Video Blog 181 - Agilent 34401A Multimeter TFT Conversion

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Published: Friday, 24 April 2026

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DISCLAIMER: This video is educational only, so if you decide to do the same yourself then you are on your own, I can't be held responsible for any problems/issues/damage/injury that may occur if you decide to follow this blog and try it yourself.

VIDEO BLOG

Off the back of my other TFT upgrades some folks asked for the HP/Agilent/Keysight 34401A, and I just happened to have one that needed a better display since the VFD was quite worn.

Thanks to the person at the following GitHub address that had managed to decode the HP/Agilent/Keysight protocol ot set me on my way:
https://github.com/openscopeproject/HP34401a-OLED-FW/tree/master/src

Download the Pcb Gerbers, Hex file & Instructions/BOM here:
http://www.ianjohnston.com/misc/34401ATFTConversion.zip

I tested the suspect BluePill board and it looks like it is running the correct clock rates etc:
- SYSCLK = 72 MHz
- HCLK = 72 MHz
- PCLK1 = 36 MHz
- PCLK2 = 72 MHz
- clock source = 8 = PLLCLK
- HSE ready = non-zero
- PLL ready = non-zero
So, there is an alternative hex file in the download which prints the BluePill main loop rate to the TFT on booting the meter - 34401A_VS_Display_with_test.hex
The results speak for themselves, a huge difference in main loop speed:
- Good BluePill = 43596 loops/sec
- Bad BluePill = 849 loops/sec
A result of around 849 strongly suggests fake/clone silicon, flash wait-state problems, bus stalls, bad crystal stability etc.

The Visual Studio source code is here:
https://github.com/Ian-Johnston/34401A_VS_Display

YouTube Video: