How to test meter replacement VFDs before soldering them? - Page 1
Forum section: electronics / testgear

author: hp3310a
author_rank: Regular Contributor
timestamp: September 11, 2025, 03:49:34 pm
content: I was wondering if there was an easy way to test replacement VFDs for meters - specifically for the 34401a and 34410a multimeters (those have different layouts, I know)? Something that illuminates all the segments and announciators would be perfect.I don't get a lot of useful (to me) clues from the 34401a schematics (relevant portion below). Only that -18V, -13V and +18V are in the mix. But what is the anode voltage, -13V or +18V? Do I just connect all anode pins to +18V (say) and grid pins to -18V or -13V? And what is pin 1 and is the numbering scheme the same as with ICs?

author: Kean
author_rank: Unknown
timestamp: September 11, 2025, 04:32:45 pm
content: You need an AC voltage for the filaments (DC will give inconsistent brightness), plus a DC voltage (typically in the range of 12-20V) for the grids and anodes. These two supplies also need to have a common 0V connection. The negative anode/grid bias voltage is to ensure they are turned off (rather than 0V or floating - and usage depends on the AC filament vs DC level), but may not be needed for a simple tester. All drive voltages should be current limited (which the typical driver ICs can do for you).Have a look at the docs from Noritake which give example wiring:https://www.noritake-elec.com/technology/general-technical-information/vfd-operationIdeally you would have some basic circuitry to cycle through the grids, and have the anodes all wired to voltage - maybe with switches of PCB jumpers to selectively enable them. The Noritake doc shows both static and multiplex configurations.If you have any microcontroller experience, you might want to make use of a MAX6921 or PT6311 for something more versatile. At least check out their datasheets for some detail on operation. These ICs make multiplexing and dimming relatively easy, and sample code shouldn't be hard to find. I don't think either of these chips can handle the full 34401A display due to the total count of anodes and grids, so maybe a MAX6922, or a pair of PT6311. I've used the PT6311 before, and I have some much older equipment I built were I used ULN something driver ICs.Here is another good article:https://www.thevfdcollective.com/blog/pt6311-vfd-display-controller-power-supply-and-arduino-library

author: Ian.M
author_rank: Super Contributor
timestamp: September 11, 2025, 04:43:40 pm
content: I'd be tempted to simply power the filament at the correct voltage*off my bench supply and use 2x PP3 9V batteries in series, with negative connected to bench supply negative, to take *ALL* the grids to +18V and a flying probe to connect each segment or annunciator anode in turn. Yes, its not ideal as there will be some brightness variation due to the DC voltage drop across the filament, but not enough to matter for testing. If you use a second channel of the bench supply for the +18V, put a resistor in series in case you mistake the connections - 470 ohm will keep the short-circuit current under 40mA which should save your filament if you inadvertently connect across it!The filament pins can be identified because they have a DC resistance between them. Every other pair of pins should read open circuit on a DMM with the filament cold.* If you can see the filament glow in subdued room lighting you are overdriving it. It should however be possible to see the glow in a totally dark room.

author: Kean
author_rank: Unknown
timestamp: September 11, 2025, 05:03:55 pm
content: Quote from: Ian.M on September 11, 2025, 04:43:40 pmDepends if you are testing for anode/grid faults, or aging effects on brightness. For testing brightness you probably want a center-tapped AC filament drive, but otherwise a good point.I should have mentioned the filament drive can be mains frequency from a low voltage transformer (often center tapped), but it also commonly implemented via a high frequency (10's of kHz) DC-AC converter. Both of these AC options, plus the DC filament drive and how the bias is ideally arranged, are explained in the Noritake page I linked above.Knowing that the brightness issue existed with DC filament drive, I have probably never tried it myself so I don't know how bad it actually is.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 11, 2025, 05:05:05 pm
content: 34410 34411 have if i recall logic circuitry in the vfd not sure if 34401a is the same i have a blackout lolli would just put the pins in the pcb holes and kinda force a small angled pressure on the pins to force a contact and see once all is connectedon 34401a i remember you have a full vfd test while powering on, i think 3441x models too

