Today I received a used LCD monitor that I bought via a forum. My
nephew lives in the same city as the previous owner and checked
it out for me before paying for it and then shipping it to me. It
must have received some hard bumps during transit because it's
not working properly now. The model is BenQ G2420HD.

It powers up and detects input signals - and goes into standby
when it doesn't receive one - but the screen stays lit up for
only about 2 seconds after power-on. It displays whatever image
is present when powered on - whether it's the manufacturer's logo
or a desktop image - and then abruptly goes dark. Even while it's
lit, the display is rather dim. Not quite hard-to-see dim but
considerably dimmer than normal viewing brightness level. It
seems to be uniform over the whole screen.

I'm assuming that it's a backlight problem, but I have enough
experience to know that things aren't always what they seem. I
used to do a lot of repairing work in the past but I'm rusty and
much of my experience was before everything went digital. I have
a general idea of how LCDs work but not the details of a modern
monitor. I opened the back and checked for obvious things like
loose connectors, cracked PCB, leaky/bulging capacitors, etc. I
haven't gone into the CCFL unit yet.

I'll really appreciate any suggestions.