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Original post by: Stuart James ,

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I don't mean to offend anyone but you all seem to have missed the blatantly obvious and that is the LED backlight array.

LEDs age, and as such the amount of light that is emitted reduces. The LEDS in the LM240WU6 panel are made by Lumileds which is a subsidiary of Philips who in turn produced the panel under license from LG. The LEDs used are LXML-PWN1-0120 which should be operated at a forward voltage of between 3.0V & 3.6V @ 350mA and have a stated 39000 hours of life.

LEDs operate in constant current mode that is the current through the device is adjusted by altering the voltage across it. In low current LED applications this done with a simple resister. In high powered applications this is done with a controller chip such as the HV9982. Most high powered applications work as a closed loop system, that is they monitor the current flowing in the LED string and adjust the voltage across the string accordingly. The circuit Apple adopted does just that, it monitors the voltage developed across a low ohm resister in the bottom of the LED string and adjust the PWM signal to the HV9982 that will adjust the voltage to the LEDs. The suggested fix above of applying 3.3V to the PWM input will remove this feedback and will apply maximum voltage across the LEDs and is likely to result in damage to the LEDs and driver circuit.

What most people are experiencing is the backlight driver circuit shutting down due to it reaching an overvoltage point as it tries to pump more current through the aged LEDs.

[image|1377459]

The above plot shows the current through each LED with a forward voltage of 3.3V. As you can see there is a lot of variation (LED 1 shows as 0mA), none of them get near the 350mA that they should.

Conclusion, the only long term way to fix this problem is to replace the LED string as a whole or at least the weaker LEDs. The individual LEDs are available from DigiKey but they will prove difficult to replace as they are surface mount and are tied to a heat sink back plate. An alternative is to retrofit a low voltage backlight replacement string, they are available on eBay for as low as £4. These will require a 12V supply and will need to be turned on and off manually.

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