An operator interface goes dark on a running machine and the first assumption is always the worst one: the HMI is dead, order a new one. Most of the time that assumption is wrong, and there's a 30-second test that proves it.
The test
Take a flashlight (your phone's is fine). Hold it at a sharp angle against the dark screen, almost flat to the glass, and look closely while the machine is powered.
If you can faintly see your screens — ghost images of buttons, numbers, graphics — the display and the entire computer behind it are alive. What failed is the backlight: the lamp that illuminates the LCD from behind. This is far and away the most common HMI failure on machines that run around the clock.
If the screen is truly black — no ghost image, and especially if there are no power/status LEDs anywhere on the unit — the fault moved upstream: usually the internal power supply, occasionally the mainboard.
Either way, the unit is a repair candidate. But the flashlight test tells you what you're dealing with before you spend a dollar.
Why backlights die on schedule
Older industrial HMIs (PanelView Standard and Plus, Pro-face GP series, Mitsubishi GOT1000, Siemens 177/277/MP series and many more) use CCFL backlights — miniature fluorescent tubes. They're rated for roughly 50,000 hours to half-brightness. Run the machine 24/7 and that's under six years; even a single-shift machine gets there in a working lifetime. The progression is predictable: the screen dims, takes longer to come up, sometimes turns pink or flickers — then quits. The high-voltage inverter that drives the tubes fails on its own schedule too, with identical symptoms.
Newer panels use LED backlights, which last dramatically longer — but their LED driver circuits and power rails still fail, with the same dark-screen result.
Why "truly black" is usually capacitors
If the flashlight shows nothing and the unit is dead-dead, the odds-on culprit is aged electrolytic capacitors in the power supply. Heat and years dry them out; the supply starts sagging, maybe restarts the terminal at random (a classic early symptom), and eventually won't start at all. This is the signature failure of Siemens Comfort Panels in particular — TP900s and TP1200s that power-cycle themselves or won't wake up — and it repairs cleanly with a proper recap using quality low-ESR capacitors.
What the fix should cost
Both failures are bounded, well-understood bench repairs — which is why we flat-price them instead of making you file an RFQ:
- Backlight repair (or, where a proven kit exists, a CCFL-to-LED conversion so it never happens again)
- Power-supply repair with a full recap
- Touch digitizer replacement if the glass has also worn out — common to do at the same visit
At Control Board Lab that's one flat price per family — for example $449 for a Pro-face GP/AGP or Mitsubishi GOT, $499 for a Siemens Comfort Panel or PanelView Plus 6 — parts, testing, and a 24-month warranty included. Your application stays in the unit's memory through all of it.
When it actually is worth replacing
Honesty clause: if your dark screen is on a $250-new commodity panel with no configuration in it, replace it. The repair math works when the terminal is discontinued, expensive, long-lead, or — the big one — carries a configured application your team would otherwise have to rebuild and re-prove. Which, on a production machine, is nearly always.
Run the flashlight test before you order anything. Thirty seconds now routinely saves a four-figure purchase order.
Have this exact problem?
Start a repair or get a free technician review — we'll tell you honestly whether it's worth fixing.