Fluke 8840A/AF 8842A VFD Display Replacement

[bdunham7]

I've been working on a way to replace the faded or broken displays on the Fluke 884x series meters and I have a half-assed working prototype.  Before I go another round with it, I'd like some input from those of you that might want one and possibly some advice on sourcing some better parts.  The current version v1 uses individual 0.56" 7-segment displays and 2x5mm individual LEDs.  This is not a drop-in replacement at the moment and it requires a bit of nibbling on the front housing assembly and it requires some soldering, bodging and IC replacement on the main board.  The reasons for the nibbling is that the digits are larger (wider mostly) and there isn't room for the annunciator LEDs without squishing them very tight.  As is is, the annunciators are going to be smaller than the original.

The first question is color.  I have 8 test boards since DK Red somehow doubled my 4-board order.  I have a red version, a blue version and enough parts for a green version, although I think the green will be too dim under the bezel.  I think the red one is the clearest and best looking, although unfortunately the photo seems blown out and doesn't show the nice deep red that is actually there.  The digits can be read clearly from 15-20 feet away.  The annunciators are another matter, and one possibility would be to use different colors so that once you learned them, you would know what mode the meter was in without actually needing to read the text of the annunciator.  Any thoughts on color combos?

The next question is what the priorities should be. 

Is avoiding any cutting worth reducing the display size further? 

Should I use smaller digits so as to make the annunciators larger? 

Do people want a lower-cost kit that can be customized for color or a high-quality ready to install product--or even the whole job done as a service? 

One issue is the half-digit and +/- sign which is a bit clunky right now.  I tried to find a "+1." or a "+1.8." display and matching "8.8." or 8.8.8.8." displays, but could not source them.  I found a listing, shown in a photo below.  My other options are to keep the design as is and perhaps tilt the "+" sign to match the digits and reduce the size with the mask that I haven't made yet, or I could actually saw one of the digits and remove about 40% of the left half and do a bit more nibbling and adjust the mask so that it looks good.  If I'm doing a one-or-two-off project that would be fine, but it's too much work for shipping product.  I'm not sure if a typical kit buyer wants to do that much work either, but that a question that can be answered here. 

As it stands, I can probably produce a kit that will work and look nice but will require complete disassembly of the meter, removing and soldering PDIP ICs, nipping leads and running two bodge wires, nibbling the front housing and perhaps sawing a digit in half as well as soldering the through-hole components onto the board.  I would sell it with the SMT stuff already on there.  Alternatively, I might be able to produce a more expensive kit that would require removing 3 ICs, soldering in a neat replacement module or modules and then installing a complete display module, possibly without any nibbling of the front housing. 

Comments welcome, even if only to let me know that you might want one.

[Black Phoenix]

My suggestion - Provide 3 stickers (or a full screen one with a middle insert that you can remove after application) to apply in the plastic front panel were the lettered options that were replaced by LED on the replacement screen. Not that who have one don't know what each means by practice but new users will not know. Also make sure that the stickers have a strong enough glue but not as strong that to remove it is a bitch and left residue behind.

Parts for this equipments start to get rare and expensive, so anything that looks like stock and can be removed without damaging anything is a god send.

[tautech]

Or you can 3D print the annunciators as Defpom has here:
https://youtu.be/mUhtBLLV6iA

In a later vid he got a dual extruder 3D printer and they came out quite good. 

[SilverSolder]

Really cool project!   Red is the way to go...

I almost feel like I should get one of these meters just for this cool display...

You could probably get a sticker made for the lettering, that would be awesome - perhaps black vinyl, with cutouts for the writing, and white vinyl behind the letters?

[bdunham7]

Or you can 3D print the annunciators as Defpom has here:

Yes, I fully intend to make a 3D printed part that will separate all the LEDs and then a printed or laser cut front that actually replicates the lettering of the original display annunciators--so no stickers or such on the outside.  I also need to manage the diffusion somehow so that they look as good as the digits do.  The only issue there is that they are going to be smaller than the originals just because there isn't enough space, so I thought color coding them might be nice.  The board is nowhere near perfect--I was tweaking it for a while then I decided to just send it out so that I could verify that it worked electrically and then solve the mechanical issues that arose.  It works--the LEDs are even and no ghosting, so one more round and it should be a product.

