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 makes it repeat 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 signal.
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 path from the inverter to the control board.
6. The fault seems to be in control board. It doesn't have obvious cracks, burn marks or bulging capacitors.
Any help will be greatly appreciated. I drew a block diagram of the main parts:
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 makes it repeat 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 signal.
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 path from the inverter to the control board.
6. The fault seems to be in control board. It doesn't have obvious cracks, burn marks or bulging capacitors.
Any help will be greatly appreciated. I drew a block diagram of the main parts:
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