I show you how to fix a digital caliper with a blank display. Thanks to Zack Sousa for writing an Instructible on the same topic. Zack figured out how to open the caliper up which was the essential first repair problem to solve.
Magnifier 5x or magnifying visor. You'll need one by the time you are 70 I assure you.
Tweezers, electronic type
#00 Jeweler's screwdriver
Strip of paper 3/8" x4 inches or 10mm x 100 mm. For checking sensor clearance.
Single edge razor blade
Cleaning solvent for rubber electronic keypads.
Step 1: Look for Scratch on the Caliper Bar. Picture Shows a Faint Scratch.
Before you take the caliper apart, look for long scratches on the caliper bar.
If you have made a paper strip, you can slide the strip inside the caliper to locate the debris.
Step 2: Remove the Back Label. Label Is Held on With Industrial Rubber Cement.
The back of the caliper is covered with an aluminum label.
I used a razor blade, tweezers and a few drops of WD-40 or mineral spirits.
I held the razor very flat and low to cut the adhesive without curling the label.
I used a few drops of something oily like WD-40 to prevent the rubber cement from re-bonding to itself.
Step 3: Remove 4 Machine Screws. Lift Caliper Bar Off. Find the Wipers.
Get ready to note how your caliper is assembled. There are two caliper bar wipers that you need to look at before they fall onto your work surface.
Lay the caliper on your work surface with the black or blue plastic measurement unit face down.
Remove the four screws. The screws are 2mm x 7.5 mm machine screws.
Hold the blue or black plastic measurement unit with your fingers.
Gently lift the metal parts of the caliper off of the mesurement unit.
Look at each end of the measurement unit and see a plastic wiper.
The wiper has a thick leg and a thin leg. The thick leg goes into the measurement unit and the thin leg lightly touches the caliper bar.
Step 4: Remove 4 Sheet Metal Flat Head Screws. Remove Circuit Board and Rubber Switch Layer.
The measurement unit is a sandwich. Circuit board, rubber switches, LCD (don't touch) and measurement housing.
Circuit board layer is first. The circuit board shows capacitive stripes and four attachment screws.
If you noted a caliper bar scratch in step 1, look for the cause of that scratch.
Note, two of the attachment screws MUST be below the circuit board or they will scratch the caliper bar.
Remove four flat head screws from the circuit board and lift the circuit board off.
Rubber switch layer is next. Note the black rubber LCD connector.
LCD display is below the rubber switches. Do not touch it. Or nudge it back in place if it moves.
Step 5: Clean Rubber Switch and Circuit Board. Assemble and Test Until It Works.
My measurement unit stopped working because I got motor oil between the rubber switch and the circuit board.
The problem is to gently clean and degrease the circuit board and the rubber switch.
Degrease switch area: I used an automotive electrical spray that contained tetrachloroethylene. It removed the oil but also caused the rubber switch to swell up. It is toxic too. I let the rubber switch dry overnight. I lubricated the holes in the measurement unit housing with cotton swab damp with WD-40 (a lubricant that evaporates).
Then I laid the rubber switch part in place, then tapped the rubber LCD connector in place, then I laid the circuit board in place.
Test measurement unit: After assembling the measurement unit, lay the measurement unit gently on the caliper bar and press the switches. It took me three rounds of fiddling to get all three rubber switches working as expected.
Check clearances: Use the piece of shim paper to prove to yourself that all the screws on the measurement unit are not touching the caliper bar.
Add wipers: Use the magnifying glass to confirm that the two rubber wipers are seated in the measurement unit. The big edge of the wiper fits into the slot in the measurement unit case. The tapered edge of the rubber wiper very lightly skims the caliper bar.
Attach measurement unit to caliper bar.
Final check: Paper test strip slides through the caliper bar. All three switches on the measurement unit work as expected.