One of the main complain when talking about the venerable HP3478A is the lack of back-light. Some are bothered and some are not, probably depending of individual lab lighting situation. I didn't find this to be a major issue. Nevertheless, I took the challenge to add the back-light to at least one of the two instruments that I have. From the very beginning the requirements were:
- Simple
- Everything must be reversible. No cuts, in the plastics, front panel only adding new stuff. The only accepted alteration is the removal of the reflective film from the back of the LCD.
- Must look (almost)professional with even brightness.
- Using real light guide, not sanded plastic or other improvised solution.
The bigest challenge was to find a real light guide. I removed one together with all the diffusing sheets from a cracked LCD monitor but it was 4mm thick, no chance to squeeze it inside the HP LCD. One day, my daughter's tablet (an Acer Iconia W510) died and I decided not to try repairing it. It was too slow. Before I said bye bye to it, I opened it and removed the entire LCD back-light. The light guide in this case was only 0.53mm thick and the LEDs were on a flexible PCB in series groups of four. Even this one together with the diffuser sheets wouldn't fit behind my LCD so I had to change the zebra connectors with thicker ones. The donor of the thicker connectors was a scrapped LCD module from work. Now I needed something to hold together the thicker sandwich. The solution came from a hobby store, a rectangular brass tube that I filed and made it a U bracket and covered it with Kapton tape. Removing the reflective tape from the back of the LCD module was somewhat challenging, the glue was left behind but with a bit of IPA and some patience it came out nice.
Materials:
- Light guide, LED strip and difuser sheets from an old tablet.
- Very thin double side adhesive tape.
- K&S #8268 Rectangular Brass Tube.
- Kapton tape.
- Thicker zebra silicone connectors from an old LCD module.
- Breadboard and components to make a current controller.
- Aluminum adhesive tape.
Process:
- Remove the LCD from the meter and open it.
- Remove the silver film from the back of the LCD and clean the glue with IPA.
- Cut a piece from the LED strip with four LEDs.
- Solder wires to the ends of each group of two LEDs and connect the LEDs inside the group.
- Cut the light guide and the diffusing sheets to fit in the black support plastic behind the LCD.
- Glue the LED strip to the edge of the light guide.
- File two corners of the rectangular brass tube and remove one side. File and sand the edges.
- Cover the resulting brackets with Kapton tape. Mine needed two layers of tape to create enough pressure on the zebra connectors.
- Cut the zebra connectors to the same length as the originals.
- Place a piece of aluminum tape on top of the LEDs to reduce light bleeding.
- Assemble everything together and put the assembly back in the meter.
- Make the current regulator and solder the wires.
- Attach the current regulator to the side of the meter and solder the input wires.
Enjoy...
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