Upgrading the HP3478A with a display backlight
The HP3478A is a great multimeter which you can get for a reasonable amount of money. It’s got good overall performance but is a bummer when it comes to working with it in low light conditions. So I decided to upgrade mine with a backlight display. Retrofitting older technology is not always trivial but with today’s technology we have many different options so let’s see what we can do with this “oldie”!
What you will need:
1. 3D printer or access to a 3D print shop.
2. Torx T7 screwdriver
3. Torx T10 screwdriver
4. Philips PH2 Screwdriver
5. 200 Ohm multi-turn potentiometer
6. Instant glue.
7. 50 cm of Black & 50 cm of Red wire
8. 5 cm length of 3mm heat shrink tubing
9. A Dremel tool
10. Custom backlight panel 140mm x 25mm x 2.4mm (I had mine custom made by a factory – more on this later).
11. Anti-static mat (not mandatory but highly recommended to protect your instrument)
12. Patience (don’t rush it)
What you need to do:
1. For those of you who know how to pull out the board from the chassis, jump to step 21.
2. To open chassis. Put the multimeter face down on you work surface with the top facing away from you.
3. Loosen the two captured Philips screws and remove the T10 screw on the underside.
You should now be able to remove the cover (P001).
4. Put the unit back on the work surface.
5. Remove T10 screw holding the electrical ground wire ) on the left side (Green/Yellow and the common chassis ground on the right side (Brown wire).
6. Disconnect HP-IB, Ext. Trigger and VM Complete wires from the board.
7. Disconnect rear terminal posts (note the colour coding).
8. Remove T10 screw holding the 5V regulator on the common chassis side (same side as the transformer).
9. Remove the two bolts holding the transformer to the chassis frame.
10. Loosen the two T7 screws holding the power switch to the frame. Slide the switch towards the top of the slots. That should leave enough room to¬¬ disconnect the pushbutton arm from the switch (P002). WARNING: Be careful, after 30 years, the plastic may have become fragile.
11. Push in the two rubber clips holding the wires going to the rear terminals.
12. With a thin hard blade, pry the plastic bezel up until you can release the two notches holding the front panel. Same for the two bottom notches (P003). Once released, you should now be able to slowly slide the board towards the front of the unit. I found that pushing on the transformer (again towards the front) was the easiest. The board should now slide outwards. You need to push it out initially by about 4-5 cm.
13. While pushing out the board, you will notice that ground wires will be holding you back, just slide them through their respective holes as you continue to push the board outwards.
14. You should now have the board completely out of the chassis.
15. CAUTION: The rear terminal wires are now floating above the board (P004). Since we will need to power up the unit for testing at various stages of this upgrade, you must make sure the wires do not touch the circuit board otherwise you might destroy your meter!
16. Disconnect the front terminal wires (Orange/Grey, Red/Black, Yellow and Brown) from the main board (P005).
17. Position the board so that it rests on its left side (transformer sitting on your work surface and slightly hanging over the edge of your work surface (P006).
18. Remove the two T7 screws holding the front panel to the main board (underside).
19. Return the board to the horizontal position and lay the front panel flat in front of it.
20. Remove the two Philips screws holding the display module (P007). Pull the module out and move the rest of the front panel assembly to the side.
21. The display assembly is a “sandwich” made up of the LCD module, the mounting bracket and the Display controller PCB. To disassemble it, you must pull the two black plastic “U” channel away from the module (P008). WARNING: The display controller is static sensitive.
22. With the display module on the work surface, lift the display controller (P009), then the black mounting bracket (P010).
23. Carefully remove the reflective foils from the LCD module (P011). (WARNING: glue is quite strong, resists even Acetone). To remove the glue, I pressed on the glue a piece of electric tape and ripped out the glue in multiple passes. Be patient, eventually all glue will be removed.
24. The connector on the Display PCB actually protrudes through the board. We will need to push that out and make it flush. Since the connector pins won’t go far enough to provide physical restraint, we will now need to solder the contacts in place (P012, P013).
25. Next we need to make a small groove in the display bezel to allow for the light panel LEDs to be connected (P014, P015).
26. Re-assemble the display module: lay the backlight module first between the rows of connectors. Align the controller PCB to the LCD display (matching corners). The backlight module should be centered (about 5mm of the panel will protrude on each side (P016). Take the sandwich between your for finger and thumb and gently squeeze the sandwich. This will allow you to slide the “U” channels back into place (again centered).
27. Print the retaining bracket (I used white PLA that I had no hand).
28. To install the bracket, first bend the LED leads so that the left side will be going around the bracket post while the right side LED leads will simply be bent upwards (P017). Slide back into the bezel (don’t forget to put the clear plastic lense back in before the display module).
29. Mark the polarity of LEDs on the 3D printed bracket.
30. I added a potentiometer on a perfboard (P018) so that I could adjust the brightness but that is optional. A fixed resistor of 270 Ohm would also work well (with a +5VDC source). I diverted the needed power (45mA) from the digital section regulator (P019).
31. Re-assembled (P021).
32. I ordered custom made panels from Shenzhen Unionleds Photoelectric Co.,Ltd. Because I had to order a minimum quantity, I still have a few spare ones (different colours). If anyone is interested, pm me. Panels cost around 12USD each. P022_Amber, P022_Blue, P022_Green and P022_White (Sorry no white left).
Here’s mine in operation in P023
Happy Moding!
YadiYada
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