Author Topic: Fluke 8840A restoration  (Read 1367 times)

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Offline bartekM

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Fluke 8840A restoration
« on: October 21, 2020, 08:59:02 pm »
Hello everyone! That will be my first topic here :)

Few months ago I bought a broken Fluke 8840A in good (visual) condition for ~$50. Internal condition was unknown at the time. So... disassembly time! First impression was great but after some time I realised that the construction of 8840A is a nightmare. In order to remove the display and keypad I had to remove every PCB and cable...


1) Main board looks OK. No damaged capacitor, no visible short circuits, all ICs in place
2) TrueRMS & IEEE488 addons in great condition
3) Testpoints voltage in spec
4) VFD display... broken

After short research I have found Chinese project with LED replacement for VFD - http://bbs.38hot.net/forum.php?mod=viewthread&tid=828474. Unfortunately there was no schematic or gerber files. In addition I didn't like the way they changed display layout. The decision was made to build my own LED display!
Requirements:
-Only discrete components allowed - not ICs
-Mimic the look of the original VFD display
-Low cost (as always)

Original driver outputs 0-30V signals that drive VFD anodes. High signal (30V) is used to drive both digit selector and segments. I used that fact in my advantage and with 1:2 resistor dividers NMOSFETs act as high and low switches for LEDs.
DC powers supply is generated by rectifier + capacitor powered from cathode heating voltage (~5V AC).

In version 1 I accidentally mirrored one 4 digit LED and half of the transistor but after many wire fixes it worked. That allowed me to perform an autotest on 8840A that succeeded without any error :)


Final version 2 got an additional PCB with “text mask” and blak 3d printed “light mask”. Thanks to that single LED lights up only one label text.


Finally I ended up with perfectly working Fluke 8840A for under $100. Now I need some references to make sure that calibration is not necessary. Only thing left to do is to replace old capacitors and find another broken device to repair.


[EDIT]
Gerbers added :)  Main board    Mask
BOM:
- All LED resistors near MOSFETs are 0402 10k
- All other resistors = 0402 47Ohm
- All MOSFETs = NMOS DMN6075S or similar
- C500 is used to lower rectifier ripples and anything at least 1000uF works
- D500, D502, D503, D504 - BAS3010B03WE6327HTSA1 - any SOD323 Schottky with at least 1A is OK
« Last Edit: October 21, 2020, 09:46:49 pm by bartekM »
 
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Offline Grandchuck

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Re: Fluke 8840A restoration
« Reply #1 on: October 21, 2020, 09:10:52 pm »
Welcome to the forum and nice work on the repair!
 

Online tggzzz

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Re: Fluke 8840A restoration
« Reply #2 on: October 21, 2020, 09:14:59 pm »
Welcome to the forum, and, I suspect, welcome to the voltnuts rathole :)
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline lowimpedance

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Re: Fluke 8840A restoration
« Reply #3 on: October 21, 2020, 11:46:15 pm »
Nice work and thanks for sharing, will come in handy for many others ,(including me), when their 8840's displays become unreadable ........and they will !.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline bartekM

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Re: Fluke 8840A restoration
« Reply #4 on: October 22, 2020, 12:52:35 pm »
Nice to hear that :)
Here you have schematic and board renders.
 
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Offline tomeo.gonzales

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Re: Fluke 8840A restoration
« Reply #5 on: October 22, 2020, 06:55:15 pm »
Nice work.
Do you intend to sell it as a kit? (PCB, components, 3D print mask)
I want to buy 2 kits if you want to sell.
Maybe with other LED colors? (green or blue probably will be closer to the original VFD)
In the PDF schematic the resistors to 7 segment displays are 33 Ohm and the resistors to the SMD LEDs are 47 Ohm
In your description all the resistors to LEDs are 47 Ohm
Can you please clarify this?
And please make the 3D print mask files available.
 

Offline bartekM

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Re: Fluke 8840A restoration
« Reply #6 on: October 22, 2020, 09:41:34 pm »
Never thought about selling it but if there is a demand there is a sale :)  Total parts cost (PCBs+components+3d print) is around $25 plus soldering time ~15$ plus shipping $??.
I used KW4-521 and KW2-521 available in red, yellow and green variant. Both 33R and 47R work great but 33R should be better with green LEDs. In fact it only depends on how bright you want it to be.

I forgot about one thing. My display is a little bit to wide and small cuts in front panel plastic have to be made. Fortunately plastic is soft like butter. After assembly everthing is hidden under front plexi (?).

Here you have STL file for the mask. I printed it with black PETG (PLA is ok), 0.2mm nozzle and 0.15mm layer heigh. Infill set to 0%, 4 perimeters, 0.25mm extrusion width.
« Last Edit: October 22, 2020, 09:45:54 pm by bartekM »
 

Offline tomeo.gonzales

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Re: Fluke 8840A restoration
« Reply #7 on: October 22, 2020, 11:23:08 pm »
I prefer green.
It is possible to sell me to unsoldered kits? (50$)
I have plenty of time for soldering   :)
What are the shipping costs for Romania?
Do you accept PayPal?
If yes, please PM your PayPal details.
 

