Voltage to backlight falls when backlight fails. All voltages measured at motherboard connector to backlight power supply read the same (referenced to ground) whether light is on or off. Simulating a lid closure (with magnet by reed switch) and then simulating reopening (remove magnet from reed switch) causes backlight to come back on (each time) for about 10-30 seconds before failure. Reed switch has been isolated and known good.
Question: Is there other circuitry on the motherboard that may cause this problem (even with the voltages measuring the same at the motherboard connector) or is the problem in the backlight power supply located in the monitor frame?
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From what you are describing, it sounds like the inverter board is failing. This board sits under the LCD screen and is responsible for taking the DC voltage from the battery and AC adapter and converting that voltage to high voltage AC power for the backlight lamp that sits inside the lcd screen. Without that high voltage the screen goes dark. Most inverters do not fail right away, there are voltage fall offs etc. Inverters are approx. 15-20 bucks on ebay, for most laptops, they can be changed out simply by removing the bezel around the screen, unplugging the two connectors on the inverter and changing it out. In most cases except Dell, the inverter is just sitting in there and is not held in by screws.
So try that out. I just got a toughbook cf-51 and will try it soon myself. Mine came with an XP reg code so maybe that can run from a virtual machine inside of linux. My advice is to resist the urge to drop yours onto cement from a couple of feet. Not quite that tough. The hard drive caddy is "the catch", for certain. Too lazy to look into what the hdd connector is for that, maybe there's a cable or converter somewhere.
Sue, it's not the inverter board. People who don't really know much
about laptops will often diagnose them incorrectly, especially computer
repair shops. The problem you're experiencing is most likely related
to your motherboard. Unfortunately the most expensive part to replace on most laptops.
Your best bet is to probably just have any important files from the laptop extracted by a repair shop and then selling the computer "As-IS" on ebay. Or you could try another local repair shop to take a look at it. I wouldn't pay any money for a diagnostics since it would take most techs about 5-10 minutes to confirm what I just typed.
Your best hope is that the CPU heatsink is dusty and the CPU is overheating and therefore shutting of for thermal protection. Try to clean it deeply with canned air caring not to short the fan speed sensing with the condense caused by the expanding air(do not hold the nozzle too close to the fan). If it keeps doing it, then it's probably because of the motherboard's voltage regulator and you need to replace the motherboard or at least the voltage regulator if it is not on-board(usually it is onboard.)