In a recent post, I documented the replacement of a difficult to read VFD display with a modern TFT display in my Advantest R6581T. It was unequivocally worthwhile and has been working extremely well. Nowl, I am at it again, but his time the subjects are the HP 3478A 51/2 digits DMM and the HP 3457A 6/12 digit DMM. Both of these DMM’s use LCD readout displays, and while they are readable in good lighting, they have no backlight, are difficult to see from certain angles, and are fatiguing (for me) to look at. Given the results I see with the R6581T replacement, I had replicate this success with the other DMM’s.

I thank Ian Johnston who developed these replacements and generously made them available to the community. Here is Ian’s video of the 3478A Display Replacement and here is Ian’s video of the 3457A Display Replacement.

The process for replacement is as follows:

• Obtain the Gerber files, code, and build notes from Ian’s GitHub pages
• Have the PCB’s made
• Order the parts
• 3D print the TFT LCD holders
• Populate the PCB’s
• Program the STM32 Blue Pill
• Remove the old display
• Install the new display and holder
• Test and enjoy the new display

The process involves PCB ordering, populating, soldering, programming and assembling.

Here are the old LCD displays in situ:

The 3478A and the 3457A use the same display use simlar LCD displays and protocols. The new display PCB’s are therefore similar. They layout is slightly different in order to accommodate the ribbon connector.

The 3457A display is readily accessible and quite simple to remove:

I found that the 3457A has at least two main board part numbers which have very different part layouts and part designations: 03457-66501 and 03457-66511. The OEM display Vcc supply resistor on the older board (66501) was R540. This is the resistor that needs to be bypassed in order to supply the correct voltage to the STM32. On the newer main boards this resistor is R615.

The new display board installed into the 3457A. I had not bypassed R540 with a permanent jumper in this image.

The 3478A required much more disassembly than the 3457A in order to remove and replace the display:

There are at least two different revisions of the 3478A which have very different case and frame assemblies. Ian ran into this as well.

The RIFA AC Line filter capacitors were replaced. These had degraded significantly.

3478A Display Board Assembly:

And the final result captured in low light so that the TFT LCD display really stands out:

A couple of final notes: I had previously replaced the electrolytic capacitors and memory backup batteries in the 3457As. Therefore they did not require any additional service. However, the 3478As had both the original RIFA Filter capacitors and the original electrolytic capacitors. Since I was well into the disassembly of these units I replace the lot. I had replaced memory batteries previously.