Offline bikeNomad

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Re: Fluke 8840A restoration
« Reply #8 on: October 24, 2020, 09:06:12 pm »
Nice work! I'm just finishing up my own Fluke 884x display using LEDs; I went a different direction. I didn't use any transistors, just a microcontroller driving the LEDs directly (not even any external current limiting resistors on the LEDs or resistor dividers on the 30V signals). I have an adjustable power supply to adjust brightness (the micro can work up to 5.5V).

I cut masks on my laser cutter out of 6mm acrylic.

I used white LEDs that are bright enough to allow for a colored plastic sheet in between the mask and the diffuser.

Today I'm experimenting with various materials for the diffuser.

Also, using a microcontroller will let me add a continuity buzzer (by interpreting the display segments).

The attached photo is what it looks like using a diffuser made from inkjet-printed copier paper; I want to find something better.
« Last Edit: October 24, 2020, 10:22:33 pm by bikeNomad »
I'm an autodidact who believes in Sturgeon's Law and wants to continue contributing to the creation and improvement of the other 10% of everything.
 
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Offline nukie

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Re: Fluke 8840A restoration
« Reply #9 on: October 25, 2020, 03:56:19 am »
Very nice project but Im worried about the old chips in there losing memory...
 

Online Noy

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Re: Fluke 8840A restoration
« Reply #10 on: October 25, 2020, 08:08:17 am »
Thats not a big problem. Some people (me included) saved the Rom content fron different versions. As long as no special fluke part or the very rare CPU is broken you can reflash the Roms. The bin files are mostly uploaded to the one special website (i forgot the name :-D)
Also recaping is normaly not needed. My device is from 1998 (i think) or 1989??
And it was 2015 last calibrated and everything in spec. So i didn`t recap it and everthing was looking fine as i read the roms this year.
Also new (nos) roms are available from eBay for 5-10€ each.

With a TL866 you can also read al roms from your device if its version is not already present in this website..
 

Online Noy

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Re: Fluke 8840A restoration
« Reply #11 on: October 25, 2020, 08:20:52 am »
Btw great project like it more than the original VFD... :-)
And i think not using a micro + dcdc converter is the better way to not disturb measurings with switching noise and other stuff.
Maybe somebody will rework the PCB to get it into the original size to not butcher the case and to get a 1:1 replacement part.
It looks like there is plenty of free unused space. Currently my VFD is still in good condition so no need currently but if i need it someday i will build your version :-)
Fluke with keithley style display (best parts from both worlds ;-))
 

Offline bikeNomad

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Re: Fluke 8840A restoration
« Reply #12 on: October 25, 2020, 05:02:04 pm »
I agree. I think that by using a linear regulator to supply power and having the microcontroller sleep as much as possible (using a hardware timer to wake the program in order to read the next group's segment data) that my board will be quiet; I'm interested in seeing whether the noise is a problem. I can compare readings taken with the display processor running with those taken when the processor is held in reset (so no switching at all).
I'm an autodidact who believes in Sturgeon's Law and wants to continue contributing to the creation and improvement of the other 10% of everything.
 

Offline bartekM

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Re: Fluke 8840A restoration
« Reply #13 on: October 25, 2020, 11:11:05 pm »
I was gone for the weekend and the discussion developed. Nice!
Great work bikeNomad !!! I thought about that approach at the beginning but I am lazy and if I don't have to write a code I don't :) In fact you can use your uC to decode display readings into RS232/UART strings and send it to PC. Just saying.

I wanted my display to be readable from a distance so I've made it as big as possible and overdo a little bit. With KW4-401 display (instead of KW4-521) I could make it smaller and no cuts will be necessary. Interested?
 

Offline bikeNomad

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Re: Fluke 8840A restoration
« Reply #14 on: October 26, 2020, 12:23:47 am »
Thanks!

I do have four microcontroller I/O pins (including a UART) going to the programming/debug connector.

I like the bigger digits on yours but wanted to keep more or less the same look as the original.

I've already written the code to recognize the digits being displayed (needed for my continuity buzzer idea) and thought about sending out the values on the UART but realized that I could also use the GPIB interface, since I have a USB-GPIB adapter.

As for coding, I wanted to try a new project with a new enough compiler that I could use the new(er) C++11/14/17 features.

The GCC Arm compiler did a great job in release mode, spitting out a binary of about 4KiB.

I have yet to put the processor to sleep, and I'm going to have to solder a wire on the RESET pin (which I forgot to connect to anything). The next revision will bring this out to a test pad.

That way I can test for noise contributions with and without the display running (using the GPIB interface to read the values).

Here's a photo of my display (on the top) and a decently bright Fluke display on the bottom. The colors are a bit off; my display isn't quite that blueish. But it is adequately bright (better than the original display in this particular meter by quite a bit).
I'm an autodidact who believes in Sturgeon's Law and wants to continue contributing to the creation and improvement of the other 10% of everything.
 

