022, 03:03:32 pm
content: Judging by the faceplate, your model likely has the separate VFD driver chip (vs. the MCU+VFD single chip); this is good. According to the Agilent/HP Service Manual 6th Ed, the VFD driver chip is a 1820-5330 which cross-references to a SN75518FN which is a PLCC44 packaged part (the one on the right in image below).I didn't see any PLCC versions on eBay but there are many 40 pin DIP versions available there. If you're desperate, you could always solder some fine  wire to the PCB and route them around the back to the 40 pin DIP (dead-bug style).Agilent/HP Service Manual 6th Ed (see last page for the VFD schematic):https://www.ecb.torontomu.ca/guides/instrument-manuals/Agilent-HP_34401A_Service_Guide.pdfComponent cross-reference (search for 1820-5330):https://www.sphere.bc.ca/download/hp_xref-free.pdfSN75518FN datasheet:https://datasheet.octopart.com/SN75518FN-Texas-Instruments-datasheet-117963.pdfeBay.com:https://www.ebay.ca/sch/i.html?kw=SN75518

author: Veteran68
author_rank: Frequent Contributor
timestamp: November 29, 2022, 03:33:31 pm
content: Thanks all for the responses!I like the idea of an OLED hack but I'd need to see some trustworthy reviews to risk such an invasive mod.When I get home I'll dig a little deeper into what specific revision I have.Yes, the VFD is still plenty bright, and as far as I can tell it's still reading normally. I have had occasion to misread at a glance and have to do a double-take to realize there were extra segments. Not the end of the world, just annoying. Especially to my OCD.If this proves too difficult or risky, I'll just live with it as long as it doesn't get worse.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 29, 2022, 03:44:00 pm
content: the vfd will eventually deteriorate over the years ... gradually dim ... etc ... it's a sure factSame problems for many vfd based instruments, since some of them are powered for very long periods ....There was a project here for a remote display for the 34401a,https://www.eevblog.com/forum/testgear/external-display-for-agilent-34401a-(or-any-dmm-with-rs232-output-stream)/25/You put the 34401a in talk mode over serial port and use an external display to help visualize the meterit could be more practical for youIt depend of your electronic knowledge / experience(s) you can add anything for a display with arduino'sI tried it with an 4x40 display loll but it is kinda a bit slow refresh rates vs the VFD ... you have some arduino libraries to double de digits sizes over 2 characters linesBut enough for normal use, @CellularMitosis give the arduino code

author: pqass
author_rank: Super Contributor
timestamp: November 29, 2022, 03:57:24 pm
content: GOOD NEWS.....I was reading this threadhttps://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/msg2690097/#msg2690097and saw the attached image (see below).Notice the VFD driver is aHV518PJwhich is still available from digikey! The function and pinout appears to be the same as the original SN75518FN.See:https://www.digikey.com/en/products/detail/microchip-technology/HV518PJ-G/4902550

author: wasedadoc
author_rank: Super Contributor
timestamp: November 29, 2022, 05:57:21 pm
content: Quote from: coromonadalix on November 29, 2022, 03:44:00 pmDon't count on that not applying to OLEDs too.

author: Veteran68
author_rank: Frequent Contributor
timestamp: November 29, 2022, 07:03:48 pm
content: Quote from: coromonadalix on November 29, 2022, 03:44:00 pmOf course. I realize that, but the issue with mine isn't dimming. As you can see in the photo, mine is still very bright. It just has these random spurious segments that come and go. And when they appear, they seem to be brighter than surrounding segments, as if they're being overdriven (or driven by a short).This meter looks to have been very lightly used, as it was in largely pristine condition when I bought it 2-3 years ago and it's seen very light use since then. So I believe the VFD itself still has a lot of life left in it, particularly as it seems from the responses and further research that this issue is likely not a VFD issue, but a driver or supporting component (bad cap?) issue.Quote from: pqass on November 29, 2022, 03:57:24 pmThanks for the follow up! I'm at work right now but am gambling on that being a compatible driver for mine. Since I had some stuff in my DigiKey cart already that I hadn't pulled the trigger on yet, I just added one before placing the order. If it's not the issue, it's only 7 bucks, and I'll toss it in the spare parts bin for a potential future project.

author: alm
author_rank: Super Contributor
timestamp: November 29, 2022, 08:10:42 pm
content: Quote from: coromonadalix on November 29, 2022, 03:44:00 pmAnd OLED displays don't even need to be powered on to deteriorate

author: coromonadalix
author_rank: Super Contributor
timestamp: November 29, 2022, 11:38:09 pm
content: Quote from: wasedadoc on November 29, 2022, 05:57:21 pmYou need to check some oled meter displays substitutes threads, some of them are dying faster than the 34401A VFD itself ...