[lowimpedance]

 One other possibility would be to use any graphics program (COREL DRAW etc) to make an annunciators layout and print in negative to a laser film so black background with almost clear letters.
And then position on front bezel with cut out for the 7 seg. displays. The photo of my 8840 LED conversion I did many years ago is below. Sadly I cannot find the file I used for the print. Still room for improvement if I could be bothered !. I also did it the hard way on vector board and wire wrap !!, definitely should have used a PCB.....but for a 1 off I though it wasn't worth it.......wrong.
 The displays I used were HDSP-H150 series which have the +/-1 as well. I think the display would benefit from the integrated  +/-1 matching display, also that would give you more space for the annunciators.
Depending on which method you use for displaying the mode perhaps some black tubing around each close LED to reduce side glow showing on adjacent unlit annuciators.
 As for color all red is traditional, but perhaps if the +/-1 display is available in blue then perhaps have the main numeric blue and the annunciator red.........sacrilege i know 
Will be interesting to see what mods you have done to the cct to accommodate your display mod.

edit. I should also note that my front panel bezel was also smashed along with the VFD thus requiring a whole new bezel assembly idea.

[bdunham7]

The displays I used were HDSP-H150 series which have the +/-1 as well. I think the display would benefit from the integrated  +/-1 matching display, also that would give you more space for the annunciators.

I'm actually using (Broadcom) Avago HDSP AlGaAs displays which look very good, but I have been unable to source a "+/-1." type and that would be really, really helpful if I could.  Otherwise I'm looking at sawing a digit, because I think I want to go with the high-effort, high-quality option. 

[lowimpedance]

 Hmm just had a look and it seems the +/- 1 displays are not popular enough to stock anymore   . Only ones I can find are LUMIX branded at Mouser.
So yeah looks like plan B.

[robert.rozee]

a couple of ideas for showing +/- come to mind:

1. invert the vertical bars of the '+' to generate a '-' sign, and display nothing for '+'. you could then use an extra full digit to display the '-', which would then perfect match the rest of the digits.

2. as 1., but use a single display for the '1' and '-' with a bit of black paint on the right-end of the '-' to give a gap between it and the '1'.

you could also look at decoding the anode and grid drive signals using a micro, then 'recreate' the original display on a LCD matrix display. i believe there have been other threads discussing this (and other solutions) for the HP 34401A meters.


cheers,
rob   :-)

addendum, see:
https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/?all
and:
https://www.eevblog.com/forum/testgear/fluke-8840a-restoration/?all

[lowimpedance]

I forgot the 8840 restoration thread completely !. I recon the thin PCB text mask is the way to go to solve the annunciator and + -  display in one go.

[bdunham7]

Thanks for the thoughts.  I've seen the previous attempts to make replacement displays for these meters and I have two additional goals for the project.

The first is to keep it simple and in keeping with the original design--so no MCU/LCD type solutions.  The original VFD is driven by 3 NE594 drivers, all powered from the +30V regulated supply.  A VFD drives both the segments and the digit select lines positive to turn the display on.  Obviously an LED display needs one of those to be negative.  I figured out that I could  replace the digit select NE594 with a ULN2803A, which is pin-for-pin compatible but switches the outputs to ground instead of V+.  This worked on a breadboard and the remaining NE594 drivers are stout enough to drive the LEDs at 15mA (so far) but the current was too high for the +30V supply.  So the fix for that was to nip the V+ leads of the two NE594s and bodge them over to the pre-regulator side of the +5V supply, which is a bit over 10 volts and can supply plenty of current.  That's all so simple and works so well that I just can't see going for a more sophisticated approach.

The second was to make a display that is as good or better than the original VFD--and that's not easy because a good unfaded VFD on these looks fantastic.  There's no way to replicate that VFD color, so I tried some other colors.  I know a lot of people think the blue LEDs look good, but I find them difficult to focus on and bothersome to the eye in the long term.  I could not find decent green versions, so I got these AVAGO deep red models and they look great with better visibility than the original VFD, including the maximum viewing angle.  My remaining challenge is to get the rest of the display to look as good as the digits.  I want to avoid the 'light through a hole' appearance and any hint of bleed-through or shadows.  I'm going to use a laser-cut mask in a 1.0 or 1.5mm ABS black sheet, but what would really finish the job is if I could find a hot glue or resin or something that matched the milky-white appearance and properties of the material used in the digits themselves.