Online robert.rozee

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Re: Fluke 8840A restoration
« Reply #15 on: October 26, 2020, 12:36:17 pm »
Never thought about selling it but if there is a demand there is a sale :)  Total parts cost (PCBs+components+3d print) is around $25 plus soldering time

very nice piece of design    :-+

i have an 8842 myself, thankfully the VFD seems to be in pretty good condition with good brightness.

might i suggest a few changes in order to make this a project easier for others to build:

1. use larger resistors, 0805 is a good size to make soldering easier for those not used to SMD parts. it looks like your current PCB layout uses 0402 or 0603 size resistors.

2. if possible, make the PCB no wider than 100mm. this allows for much cheaper PCB fabrication in china.

with the above two changes, you can just release the gerber files and folks can then get their own PCBs made.


cheers,
rob   :-)
 

Offline bikeNomad

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Re: Fluke 8840A restoration
« Reply #16 on: October 26, 2020, 02:49:32 pm »
I did get the boards made and (mostly) assembled in China (at JLC PCB), making sure especially that the LEDs were in their 'Basic' inventory. There was no way I was going to hand-solder those LEDs. They had no additional charge for the 130mm wide board. They attached 71 LEDs, a voltage regulator and a couple of diodes and caps, and a handful of resistors. The boards arrived a week after I ordered them.

The only assembly I have to do is to solder the microcontroller, electrolytic capacitor, trimmer resistor, pin headers and programming connector.

I'd be willing to do an order of 30 boards and offer a kit with the board, remaining parts, laser-cut mask, diffuser material and printed words for $30 for the people here in the forums if there's enough interest. If I have to solder the microcontroller on it might be somewhat more. I'm not sure how best to go about programming the microcontroller off-board, since I don't have an appropriate ZIF socket.

I still would like to get a single sheet of thin translucent stock printed for the words, since I'm not yet happy with the results from either my label printer, laser printer, or inkjet printer.

I'll also be documenting the details of how I made the board, since I think the level conversion and LED driving methods I used may be unfamiliar to some people.

I'm an autodidact who believes in Sturgeon's Law and wants to continue contributing to the creation and improvement of the other 10% of everything.
 
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Offline bartekM

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Re: Fluke 8840A restoration
« Reply #17 on: October 26, 2020, 03:31:59 pm »
Quote
1. use larger resistors, 0805 is a good size to make soldering easier for those not used to SMD parts. it looks like your current PCB layout uses 0402 or 0603 size resistors.
Yes, I used 0402 resistor. I can change LED resistor from 0402 to 0805 without problem. Dividers for Mosfets are more tricky but I can try.

Quote
2. if possible, make the PCB no wider than 100mm. this allows for much cheaper PCB fabrication in china.
With smaller 7 segment displays it should be possible to fit in 100mm.

So many request so little time... I will let you know if I was able to implement all or most of your requests :)
 

Online Noy

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Re: Fluke 8840A restoration
« Reply #18 on: October 26, 2020, 04:28:59 pm »
Or publish your project. Best in kicad. Then everyone can modify how he / she likes it.
 

Offline Rollin Hand

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Re: Fluke 8840A restoration
« Reply #19 on: October 26, 2020, 07:10:42 pm »
I m interested in the kit keep me posted at rubyg@juno.com
 

Offline bartekM

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Re: Fluke 8840A restoration
« Reply #20 on: October 28, 2020, 04:03:11 pm »
Quote
Or publish your project. Best in kicad. Then everyone can modify how he / she likes it.
Can't do. Project was made in Altium and kicad is not my friend :)

Based on your requests I've made few changes:
1) KW4-521 and KW2-521 LED displays was replaced by single digit OSK1039A:
   *smaller digits 13.2mm vs 10mm
   *blue light
   *darker (probably)
2) thank to smaller displays not a single cut is needed!
3) PCB is still larger than 100mm because of the pins/connector width
4) new PCB will have black soldermask

Trace routing is still in progress. Give me some time for that. If anyone want previous version with bigger displays I still have 3 PCB sets for that. If you want new version you have to wait few weeks.

I can tell you that developing this VFD replacement was a great fun! There is nothing better than bringing high class metrology equipment back to life. After this project I will definitely start looking for a new patient.
 
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Offline tomeo.gonzales

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Re: Fluke 8840A restoration
« Reply #21 on: October 28, 2020, 10:06:29 pm »
I'll wait for the new kit without any cuts needed. Please count on me for two kits.
 

Online Noy

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Re: Fluke 8840A restoration
« Reply #22 on: October 28, 2020, 10:18:13 pm »
Me too, waiting for the new PCB. Maybe a kit (price depending) to save it for the time my vfd will break off.
 

Offline giosif

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Re: Fluke 8840A restoration
« Reply #23 on: October 28, 2020, 10:21:47 pm »
I too am interested in one, possibly two, such kits (depending on the price).
 

Offline martinr33

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Re: Fluke 8840A restoration
« Reply #24 on: October 29, 2020, 03:58:31 am »
There's an acrylic material described as "transparent black". It has a black appearance, but actually is transparent. Needs a brighter display to use it, but the contrast is superb.
 


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