wrote:
I had a Samsung 2443 sitting in a cupboard for ages. I plugged it in to a
laptop using VGA cable, and screen is blank. If I shine a torch onto the
screen, I can see the image, meaning the backlight is shirking its duty.
However, if cable unplugged, you get the moving box telling you there is
no input, check cable. So the backlight is working.
I can't see anything in VGA pinout that would tell monitor to turn out
the backlight.
It works using the DVI for about 10 seconds, then backlight goes out,
but the power indicator LED stays blue.

https://www.badcaps.net/forum/showthread.php?t=36555

Here is a composite picture with some labels.

https://i.postimg.cc/P5dB8HxL/SAM244...adcaps-net.jpg

The IC in that picture, likely controls the drive to the
transformer. It could have an "intensity" input. The OSD
may have a control which sets the backlight level, and the
logic board would send a "signal" on one of the wires
to the power board, asking that IC to adjust the level.

To adjust level on a CCFL, you run the transformer at 25KHz
as usual, but you send "bursts of sine waves" at the tube.
This adjusts the duty cycle, while maintaining gas "ignition"
and conductance in the tube. Such a control method allows
a larger reduction in backlight intensity than the
old "adjustment knob" method used on much older equipment.
So while you might have concluded by looking at the circuit
"it's over-engineered", that IC is likely designed with
backlight modulation in mind. They could either send a
25KHz PWM intensity signal from the logic board, or
they could send a DC level from the logic board requesting
a particular level.

The CCFL tubes require an AC sine wave with no DC offset.
The 22pF caps are good conductors at 25KHz, and there is
little drop across them. One thing to remember - since the
coupling caps are 22pF, even the "foil" or "reflectors"
around the CCFL are part of the circuit. The foil and
reflectors have capacitance too, and are sometimes used
to enhance the electrical performance of the coupling.
There is a whole book written about the dos and don'ts
of placing stuff around CCFL tubes :-) Many companies
botch this detail when making equipment. Users also
screw things up when replacing CCFLs.

When you make adjustments in the OSD, the logic board
remembers that for next time, and when the monitor powers
up, the logic board will put the appropriate signal on
the one wire in the cable.

The above isn't entirely consistent with your symptoms,
because it seems the inverter and backlights work properly
when the OSD rectangle is displayed. And really, the driving
circuits do all the same things when drawing that, as is
done when the computer image is on the screen. The difference
is, the logic board draws slightly less power when there is
no DVI/VGA input. Implying that maybe the primary voltage
on the power board pops up a bit when the OSD-rectangle
is present, and collapses a tiny bit when the computer
image is drawn.

If you could see the OSD and adjust the intensity, you
might squeeze another couple weeks of operation from the
monitor. It's unlikely you can see it though, to make the
adjustment. The higher you set the intensity, the harder
that will be on those two bad solder joints.

Best guess,
Paul