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"Vinegar syndrome" of LCD screens (and how to rescue)

·4 mins

Today I learned: “Vinegar syndrome” of LCD screens (and how to rescue)

Short background: LCDs are essentially composed of five layers - a middle liquid crystal layer, sandwiched between two glass plates, each with a linear polarizer film on the outside (polarizer-glass-crystal-glass-polarizer).

The polarizers are 90° to each other. Light entering from the back is polarized and naturally blocked by the front polarizer. Depending on voltage, the liquid crystal can reorient the light to match the polarization of the front filter, allowing the light to pass (white) or not (black)1.

With time (7-10 years), and depending on storage conditions, the cellulose acetate polarizer undergoes an organic reaction2 and turns cloudy and acidic, losing its polarizing properties. When peeled, it smells of vinegar - hence the name (one of the reaction’s products is acetic acid - a.k.a actual vinegar you use in the kitchen).

The screen will develop “white spots”, sometimes to a point where the screen is unreadable in affected areas.

Screen is barely readable…
Screen is barely readable...

The screen can be rescued by peeling the damaged polarizer, removing the glue with isopropyl alcohol, and attaching a new linear polarizer after carefully orienting it to match the original polarizer. Easier said than done - look up many YouTube videos on this fix for the specifics.

Back-side, after disassembly of the module. Looks like there might be a crack in the glass?
Back-side, after disassembly of the module. Looks like there might be a crack in the glass?
Scraping off the damaged polarizer was agonizing. You have to be firm to remove the glue, but very delicate not to scratch/crack the glass or damaging one of the delicate connections to the board.
Scraping off the damaged polarizer was agonizing. You have to be firm to remove the glue, but very delicate not to scratch/crack the glass or damaging one of the delicate connections to the board.
Finally! It wasn’t a scratch after all.
Finally! It wasn't a scratch after all.

How did I get here?

About a year ago I bought a second-hand oscilloscope3 with a small white blemish in the middle of the screen. It was a good deal due to the defect. With time, the blemish grew bigger and bigger, to a point where the waveform was barely visible.

I emailed the manufacturer an inquiry about a replacement LCD, and got an answer that they no longer offer maintenance services for that model. However, they did have a similar (not exact) part number that they can send me for $160 (+$60 in shipping) - but they don’t have the scope on-hand and cannot test the screen so I have to assume all risk. I decided to pass.

As I was unable to find details on the LCD module on the Internet, I emailed the LCD manufacturer to get the device’s datasheet, hoping I could find a cheap compatible module on AliExpress or something. Alas - this is an industrial module (320x240 5.7” RGB), and while AliExpress had a lot of monochrome modules with that spec for a reasonable price, the RGB ones were scarce, expensive, old, and with incompatible pinout which would require me to build an adapter.

I’ll skip the part where I considered a conversion to monochrome (the scope’s board had a B/W header, but I had to figure out the pinout), or connecting a small tablet to the USB port in host mode and use the scope’s display dump ability to show the waveform on the tablet (near real time, but not quite. Also proprietary protocol in need of reversing). I will also skip the idea of sampling the LCD timings with a Raspberry Pi and showing the image on a cheap, modern, consumer-grade LCD module. All of the above were too much work.

So I decided to try my luck with replacing the polarizer - even though the screen looked like it had a crack in it. It took almost two hours to carefully remove the damaged polarizer (the glue became brittle and horrible), but I was able to return the module to a working condition.

The sharp-eyed readers (that got this far) would notice the colors are now pretty washed. I probably used a low-quality polarizer which lets too much light pass. Also - there are some air bubbles trapped between the glass and the polarizer. Nevertheless, the screen is now readable, and this is good enough for my hobby-grade oscilloscope.

Total $$$ saved - about $100. Total time saved on shipping - 3-4 weeks. Time spent on this problem - about a week. What I learnt in the process - priceless 🙂

Much better now! Though the low-quality polarizer is showing. I might replace it some day.
Much better now! Though the low-quality polarizer is showing. I might replace it some day.

  1. LCDs are a complicated matter with many variations, so I’m keeping it simple, although inaccurate ↩︎

  2. Cellulose acetate film - Decay and the “vinegar_syndrome” on Wikipedia ↩︎

  3. 150MHz GW Instek GDS-820C ↩︎