HP 3478A: Display upgrade
Many complain about the original LCD of the good old HP 3478A bench DMM and other HP equipment of this era, because of its rather poor readability. So did I.
Quite many projects have solved this issue using different approaches: From adding backlighting to the original LCD, using multiple OLED displays - one for each digit - to replacing the display with Nixie tubes. Despite some of the projects achieving great results I didn’t want to replicate any of those, often because of the complexity/quantity of parts to be soldered, aligned and so on.
Caution
The elevated voltages inside this DMM can cause damage, severe injury, and death. If you are not authorized or qualified to work on such equipment, do not do it. If you follow along, you do so at your own risk.
Then Ian Johnston (https://www.ianjohnston.com) came up with and shared a solution based on a blog post from eevblog user xi that works really, really well and is easy to replicate: A single high resolution TFT display with a dedicated LCD controller board, controlled by an stm32 blue pill dev board and mechanically and electrically “glued” together with a simple two layer board that uses the mounting solution of the original display. The new display itself is held in place by a 3D printed part. Cool!
I started with the .hex file Ian provided and it worked out of the box. There was a minor issue at least with my display, where a tiny line in the upper right corner was permanently lit. Not too big of a deal and I surely could have left it alone. But where is the fun in that?
Ian made the source code available, or to be more specific, the code for his 3457A variant of the display. I modified the code to be able to use my usual STM32 tooling and tweaked a couple of minor things like the annuciators.
And the mentioned issue with the tiny line is also gone.
By the way: Ian used the older version of the 3478A with the aluminum chassis for his development. The 3D part is not a perfect fit for the newer version shown here and seems to require/benefit from a couple of small modifications to the 3D model (or the plastic front panel alternatively).
