"Keimah" wrote in message
...
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.


The first thing to check is whether the image is still on the screen after
it has gone dark. You can usually determine this by shining a strong light -
like an LED flashlamp - at the screen at a highly acute angle and looking
closely and carefully. If the image is still on the screen, then the problem
is backlighting, but as to whether this is related directly to the inverter,
or is to do with the lamps themselves, is rather more difficult to prove, as
the inverters usually have current sensing on their outputs, and if
insufficient current is being drawn - if for example you had a faulty or
broken lamp - then the inverter will shut down shortly after it starts. A
bad lamp may be the reason that it looks a bit dark before it does shut
down.

Alternately, one half of the inverter may have gone bad. You will usually
find that it has two outputs - one at either end of the board - which are
normally fed from two independent drive circuits. If either fails, it will
shut down the whole inverter because of the current monitoring. The
inverters are not all that easy to check, as you need to 'fool' them into
thinking that they have a correct load on the end of them by substituting an
appropriate value resistor for the lamps.

The inverters are often available very cheaply on eBay, and this is
sometimes the easiest way of proving whether or not the fault lies with the
inverter or the lamps. If it turns out to be lamp related, then it's pretty
much a write-off anyway, because although they are available, they are often
almost impossible to fit, and are extremely delicate and easily broken when
being handled.

Arfa