author: hp3310a
author_rank: Regular Contributor
timestamp: September 12, 2025, 06:23:12 am
content: Many thanks for the replies!Quote from: Kean on September 11, 2025, 04:32:45 pmThanks for this, very interesting and highly "illuminating" read.Quote from: Kean on September 11, 2025, 04:32:45 pmSounds doable, the fixture for the VFD will have to be gentle enough so it doesn't destroy those long skinny pins. They went through enough already being bet all over the place during packaging.Quote from: Kean on September 11, 2025, 04:32:45 pmThanks, I'll probably not go down the road of building a board for testing (I don't plan to change VFDs on a regular basis) but I will read this nonetheless.Quote from: Ian.M on September 11, 2025, 04:43:40 pmThanks, that's what I'm probably going to try first. I have a PSU that can do +18V and -18V, so I'm not sure I'd need battery sourced 18V, I take it this was for the case that no PSU is available? From the sound of it it's best to start a lot lower, not at the full +-18V.I only want to know if the filaments work at all, since the VFDs are new I assume they would illuminate with equal intensity when controlled correctly.Unless uneven illumination is a common thing with those VFDs? So far I only replaced one VFD trusting it will work... and it works great with evenly illuminating filaments.Quote from: coromonadalix on September 11, 2025, 05:05:05 pmInteresting idea, I'll keep that in mind. Yes, the power-on self test lights them up on both models. The downside of this is it requires desoldering before knowing the replacement works.

author: Ian.M
author_rank: Super Contributor
timestamp: September 12, 2025, 06:30:07 am
content: I hope you are aware that the filament voltage will be *MUCH* lower than 18 V. The battery sourced 18V was if you don't have two independently adjustable PSUs or PSU channels.

author: Kean
author_rank: Unknown
timestamp: September 12, 2025, 06:38:29 am
content: Quote from: Ian.M on September 12, 2025, 06:30:07 amWas about to mention the same thing. Filament often about 2-3V RMS.Actually, looking at the 34401A service manual, it is 5V RMS - but you can probably go a bit lower for testing, and probably ignore the negative supply completely.Quote from: 34401A Service Guide Ed 9, 2014 pg 106Quote from: 34401A Service Guide Ed 9, 2014 pg 107

author: Ian.M
author_rank: Super Contributor
timestamp: September 12, 2025, 08:06:36 am
content: So if there's -12V cathode bias and +18V on an 'ON' segment anode (or digit grid) the display actually runs on 30V HT. You should see something with +18V relative to ground on one end of the filament but nowhere near full brightness.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 12, 2025, 10:28:37 am
content: Quote from: Ian.M on September 12, 2025, 06:30:07 amQuote from: Kean on September 12, 2025, 06:38:29 amOK, thanks guys! Very good to know. I'll start lower.Quote from: Kean on September 12, 2025, 06:38:29 amOK, thanks, got the service guide now, too.There's one unknown for me before I try and hook it up to the power supplies and that's GND. Where does it go on the VFD? The schematic (34401a) doesn't spell it out (at least not to me). The only pins that are not connected are 15 and 17. So probably one of those, but which one? I can't see any markings on the VFD itself.

