Excellent PLC
Author: Lucy Grant – Industrial Automation Blogger, likes turbines and coffee
So I was at a medium-sized hydro plant last week, checking on some older Bently Nevada units, when I stumbled across something… unusual. One of the 120M8155-01 touchscreen displays looked like it had been through a color filter from Photoshop. Whites were beige, blues were greenish, and alarm banners looked slightly “off.” Operators were confused but the system still worked—data updates were normal, buttons responded correctly.
What Happened?
The weird colors didn’t appear suddenly—it was gradual. At first, only whites looked slightly yellow. Over a few months, the tint increased.
After checking the device, I confirmed:
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The LCD image itself was fine; no dead pixels, no flicker
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Touch inputs worked perfectly
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Color shift worsened when cabinet temperature climbed above 45°C
My Diagnosis Process
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Visual inspection: Nothing obvious at first glance.
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Power check: 24VDC stable, no brownouts.
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Thermal observation: The backlight diffuser panel had noticeable heat aging, especially near LEDs.
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Test on bench: Removed display and powered it up in a cooler lab. Colors returned to normal.
Bingo. The culprit? Backlight diffuser aging due to heat exposure. This little plastic sheet had yellowed over time, tinting the whole display.
Fix Options
Depending on budget and time:
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Replace entire HMI panel: Fastest, but expensive.
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Replace diffuser / backlight kit: Slightly cheaper, needs careful disassembly.
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Add cooling + live with it: If you don’t mind the yellow tint and operators can still read data.
At this plant, they opted to replace the module since the maintenance window aligned perfectly.
Lessons Learned
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Keep panels away from direct heat sources and high ambient temperature.
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Even industrial HMIs suffer from long-term heat aging.
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Check your backlight diffuser if colors start drifting; it’s rarely a firmware issue.
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Simple monitoring of cabinet temperature can prevent long-term discoloration.
Honestly, it’s not a common failure, but when it happens, operators panic. A few adjustments and you avoid unnecessary replacements.
— Lucy