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Never asked on stack Exchange before, and only have very basic knowledge in electronics (mechanical engineer by trade).

Can anyone suggest if the LCD on this vernier caliper is repairable? One column of the display isn't working. So for example in the photo, it should display 81.28 but instead shows 31.28. It's my favorite caliper so would love to be able to repair it.

I have disassembled the circuit board etc and cleaned all contacts with switch cleaner and a cotton bud. The circuit board clamps against an elastomeric connector on the LCD. There was some dirt in here when I first disassembled the board. Cleaned everything, reassembled but it's the same. One thing I notice is that the elastometric connector is partially disbonded from the glass? of the LCD. Can't imagine that it's supposed to be like that.

Anyone got any suggestions?

Thanks in advance. enter image description here

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    \$\begingroup\$ The elastomer is not normally bonded to the glass, other than by accumulated gunge. Remove (carefully!) and clean both with alcohol, then reassemble. Good luck. \$\endgroup\$ – Brian Drummond Jan 18 at 12:38
  • \$\begingroup\$ I've been using these same calipers my whole engineering career. I hope you can fix it. \$\endgroup\$ – Drew Jan 18 at 21:45
  • \$\begingroup\$ Flipping the zebra strip (swapping long ends) would be diagnostic. \$\endgroup\$ – amI Jan 19 at 3:27
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enter image description here

Figure 1. An LCD connector. Units are cm. Look closely for the stripes in the middle layer. Source: Elastomeric Connector.

The elastomer strip is known as a "zebra strip". It has a sandwich construction with insulating bread and filling consisting of alternating strips of insulation and conductive plastic - probably a carbon filling. Usually the strips are much narrower than the PCB contacts so that there is no danger of the conductive strips shorting adjacent PCB pads while guaranteeing that at least one strip will actually contact each pad.

One thing I notice is that the elastometric connector is partially disbonded from the glass?

I've never seen them bonded. They're usually just slightly compressed between the LCD and the PCB so it should come off with a gentle bending sideways. Try cleaning it with whatever contact cleaner or alcohol you have and assemble carefully ensuring that the LCD and PCB are vertically aligned.

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The failure could be the driver, Zebra Pad or CMOS input pin.

You would need to identify which of the 3 areas was the fault in order to see if it was repairable with the Pad being the only repairable option.

A typical Zebra Elastomer may be chosen from 150 to 500 conductors per inch. A typical gold plated conductor strip may be 7 pads /cm or 17.5 pads /" with up to 8 pads mating with conductors leaving a SNR of 7:1. If elastomer carbon resistance varies 50% this reduces to 3.5:1 SNR. CMOS threshold is know to drift 33% reducing to margin to 0.5 to 1.

If you were able to bias the input to activate the LCD segment, then the fault might be corrected by bias. Otherwise defective CMOS input.

It can be debugged with scope and some idea which pins are involved. The output signal would look like a 2 or 3 bit quantized level after the Zebra pad.

Not necessarily repaired.

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