author: Kean
author_rank: Unknown
timestamp: September 12, 2025, 11:51:43 am
content: Quote from: hp3310a on September 12, 2025, 10:28:37 amIn this simple case GND would be connected to one side of the filament (doesn't really matter which), and the difference between "off" drive and filament is then Vbias. As we are not using a negative voltage for off, then Vbias becomes 0V although the "off" anodes and grids are probably floating.There are two common configurations I've seen. The configuration used on the 34401A pulls "off" anodes and grids to -12V with a resistor (in the driver IC), and "on" anodes and grids are driven to +18V. One side of the filament is on 0V, the other to 4V (it is actually 5V RMS across the filaments with a center tap to 0V). Another configuration is to use only use positive voltages, so 0V is "off", +12V is common with the filaments, and +30V is "on" - this may simplify level shifting circuits. Voltages will vary with actual display brand and size, and voltage levels will vary driver IC or level shifting circuit.As it has been a while since I did this in practice, I just did a quick test with a spare bare VFD intended for calculators, 11-digit 7-segment. Apparently a Futaba 11-ST-26A, and is Jaycar part ZD1880.I set up a couple of PSU channels to 3V DC and 18V DC, with a common ground.The first channel at 3V DC was connected across the filament, and current draw was about 15mA.I then connected the 18V rail to Grid 1, and also to Segment G (center horizontal bar) and this lit up the expected segment.If I also bridge Grid 2 to 18V, then segment G on the first 2 digits lite up.If I also bridge Grid 11 to 18V, then segment G on the first and last digits lite up - and seem pretty similar in brightness.All other anodes and grids were floating, and just the resistance of my fingers was enough to make additional digits or segments light up if I bridged pins (but generally not at full brightness).The (crappy) datasheet from Jaycar is attached ... which I see now recommends 12V for Vgrid & Vseg on this display. I have a better datasheet in paper form, which doesn't give a drive voltage, only the filament voltage, but does recommend UDN6118A driver ICs.I hope this make sense. I didn't take any photos, but there are some of the same display and setup on this posthttps://electronics.stackexchange.com/questions/230044/help-driving-a-vfd-displayMost of my VFDs are on modules which have filament drive circuits, and ICs for multiplexed anode & grid drive. These are quite expensive new, but I bought a bunch at clearance as I think they look great on my custom made gear.In my first job repairing cash registers (National Semiconductor / Datachecker DTS), we used plasma displays with orange segments and they used a -120V drive voltage if I recall correctly. The control circuitry including CPU and I/O chips was all PMOS (running at 17V), so the design used +5V and -12V to more easily interface with common logic ICs. I see these are commonly called Panaplex displays (brings up good pics on Google), and the ones we used were 14-segment and maybe 16 or 20 digits.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 12, 2025, 03:53:38 pm
content: Thanks again guys, it works!For a simple test this is definitely sufficient:Red on pin 1 is FIL1 on +2V DC, Black on Pin 16 is FIL2 on GND. Yellow and White connect to all other pins (except Pin 15) are on +10V (GND is connected to GND of the filament voltage). The uneven intensity presumably is because the filament voltage is DC and not AC.I have two displays and the second one is much darker at the same voltage (+2V/+10V). Anode and grid voltage needs to go up to +18V to be as bright as the first display.Not sure if that means the the first one is actually worse because it will burn up more quickly when the much higher production voltages are applied?

author: Kean
author_rank: Unknown
timestamp: September 12, 2025, 04:49:26 pm
content: Quote from: hp3310a on September 12, 2025, 03:53:38 pmYes, it is a combination of polarity and filament resistance leading to a change in the bias level.If you reverse the filament polarity, does the increasing brightness gradient change from L->R to R->L ?

author: hp3310a
author_rank: Regular Contributor
timestamp: September 12, 2025, 04:55:24 pm
content: Quote from: Kean on September 12, 2025, 04:49:26 pmI tried that but the gradient direction remains right-to-left.

author: Kean
author_rank: Unknown
timestamp: September 12, 2025, 05:03:22 pm
content: OK, thanks for reporting back. I wasn't sure if it would work, it was just a hunch based on how I thought this display was constructed.

author: Ian.M
author_rank: Super Contributor
timestamp: September 12, 2025, 05:41:34 pm
content: Note that under-running the filament can result in cathode poisoning, and in combination with excessive HT can also strip the emissive oxide coating, so for anything other than a short test make sure the filament supply voltage is in-spec. If you don't have the specs or don't trust the voltage given on a schematic, bring the voltage up slowly till you can see the filament faintly glow, then back if off a little so the glow is barely (but still) visible. Repeat when you have some segments set up with drive as you want to be operating a bit past the knee of the emission curve on the plateau, not on the slope up from zero, so when turning the filament voltage down you don't want to see a significant drop in segment brightness. See fig.10 at the Noritake link.At 10V HT and 2V filament supply you only have 8V cathode to anode and grid at the positive end of the filament so it isn't surprising you have an extreme brightness gradient. Try around 25V to 30V for your HT and some more filament voltage (as just described) while testing.

author: floobydust
author_rank: Super Contributor
timestamp: September 12, 2025, 06:18:09 pm
content: It's normal - there is always a brightness gradient with wide multichar VFD's.With AC filament drive, it's just that the eye can't see it due to the flickering."... the transformer is the most popular device utilized to supply the filament voltage (Ef) with a 60(or 50)Hz sine wave which also has a center-tap for cathode bias as shown in Fig.11. The center - tap technique is used to prevent luminance slant i.e. difference in brightness from one side of the display to the other.""... If a DC filament drive is adopted, a potential difference between the anode and grid voltage will be apparent as a luminance slant across the display as shown in Fig.14. This shows brighter luminance at one side of the display due to the DC voltage drop."https://www.noritake-itron.jp/eng/cs/appnote_vfd/18/Also normal is for the filament to have a dull orange glow, visible in a dark room.The HP34401a is bad for "cathode stripping" of the VFD on power-up it's all segments on before the filaments are warmed up.My measurements it's typically 5VAC at 130-140mA (hot) and for rejuvenating the display by boiling off the cathode contamination I'll go up in voltage.

