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Default BenQ G2420HD LCD monitor cannot stay lit up

On Friday, 9 March 2018 13:44:56 UTC, Pimpom wrote:
I have a BenQ G2420HD monitor that was damaged during
transportation. Two of the four CCFL tubes were broken and I've
solved that problem but it still has a fault. These are the symptoms:

1. When switched on, it lights up and presents a normal display
FOR ONE SECOND, then goes dark for 3 seconds, lights up again for
one second and then goes dark permanently. Power cycling repeats
the same behavior over and over.

2. It detects the video and sync inputs. The pilot light stays
green as long as there's a signal input (even when the screen
goes dark by itself as described) and turns amber when there's no

3. The LCD panel syncs and displays correctly even when the
screen goes dark by itself. I verified this with an improvised
backlight. It is only the backlight that is misbehaving.

4. The light/dark cycle is caused by the 3.3V ENA signal from the
control board to the inverter coming on and off. It's too regular
to be a thermal issue.

5. All power supply voltages (17V, 5V & 3.3V) are OK. The control
board controls the inverter with *one-way* ENA and DIM signals.
There is no feedback from the inverter to the control board.

6. The control board doesn't have obvious cracks, burn marks or
bulging capacitors.

Any help will be greatly appreciated.

bad electrolytics are often not visibly damaged, test them for ESR if practical. Or holding a similar one one in parallel can find a bad one by making it work ok.

If a 3.3v signal line is the cause, you could always cut & tie it high or low.