Products > Test Equipment

Fluke 8840/42 Series Universal Replacement Display

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james_s:
It would be great if you could just trim to size the way you can with EL strips. Maybe some day.

Kjo:
Once I get some new colored bezels made I'll post what the finished meter looks like.

Kjo:
While I have had this display design working for some time, I have had an insidious, subtle and consistent problem. While the 2 meters I use as testbeds (8840A & 8842A) are fully functional and pass all self tests with both a VFD and the newer blue LED replacement display, I would get test failures with the OLED design. Specifically tests 4, 6, 7, 15 & 22(8842A). These typically point to the A/D which I know is fine!  |O

I at first suspected my level converter design was at fault for possibly overloading the +30V supply. But I found the errors remained after completely removing the level converters (no connection to the mainboard +30V grid and segment signals! They remained after removing the +5V from the OLED controller PCB. The only connection to the mainboard was the pass-through connections to operate the PB switches and the controller PCB ground to the "digital ground" by the power transformer.

Ok, maybe there is something obvious here, but it took some time for me to realize that there has to be a un-expected short between the analog ground and digital ground. When I removed the OLED controller ground from the mainboard the errors disappeared! But where was the short?

I went back over my design and the schematics of the Flukes looking carefully at the logic control and the VFD board wiring. On all of the schematic versions I have of the VFD, pin 16 of J203 on the display has a 7 in the grounding triangle. That is digital ground. But a careful look at the digital main PCA schematic shows J203 pin 16 with a 2 in the triangle! That is analog ground. A quick ohm meter check confirmed this.

What I had done is follow the VFD/PB schematic and on the pass-through wiring for J203 from the main PCA to my OLED PCB to the VFD/PB PCB I connected pin 16 to digital ground everywhere. Voila, analog to digital ground loop! I simply lifted the pin 16 circuit completely, since the VGD PCB ground plane is rather un-necessary now I think, and all tests pass!

The moral of this story is 1) these old hand-written schematics are not always correct, 2) analog and digital grounding issues can be subtle. The impedance and resistance between them may be small, but the currents can play havoc with such a sensitive meter.

kjo - KO3Y

Kjo:
Here is a photo of my replacement display along side a couple of pretty good OEM VFD displays.
All three have the OEM filter screen. The OLED display on top is using green OLEDs. I have a
few others using blue, white & yellow that I am experimenting with.

The controller behind the display has quite a few user programmable options as well as a demo test mode.
Kind of expensive to build compared to the AHOL blue LED replacement, but you
do get much better annunciators and units!

Kjo:
The 8840A, 8842A & 8840A/AF are about 40 years old in 2022. What a great design to be still in demand after all that time.
But the VFD selected had not held up to the ages as well as the rest of the design. This thread is about the replacement
design I have been working on for quite some time. It cant compete on price with the likes of the AHOL Ukrainian LED design that was
available for a while, but it is a whole lot smarter!

In addition to decoding the grid and segment signals, I have added a feature function mode that was missing from the original design.
Because the controller knows exactly what units, mode, and scale the meter is set to, I have added a continuity test mode
that drives a small PCB mounted sounder. By selecting OHMS, M or F rate and 200 ohms range the function will activate
the sounder if the resistance is below 20 ohms. (the "S" rate has an unacceptable delay caused by the ADC sampling algorithm.)

Further, the continuity mode can be toggled ON or OFF by activation the "AUTO" and "EX TRIG" functions. (See photos upper right)
Also, the "B" indicates that the currently loaded firmware is using a bootloader. If standalone firmware is loaded the "B" becomes a "V".

Kevin KO3Y

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