Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Help with LCD monitor problem

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.


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Default Help with LCD monitor problem



"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

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Default Help with LCD monitor problem

On 4/23/2015 1:29 PM, Randy Day wrote:
In article , says...

[snip]

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.


Assuming that your buyer viewed the device on a computer before
purchasing it,
and it was OK,
Something happened in shipping.
A broken backlight is more likely than a bad cap in that scenario.
If one of the backlights is damaged, you'll see the other one light
until the protection circuit shuts off the inverter.


You are correct, that is the symptom of a
backlight problem. Whether it's the CCFL
or a cap in the inverter is the question.
If the monitor has dual lamps, they
wouldn't both go bad, so I'd say inverter.

The inverter circuits I've seen don't
have too many caps, so, as a first attempt,
I'd suggest replacing them even if they
look ok.

You probably don't need to replace every
cap in the power section; the inverter
is usually on its own, away from the other
supplies, since it provides high voltage.


Depending on exactly what you mean by "inverter"...
I've seen more than one where the cap in the supply
to the inverter was the cause. High ESR...overvoltage peaks...
took out one of the fets in the high voltage inverter.

Good Luck.


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Default Help with LCD monitor problem

On Fri, 24 Apr 2015 01:16:00 +0530, "Keimah"
wrote:

It
must have received some hard bumps during transit because it's
not working properly now. The model is BenQ G2420HD.


Nope. Shock would kill it completely, usually by breaking either the
LCD or the CCFL backlighting tubes. More like it wasn't working
before it was shipped and you were sold a monitor with a known
problem. It happens all the time on eBay. Send the seller a nasty
message demanding your money back, unless you want to fix it yourself.

Follow Arfa Daily's troubleshooting advice with the flashlight. If
you can see a faint image, then tear the monitor apart and look at the
power supply inverter PCB. Look for bulging caps, exploded parts, and
a burned PCB. All are common and repairable. What's not easily
repairable is a shorter LCD backlighting transformer.

You can buy a rebuild kit or a replacement board:
http://www.ebay.com/itm/151471890636
I could not find a capacitor and semiconductor replacement kit for the
BenQ G2420HD. Sorry.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558


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Default Help with LCD monitor problem


"Randy Day" wrote in message
...
In article ,
says...

[snip]

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.


You are correct, that is the symptom of a
backlight problem. Whether it's the CCFL
or a cap in the inverter is the question.
If the monitor has dual lamps, they
wouldn't both go bad, so I'd say inverter.


I haven't stripped it down to where I can see the CCFLs
themselves but since there are four 2-pin connectors coming from
the lamps to two inverters, I assume that it has dual lamps. Id
that a reasonable deduction?

The inverter circuits I've seen don't
have too many caps, so, as a first attempt,
I'd suggest replacing them even if they
look ok.


OK. I'll also check the high-voltage coupling ceramic caps for
leakage and nominal values.

You probably don't need to replace every
cap in the power section; the inverter
is usually on its own, away from the other
supplies, since it provides high voltage.

There are what appear to be two independent inverters on the main
power supply board.

Good Luck.


Thanks.


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