Excellent PLC
Posted by: Chris “CJ” Montrose – freelance maintenance tech, mostly living out of tool bags and airport lounges.
I’m currently working a short contract at a refinery on the coast, and I ran into a weird one with a Bently Nevada 120M8155-01 display. I figured I’d write about it while I wait for my laundry to finish spinning at the hotel.
Here’s the situation: the HMI looked like someone put a yellow filter over it. No kidding — whites became beige, blues became greenish, and the alarm banner looked like it had a nicotine habit.
Funny part? Everything still worked. The operators could still poke around the menus and see vibration numbers. It just looked… wrong.
Symptoms in Plain English
This is what we saw:
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Overall yellow/golden tint on the screen
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Low contrast (dark areas not dark enough)
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Slight color shift depending on viewing angle
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Completely stable image, no flicker or tearing
If the panel was dying, I would expect flickering or horizontal lines, but nope — just color weirdness.
My First Two Suspects
Whenever I see odd colors on LCD panels, I mentally split the issue into two buckets:
Bucket 1 — Display Electronics (driver, TCON, cabling)
Bucket 2 — Backlight (LED strips, diffusers, aging)
Yellowing is usually Bucket 2, so I started there.
Tests and Clues
Here’s how it played out:
✔ I shined a bright white LED flashlight across the surface — the white reflected fine.
✔ I opened the enclosure and checked internal temp — around 45°C, which is high but not crazy.
✔ I noticed faint discoloration around the diffuser panel area (behind the LCD).
✔ Supply voltage was good, stable at 24.0V DC.
✔ No graphics corruption — UI icons looked crisp.
This basically ruled out TCON failure, GPU issues, and cable faults.
The Real Culprit: Backlight Diffuser Aging
After popping the panel open (carefully, because those ribbon cables are sneaky), I found the expected answer:
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The LED backlight was still bright, but the diffuser film had yellowed from heat exposure.
These Bently displays live in cabinets that cook all day. The diffuser plastic doesn’t like long-term heat. It slowly tints, making the panel look like a smoker’s teeth.
Fix Options (Realistic Ones)
There are three ways you can deal with this, depending on budget and availability:
Option A — Replace the entire display module
Fastest, costs more, no messing around.
Option B — Replace the diffuser film/backlight kit
Cheaper, but availability varies and requires delicate disassembly.
Option C — Add cabinet cooling and live with it
If the client doesn’t care how it looks as long as data shows up, this works.
At this refinery, the supervisor requested Option A because shutdown was coming up anyway.
What Made It Happen?
Three contributing factors:
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High cabinet temp (35–50°C routinely)
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Continuous 24/7 runtime for multiple years
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Passive ventilation only — no fans, no AC
LCD modules aren’t huge fans of those conditions.
If You Run These Panels, Here’s My Recommendation
If you don’t want yellow HMIs in a few years:
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Add a small DC fan or panel cooler
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Don’t mount HMIs over heat-producing drives or relays
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Check temperature inside cabinets in summer, not winter
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Replace filters so dust doesn’t insulate heat inside
These little things matter more than people think.
Alright, laundry’s done and I’ve got an early morning startup tomorrow. Hopefully someone finds this helpful before ordering a new display out of panic.
Catch you on the next one.
— CJ