The prototypes function perfectly so if I can work out the appearance and layout issues, I'll make these if anyone wants them.  The only issue I see is that I don't think my version will be cheap, so I'm not sure if there is more demand for a thrifted version at a low price that works but just looks OK or for a great looking one that costs more.

[bdunham7]

So I drew up the annunciator mask and just printed it on paper with a laser printer for now to test it out.    One issue is that in order form me to get it lasered into a thin black plastic sheet, I have to use a line stencil font.  The same issue occurs if you use a PCB with text cut out.  I really would like to find a two-layer acrylic like they use for making placards where the laser or CNC just removes the top layer.  I would need black-on-translucent and I simply cannot find it anywhere.  I think it I'd be able to make better looking letters, but perhaps what I have is good enough?  Or is there some other process, like screen printing on a translucent material that might work for this?

[H713]

If you ever get this project finished, I think there are a lot of of us with 8840s / 8842s with dim VFDs who would love a replacement option.

[robert.rozee]

personally, i'd like to see a small adaptor board that plugs in between the main PCB and the front panel that handles the level translation without any need to modify/change any parts on the main PCB. this is simply because many owners likely do not have the skills necessary to carry out such modifications.

the display section looks extremely good, with the only real difficulty being the 'hand crafted' annunciators. my personal preference would be to:
- have a single 7-segment module dedicated to the '-' sign, and drive this with the inverted vertical bar of the '+'. this is far simpler than trying to build a '+' out of several separate LEDs.
- use DIP bar graphs (commonly used for VU meters) for the annunciators, arranged vertically, one at each end of the display window. the user can then choose to either just write the functions beside the bars externally, or produce a mask with the text labels to be fitted in front of each bar.


just my opinion, mind you!

cheers,
rob   :-)

[robert.rozee]

looks like someone on ebay is selling a complete solution, with no need to swap the drivers:

https://www.ebay.com/itm/154545708329
"LED Display Module FLUKE 8840A/42A (Diy Kit) replace broken, damaged, etc. VFD"

cheers,
rob   :-)

[bdunham7]

Yes, user AHOL from the Ukraine got a complete LED display custom-made and has released a viable replacement unit at a price I don't want to compete with.  The display is blue and isn't as large or bright as mine would have been, but it is available now and can be installed without any mainboard modification.  I can confirm that they work and look pretty decent.  The custom LED assembly is finished off beautifully, better than anything I've been able to make so far, even after I discovered laser-etchable black-on-clear acrylic.  He had a first version that didn't work as advertised, the second should be OK.  See this thread:

https://www.eevblog.com/forum/buysellwanted/ebay-led-display-module-fluke-8840a42a-(diy-kit)-replace-broken-damaged-vfd/msg3581681/#msg3581681

[AHOL]

Hello bdunham7!

And a special thank you for paying attention to the problem on time.

[Kjo]

Very appropriate to bump this topic with new developments.
Hats off to AHOL for continuing to supply blue LED modules under his conditions.

I have been reselling these modules in the USA for some time by prewiring the module
for quicker & more reliable assembly. I have now reproduced the Fluke display PCB
with ENIG (electroless nickel immersion gold) for reliable switch contacts and to produce
a complete replacement for the Fluke part number 728873. This makes display repair a purely mechanical
operation.

Currently on eBay https://www.ebay.com/itm/155620309426
soon on my website.

[nukie]

This is good. Im wonder why the designers didnt start off with LED displays in the first place then the future us don't need to suffer from these fading parts.

I am not old enough to know when the first led segment display was released but I know earlier dmm like the Keighley 196 sports the LED displays.

