1
\$\begingroup\$

I pulled this multimeter (probably built in the 1980s) out of storage last week. About half of the display is unreadable:

enter image description here

The black patch simply stays when power is off:

enter image description here

This is a common problem for old multimeters, calculators, and watches.

From Wikipedia's description of these TN LCDs, it seems that the liquid crystal just can't fall into its twisting off state. Maybe air or moisture gets in there:

enter image description here

To be clear, the Wikipedia image above uses a backlight, but this multimeter avoids the backlight with a reflection at the top (so light passes through the twisting crystal twice.)

Is there anything I can do to reverse this damage?

I have seen some videos showing that you can "massage" these back to normal by applying pressure and rubbing the screen over many hours (maybe this pushes the air/moisture out the edge,) but I hope there is a more reliable trick.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ Was something pressing on it for a long time? In my experience, this type of damage is permanent. But the LCD itself might still be available, or could try to find another unit for parts. \$\endgroup\$
    – rdtsc
    Commented Oct 21, 2022 at 18:49
  • \$\begingroup\$ @rdtsc I don't think anything was pressing on it. I also don't think that this multimeter was ever dropped, or hit by anything. It's surprising to me that the patch has such a sharp boundary (i.e., there is no band that appears only partly darkened on the boundary) in case that is a clue. I do wonder if this patch grew slowly over many years, or just appeared in full one day. \$\endgroup\$
    – bobuhito
    Commented Oct 21, 2022 at 19:00
  • \$\begingroup\$ it may be an air bubble ... if it is, then a vacuum chamber may be of use \$\endgroup\$
    – jsotola
    Commented Oct 21, 2022 at 19:30

1 Answer 1

1
\$\begingroup\$

Not an answer, just more observations from an old LCD having a very similar fault. At left is LCD with one digit driven with AC signal. At right, that digit is idle - not driven.

Note the perimeter black border - this is where the top polarizer is bonded to the top-layer glass. Judged by eye, this border has a very similar optical black colour and abrupt edge property as the spotty fault region.

Note that the digit LCD region is still operating as it should - this may not be apparent from these photos...black on black is hard to distinguish. It would seem possible that the border attachment "glue" has migrated into the viewing area.
Possibly seeped between glass layers or wicked between polarizer and glass?

No amount of flexing affects the black fault area - it seems stable. I believe its fault area has grown slowly over many years. similar LCD fault

\$\endgroup\$
3
  • \$\begingroup\$ Why is there a gap without black at the bottom border of the image? \$\endgroup\$
    – bobuhito
    Commented Oct 22, 2022 at 0:31
  • \$\begingroup\$ @bobuhito that short bottom-end is sealed with an epoxy-like substance. Could it be the entry/exit port for the nematic liquid? \$\endgroup\$
    – glen_geek
    Commented Oct 22, 2022 at 1:41
  • \$\begingroup\$ From this video and other pictures I've found on the internet, I believe this "abrupt fault area damage" does not always connect to the border. The simplest explanation, though just a guess, is that the nematic liquid slowly decomposes (all by itself or with the assistance of some contaminant), creating a second liquid that just doesn't mix well with the nematic liquid (the second liquid then slowly migrates/clumps to the front and back surfaces where it "blocks" the twisting alignment layers). \$\endgroup\$
    – bobuhito
    Commented Oct 22, 2022 at 16:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.