The most basic symptom of a failing inverter board is a dim, almost black LCD screen. This can occur immediately upon boot, or after the ThinkPad has been working properly for a while. In some cases the screen may flicker—lighting up properly for a few minutes and then turning dim again, over and over. Inverter boards that have experienced electrical failure may emit a buzzing sound from their location at the bottom of the display assembly.
Your ThinkPad incorporates an inverter board at the bottom of the LCD display to convert the lower-voltage power it receives from the laptop into the higher voltage the screen requires. The inverter board connects to a long light bulb called a backlight that illuminates the liquid crystals inside the screen's display. If the inverter board fails, the backlight will not receive power, rendering your ThinkPad's screen dark and practically useless.
Connect a separate monitor to your ThinkPad and look for a properly lit display. If the image on the external monitor appears normal, the inverter board on your ThinkPad may be failing. Frustratingly, a failing inverter board and a failing backlight both produce the same major symptom—a dim screen—so the only sure way to diagnose a failing inverter board is to test your current board with a backlight you know to work. If the new backlight does not light, your inverter board is bad and will need to be replaced.
Replacement inverter boards for your ThinkPad can be found at computer electronics stores and online retailers. Consult your user's manual or service guide to find the part number for your inverter board and search for compatible parts. The actual replacement of the board is fairly straightforward and involves disassembling the display, unplugging the current board and plugging the new one in.
Depending on the age of your ThinkPad, disassembling your display or using replacement parts not approved by Lenovo may void your warranty. If your ThinkPad is still under warranty, contact Lenovo's customer support to inquire about repair. For those with expired warranties, do-it-yourself replacement is fairly simple with the help of your service guide for disassembly, or you can contact a local repair shop to do it for you.