[Kjo]

VFD offer design flexibility not easily achievable with discrete LED modules. (Even today). I think they were also brighter and offered fixed mixed color with different phosphors historically.
Graphic LCD &  OLED can now offer similar design flexibility.
I also think they this particular Itron display had some manufacturing issues that made its reliability and lifetime bell curves much broader for some reason, leading to  fading not seen in other VFD displays. I have seen microwave ovens just as old as Fluke 8840A meters with VFD displays that are still working brightly. But I have many more 8840A than microwave ovens, so the data is poor.

[SilverSolder]

VFD offer design flexibility not easily achievable with discrete LED modules. (Even today). I think they were also brighter and offered fixed mixed color with different phosphors historically.
Graphic LCD &  OLED can now offer similar design flexibility.
I also think they this particular Itron display had some manufacturing issues that made its reliability and lifetime bell curves much broader for some reason, leading to  fading not seen in other VFD displays. I have seen microwave ovens just as old as Fluke 8840A meters with VFD displays that are still working brightly. But I have many more 8840A than microwave ovens, so the data is poor.

OLED has its own durability issues?  -  LED seems pretty reliable, I have some ancient HP units from back when the LED segments were individual dots...   they all still work, after ~50 years.

[Kjo]

OLED has its own durability issues?  -  LED seems pretty reliable, I have some ancient HP units from back when the LED segments were individual dots...   they all still work, after ~50 years.

I understand your point of view, but anecdotal experience is not useful without a boat load of data. I can counter your observation with real, modern 21st century product design using 0805 SMD discrete LEDs in user interface displays that burn out withing about 5 years. Electrolux refrigerator displays from 2012-2019 used discrete LEDs to form both 7 segment functions and text annunciation. All segments and text that is in static ON condition will eventually burn out. Segments and text that are not always ON will display full brightness. There is no repair for this failure other than replacement of the display module with a NOS unit. The design flaw is not that they used discrete LEDs, but that the bias selected caused premature failure that was likely understood at the time, but ignored.

Apple makes millions of iWatches with OLED displays. I doubt the failure rate of these displays is a more than a micro-percentage of the failure rate of Electrolux displays. My point is that in the field of amateur equipment repair, the choice and result of device replacement selection is far more variable than big time manufacturers efforts. And if they can screw up by design or by ignorance, amateur work cant expect to land with statistically better results.

kjo - KO3Y

[AVGresponding]

Premature failure in modern LEDs is as you pointed out, a deliberate design choice; they are overdriven. OLEDs on the other hand degrade even when off, though since it takes several years, smartwatches are likely to have been replaced by newer models before there's a significant quantity in the RMA chain.

In any case, VFDs and GDTs are where it's at. You can keep all your silly diodey stuff   

[SilverSolder]

OLED has its own durability issues?  -  LED seems pretty reliable, I have some ancient HP units from back when the LED segments were individual dots...   they all still work, after ~50 years.

I understand your point of view, but anecdotal experience is not useful without a boat load of data. I can counter your observation with real, modern 21st century product design using 0805 SMD discrete LEDs in user interface displays that burn out withing about 5 years. Electrolux refrigerator displays from 2012-2019 used discrete LEDs to form both 7 segment functions and text annunciation. All segments and text that is in static ON condition will eventually burn out. Segments and text that are not always ON will display full brightness. There is no repair for this failure other than replacement of the display module with a NOS unit. The design flaw is not that they used discrete LEDs, but that the bias selected caused premature failure that was likely understood at the time, but ignored.

Apple makes millions of iWatches with OLED displays. I doubt the failure rate of these displays is a more than a micro-percentage of the failure rate of Electrolux displays. My point is that in the field of amateur equipment repair, the choice and result of device replacement selection is far more variable than big time manufacturers efforts. And if they can screw up by design or by ignorance, amateur work cant expect to land with statistically better results.

kjo - KO3Y

That absolutely makes sense.   

We are rarely see "over-engineered" products like that light bulb that has been working for >100 years at some fire station in the US.  We are getting (too) good at targeting a specific design life, after which we are supposed to scrap/recycle the product and buy a new one... easy monthly payments!  Given this philosophy, is it any wonder that not much weight is given to longevity considerations?

But still...  VFDs are generally doomed to go dim over time, in my experience.  Which reminds me, I've got to fix the VFD in the microwave, it is now so dim I can hardly see it...