author: hp3310a
author_rank: Regular Contributor
timestamp: October 03, 2025, 12:08:24 pm
content: So I tried the same with a new VFD for a HP 3245a/3458a, but I can't get it to work like for the 34401a.I connected the filament pins 1 and 28 to DC +3V and GND, pins 2 to 14 to GND and pins 34 to 49 to +18V but nothing lit up. I can see pin 15 is marked as CUR but I have no idea what that is supposed to do. If I read the schematic correctly it is connected to 5V via the Q1 but I didn't want to try that. I briefly went up to +20V on pin 34-49 but that didn't do anything either (that I could see).Not sure where my mistake is, did someone try this with such a VFD and can tell me what I'm doing wrong? I'm attaching the relevant portion of the schematics just in case.

author: hp3310a
author_rank: Regular Contributor
timestamp: October 05, 2025, 02:09:03 pm
content: Reading a bit more about the driver (produces +50V) I convinced myself that the voltage needed to go up. I tried with about 30V and filament voltage of 4.1V (supposedly the driver produces AC 5V RMS), and got a few dots to light up.Then I connected more dots, put 35V to the characters and turned up the filament voltage slowly from 2.5V to 4.2V. Then I could watch to how one the dots exploded.PS: I think the CUR pin is just one annunciator that is controlled directly.

author: floobydust
author_rank: Super Contributor
timestamp: October 05, 2025, 07:09:40 pm
content: Bench testing VFD's, you always need TWO current-limiting resistors. This is what I advise.1. Filaments is series with around 10 ohms 5W- for protection against cold inrush, stupid power supplies that overshoot, and any short to +HV etc. Adjust the bench PSU so the filaments have the right voltage. 5VAC = 7VDC.2. Around 10k ohms in series with the HV for any segment you are energizing.50V is for really high brightness, multiplexed displays and they will roast unless you limit current somehow, manually driving them.

author: hp3310a
author_rank: Regular Contributor
timestamp: October 05, 2025, 08:22:48 pm
content: OK, thanks good to know for next time, if I ever feel like nuking a VFD again.

author: coppercone2
author_rank: Super Contributor
timestamp: October 05, 2025, 08:36:55 pm
content: Hmm you know honestly testing them on the bench can mess it up easily. You better have good test leads. If you solder it in, and use the solder extractor, its usually OK. But if you yank a grabber hook by accident or have a stiff wire, it can take it out. Even when desoldering, the hole of the PCB for the lead prevents 'over bending', while if its on the bench, you can easily yank it offI think that kovar seal should be pretty heat resistant. But you need desoldering tools in good order.I noticed when I replace the screen on my K2001, they are easy as hell to break while on the bench. I had one close call when I left it out on the bench to do something, and something almost fell on it.BTW, tape it on with kapton, before you solder. Then when you desolder it will stay in place. but make sure the tape is clean and well adhered. If you need to use a manual solder sucker, then I don't recommend it.Hp3310A is brave using those clips and wires on the meter!If you have to t est alot, then I guess its just a risk you need to take with them. But for a one off, I would just solder it in.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 06, 2025, 05:26:05 am
content: If you have to test a lot, it would definitely be worth investing in a proper test rig. An adjustable low voltage AC power source would be top of my list

author: hp3310a
author_rank: Regular Contributor
timestamp: October 07, 2025, 07:14:51 am
content: While the pins certainly are delicate, they can (and did) survive gentle manipulation on the bench.I think the mistake I made is to connect all characters at the same time. Maybe VFD is intended to be lit one character at a time, otherwise the dot connections get overloaded (and explode). I will give it another try today, it is broken already so it can't get worse. The filament is still intact with the same resistance as before, so something may work.Edit:It is completely dead

author: AVGresponding
author_rank: Super Contributor
timestamp: October 07, 2025, 08:54:51 am
content: Likely to be multiplexed, as well, would be worth studying a datasheet to get a rough idea on pulse duration etc