author: Veteran68
author_rank: Frequent Contributor
timestamp: November 30, 2022, 12:48:02 am
content: The revision of mine is 10-05-02. So only 20 years old, not pushing 30+ like some of them out there.As I'm going through the menus tonight, I noticed a pattern in this "leaky" segment behavior. At least for the moment, it's happening whenever either of the bottom slanted segments are used -- either "\" or "/". So V, R, and N all show the extra segments (see photo). Whenever one is displayed, so is the other, and it doesn't matter the digit's position. So those two segment lines seemed to be tied together.I don't know if that's enough of a clue for anybody that's seen this behavior to narrow down the problem or not. When I get time here in the next few days I'll take it apart and look for bad caps or transistors, and if I don't see anything obvious I'll replace the driver IC when it gets here.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 30, 2022, 01:05:29 am
content: eh i could be wrong, the vfd controller could play tricksbut the problem will be, remove the vfd carefully, change the chip, put back the vfdBut i would personally check before assuming its the vfd controller that all the supply lines and noise juuuuust to be sure they are cleanSome people reported bad capacitors on the vfd pcb supply linesyou're dealing with old puppies from 20, 25 ... years old in some cases ??my 2 cents, sure the vfd controller is not too priceythe 34401a sucessors aka 34410a and 34411a just had their end of life made in febuary 2022no original keysight vfd+pcb are available only clones ... or keysight repairs who cost like a new meter loll

author: robert.rozee
author_rank: Frequent Contributor
timestamp: November 30, 2022, 02:02:23 am
content: hmmm... when one lower diagonal is on, so is the other. looks like it could be a short. do you see ANY characters displayed where one lower diagonal is ON, while the other lower diagonal is OFF?i'd be checking the display PCB for the dreadedtin dendrils. these are fine whiskers of tin that can grow between electrical contacts, developing over a period of many years. microscopically thin, but can grow up to half an inch or so long.these dendrils can be removed simply with a paint brush, although be cautious when brushed aside that they do not land somewhere else where they can cause mayhem!cheers,rob :-)

author: coromonadalix
author_rank: Super Contributor
timestamp: November 30, 2022, 02:07:34 am
content: do the short display testHold shift and power up the meter and release shift, it will briefly light all the vfd sections ..

author: Veteran68
author_rank: Frequent Contributor
timestamp: November 30, 2022, 02:27:07 am
content: Ok, so further observations of interest. Now that it's been powered up for over 2.5 hours, those leaky extra segments have gone, come back for a few minutes, and are now gone again. But when gone, multiple segments appear "hot" or over bright, particularly the slant segments both top and bottom,but only while in the menu. At the DC voltage measurement mode, for example, brightness is consistent across all segments including the slants in the V in VDC.Looks like some thermal variation causing segment lines initially shorted to break contact, and voltage or current to fluctuate. That or the brightness variations are just a characteristic of an aging VFD, but not sure why only when in the menu. Maybe because the menu display is static vs refreshing like in measurement mode? The brightness difference doesn't show as significantly on camera as it does in person, but this pic captures some of it. Notice how bright the slant segments on the R and V are:Quote from: robert.rozee on November 30, 2022, 02:02:23 amNope. When it happens, both lower slanted segments show whenever either one should be displayed. I agree it looks like a short on those segment lines.Quote from: robert.rozee on November 30, 2022, 02:02:23 amThat's an interesting notion I hadn't considered. If such a thing occurs over time, it would certainly make sense in a 20 year old instrument. And with such a delicate short, thermal expansion could easily break the short.I have a sneaking suspicion it's a faulty component or driver IC, but I will look at everything suggested and will give it a good brushing just to cover the bases.Quote from: coromonadalix on November 30, 2022, 02:07:34 amYeah I've done the tests. All VFD segments light up, but that's to be expected -- I'm not having any issues with segments NOT displaying, just those displaying when they shouldn't.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 30, 2022, 03:33:15 am
content: you could hold for a long period the shift button and see over a long long time how the vfd would react, it would be the maximum stress given to it ?once again i have my doubts over the vfd drivers, i had an 22 ish year old meter, no dendrils but the unobtanium driver ic with some of the symptomsand put an aliexpress clone and everything went fine .. the vfd as you know have filaments, plates, heater etc ... it can create deposits inside the vfdYou have some trick like overdriving a bit the filaments to clear some internal residues ...another thread of vfd herehttps://www.eevblog.com/forum/repair/vfd-unwanted-segments-glowing/some even tried thishttps://www.eevblog.com/forum/chat/vacuum-fluorescent-display-rejuvenation/

author: james_s
author_rank: Super Contributor
timestamp: November 30, 2022, 04:12:14 am
content: Your VFD looks great, I really don't think that's the problem, so you already have the hard part taken care of. I suspect a problem with either the driver or the digital signal to the driver, maybe as simple as a bad connection. As far as displays go, VFD is one of the most robust and longest lived there is, second maybe to LED but some of those are not as reliable as you'd expect.OLED displays look great as far as modern display tech goes, but they have a spotty reliability record, there are many cases of failing OLED displays.