On Friday, March 9, 2018 at 8:44:56 AM UTC-5, 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
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 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.

If we ignore the shipping damage, it sounds like a typical back light problem. Are you sure you have the correct CCFL tubes in the display?

There's a chance one or more of the inverter transformers was damaged when the display was run with an open tube.

What you can do is place your scope probe on the plastic part of the lamp connector. Where it goes exactly is unimportant as you're going to be looking for *differences* in the waveforms between the original tubes and the replacements. In any case, don't make any electrical connection or you'll likely damage your scope input. But you must be sure to place the probe in the exact same spot on the connector as the waveform will vary greatly with just a small physical movement.

Look at the waveforms for each of the tubes during the time it's actually lit. If one or more waveforms differs from any other, the controller IC will shut down the inverter. Too large a waveform usually indicates a bad, weak, or incorrect CCFL tube. A low or distorted waveform will usually indicate a bad transformer.