Valhalla 2724A Resistance Standard VFD to LED Display Conversion

[TERRA Operative]

So I recently picked up a Valhalla Scientific 2724A Programmable Resistance Standard and found the display was a little dim as to be expected for an old blue VFD.

So, here's my preliminary attempt at making a replacement LED display...


I'm designing this as a direct drop in replacement, I'm hoping due to the multiplexed nature of the display (one LED on at a time) that I won't need to add any extra power supply as I can take 5V from the logic supply to the original PCB.
As I was unable to find any 14-segment LED display modules at the correct size, I opted to use 0402 LED's of a similar wavelength to the original VFD populated on the PCB, with a 3D printed frame to go on top to produce the digits.
I am using ULN2803 and HT62783 transistor arrays to interface from the +30 and -10 volts for the VFD to something the LED's can handle.

I've never interfaced LED's to work in place of a VFD, so anyone who has more experience please take a look and tell me what I got wrong!


I also need to do a test run on 3D printing the frame for the LED segments, some of the walls between the segments are just over 0.2mm, so we'll have to see if JLCPCB can 3D print that in resin successfully....


I've attached schematics and board layout below, once it's working, I plan to release the Gerbers and BOM etc for others to use (and I plan do a small run of boards to offer to 2724A owners here who are interested).

[sean0118]

This is a very cool project idea, thanks for sharing the schematic 

[squadchannel]

some of the walls between the segments are just over 0.2mm, so we'll have to see if JLCPCB can 3D print that in resin successfully....

You should increase it to 0.25mm.
at 0.2mm, failed with most resins.
Also, you’ll get a confirmation email from JLC, so reply with "manufacture as is."
https://x.com/squad_3476/status/1884902534533525799

test piece starts at 0.2mm. I was testing them with the resins available at the time (early 2025). that worked well was "Black Resin" at 0.25mm.