Any interest in LED replacement for Fluke 8840A/8842A VFD displays? - Page 1
Forum section: electronics / testgear

author: bikeNomad
author_rank: Contributor
timestamp: September 19, 2020, 05:07:24 pm
content: I'm making a LED-based replacement display module for a couple of my Fluke bench meters. The VFDs in these get dimmer over time.The replacement consists of a PCB, a plastic baffle with cut-outs where there are lit segments, translucent material of some sort in the cut-outs to diffuse the light, and a printed sheet with words all laminated together.There is a trimmer on the board to adjust the brightness.I may also add the ability to have a continuity buzzer, but I can't find a good place to put a switch to turn that feature on and off.On the 8840A the unused button position where the AUTO switch is on the 8842A might be usable but there aren't good front panel choices on the 8842A because of the metal plate.Maybe it would be usable to always have the continuity buzzer enabled; I don't know.If enough people are interested, I could make these available for purchase (assuming I get good results from my prototypes).What would be a reasonable price to ask?Are any of you interested?I'll be getting a batch of 10 from JLC PCB in a couple of weeks, and will have to hand-solder the missing components including the 64-pin QFP processor that is doing the level shifting, LED driving, and continuity detection.Here's the board:

author: AVGresponding
author_rank: Super Contributor
timestamp: September 19, 2020, 05:51:41 pm
content: Heresy!You should be building your own custom VFD factory instead!On a more serious note, nice that these fine meters get a new lease of life thanks to your efforts. Luckily my 8840A is still bright as new.Not sure I'd want to butcher the front panel to add a continuity buzzer switch, I just use other meters for continuity.Have you settled on an LED colour yet? I would bet it's possible to get a pretty good match to the VFD these days, white would be a second choice for me.I might well be interested in buying one, as a backup, if the price works for me.

author: bikeNomad
author_rank: Contributor
timestamp: September 19, 2020, 06:33:25 pm
content: I'm using plain blue LEDs right now because that was the only blue-ish choice at JLC PCB.I was hoping to find cyan LEDs to match the original color; we'll see how the blue ones work out.What kind of a price would work for you?

author: AVGresponding
author_rank: Super Contributor
timestamp: September 19, 2020, 06:58:30 pm
content: As a chronic and unrepentant TE addict, it varies greatly from month to month...(in proportion to the thickness (more often thinness) of my wallet)

author: giosif
author_rank: Frequent Contributor
timestamp: September 19, 2020, 07:02:49 pm
content: I too would be interested, but total cost is a key factor (cannot suggest a price, though).Would you consider selling it as a DIY kit as well?

author: bikeNomad
author_rank: Contributor
timestamp: September 19, 2020, 09:10:23 pm
content: I might consider selling it as a DIY kit. The difficult part is the translucent diffuser. I’m not entirely sure how easy that’s going to be for people to do at home. But I’m hoping to find some way that’s DIY friendly.

author: Noy
author_rank: Frequent Contributor
timestamp: September 20, 2020, 09:03:23 am
content: My 8840A is still in good condition.But as a spare part i would also be interested in one.I also don't want to butcher my meter so best would be a drop in replacement without bigger rework of the original parts. How sure are you that the measurment is not getting worse because of the regulator / uC?DIY / soldered without led would be nice, everbody can suggest led color by himself. Some nice cyan LEDs were sold in the market here some time ago.Price point should be less / same than an original spare display (form eBay). So less than 20-40€ if possible (realy nice would be part cost + a free to choose donation to cover your time / work )

author: Tarloth
author_rank: Regular Contributor
timestamp: September 21, 2020, 12:10:37 am
content: Sorry for my question, but why to use individual leds instead of 7 segments led number modules? It's hard to obtain an even illumination from a single LED without appropriate optics like the 7 segment modules has.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 21, 2020, 12:42:48 am
content: @tarloth Do you mean an already made multiplexed diplay like 10 digitsor alphanumeric displaysThe last problems would be the leds functions annunciators ??

author: Tarloth
author_rank: Regular Contributor
timestamp: September 21, 2020, 04:51:40 am
content: Quote from: coromonadalix on September 21, 2020, 12:42:48 amYes, the 5 and half digits will be made perfectly with simple old 7 segment and use the single LEDs to announce functions or states. Use a single led to digit segment and obtain a nice uniform illumination in the segments needs a no so direct optics.

author: usagi
author_rank: Frequent Contributor
timestamp: September 21, 2020, 07:57:21 am
content: How about a replacement using a real VFD?

author: giosif
author_rank: Frequent Contributor
timestamp: September 21, 2020, 09:53:50 am
content: Quote from: usagi on September 21, 2020, 07:57:21 amThen, the replacement may not be financially viable anymore (i.e. I wouldn't spend half the price of a working meter just for the replacement display).

author: coromonadalix
author_rank: Super Contributor
timestamp: September 21, 2020, 10:36:51 am
content: @UsagiThere is simply no interest for theses meters in the "china display clones community", you wont find any vfd replacements / copy, like the Hp / Agilent 34401A has ...The 34401a display is unobtanium, and a pcb with the vfd is 200$ usd, aaaand they will be unavailable in not too long ?I dont think you would pay that amount for your 88xx meter ?You'll have to salvage parts ... or buy / build the BikeNomad project

author: texaspyro
author_rank: Super Contributor
timestamp: September 22, 2020, 07:32:44 am
content: Quote from: bikeNomad on September 19, 2020, 09:10:23 pmTry drafting velum/mylar. It is clear plastic with one side that is frosted.

author: bikeNomad
author_rank: Contributor
timestamp: September 24, 2020, 01:59:20 pm
content: Quote from: Tarloth on September 21, 2020, 12:10:37 amI think I can get even illumination with my scheme using a translucent filler material (like the seven-segment displays do). Also, I've only got a maximum height of around 10mm to work with and wanted to get a color as close as possible to the 505nm original display.There's a good overview of various diffuser methods here:https://hackaday.com/2017/04/25/ask-hackaday-what-about-the-diffusers/I have a dual-extruder 3D printer and a laser cutter, so I will try easy things like making the mask from opaque plastic and then adding a sheet of diffuser material and a printed mask with the words. If this bleeds too much I'll try filling the voids in the mask with hot melt glue or maybe a translucent acrylic casting resin.

author: Tarloth
author_rank: Regular Contributor
timestamp: September 24, 2020, 08:38:20 pm
content: OK, but using 7 segment display the diffuser is trivial. Nothing more from here, I was only curious if did exist any other reason.

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: October 27, 2020, 04:07:54 pm
content: For the LED display diffuser film there is a company that makes them:https://www.toppress-siebdruck.de/de/leistungen/diffusorfolien.html

author: Miti
author_rank: Super Contributor
timestamp: October 28, 2020, 10:19:36 am
content: What about this one?https://www.eevblog.com/forum/testgear/fluke-8840a-restoration/

author: bikeNomad
author_rank: Contributor
timestamp: October 28, 2020, 02:05:40 pm
content: Quote from: volvo_nut_v70 on October 27, 2020, 04:07:54 pmThey're interesting, but I'm sure there's a NRE and minimum order involved.