LCD monitor inverter/backlight repair

[[email protected]]

Hi all,
I am trying to repair a 19" Samsung 191T monitor that appears to have
been damaged due to a storm-related power surge over the weekend.

The problem with the monitor is clearly related to the backlight: the
four backlight lamps appear "burnt" and if I shine a bright light into
the back of the panel, the image on the screen appears perfectly fine


The monitor includes two main circuit boards: a big one that the DVI
input and power connects to, and a smaller LCD inverter board. The LCD
inverter board is an Ambit BN44-00060A, which appears nearly identical
to this one: http://www.lcdpart.com/images/AMBIT_K021036.gif
There are no fuses on either board, as far as I can tell.

The LCD inverter board is connected to the main circuit board via a
12-pin connector. The voltages output by the LCD inverter board are
very close to zero, so obviously something is wrong with it :-( If I
understand correctly, the cold cathode backlight bulbs need something
like 1000V AC, is that right?

I would like to be able to verify that nothing is wrong with the power
supply part of the MAIN circuit board as well... does anyone know what
the voltages should be on the 12 input pins to the inverter board?
Since the image is apparently fine, there is no problem with the
display electronics on the main board, but there could be a problem
with the power supply on the main board... I would like to know if
there is a way to tell this.

Can anyone give me any advice on how to test the LCD inverter and main
power supply boards? There are no fuses. Thanks for any advice!!!

Dan Lenski

 

[James Sweet]

Hi all,
I am trying to repair a 19" Samsung 191T monitor that appears to have
been damaged due to a storm-related power surge over the weekend.

The problem with the monitor is clearly related to the backlight: the
four backlight lamps appear "burnt" and if I shine a bright light into
the back of the panel, the image on the screen appears perfectly fine


The monitor includes two main circuit boards: a big one that the DVI
input and power connects to, and a smaller LCD inverter board. The LCD
inverter board is an Ambit BN44-00060A, which appears nearly identical
to this one: http://www.lcdpart.com/images/AMBIT_K021036.gif
There are no fuses on either board, as far as I can tell.

The LCD inverter board is connected to the main circuit board via a
12-pin connector. The voltages output by the LCD inverter board are
very close to zero, so obviously something is wrong with it :-( If I
understand correctly, the cold cathode backlight bulbs need something
like 1000V AC, is that right?

I would like to be able to verify that nothing is wrong with the power
supply part of the MAIN circuit board as well... does anyone know what
the voltages should be on the 12 input pins to the inverter board?
Since the image is apparently fine, there is no problem with the
display electronics on the main board, but there could be a problem
with the power supply on the main board... I would like to know if
there is a way to tell this.

Can anyone give me any advice on how to test the LCD inverter and main
power supply boards? There are no fuses. Thanks for any advice!!!

Dan Lenski

These sort of things can be difficult to track down, it could be
something wrong in the supply to the inverter, as well as many inverters
will shut down if they detect a lamp fault. It really helps if you have
access to another identical monitor to swap parts around, though you can
do some work without. The inverter itself is my primary suspect, if it
has any electrolytic capacitors check those.

 

[[email protected]]

These sort of things can be difficult to track down, it could be
something wrong in the supply to the inverter, as well as many inverters
will shut down if they detect a lamp fault. It really helps if you have
access to another identical monitor to swap parts around, though you can
do some work without. The inverter itself is my primary suspect, if it
has any electrolytic capacitors check those.

Thanks James! From the healthy market for inverters, it does seem like
the inverter is a likely culprit.

Is there any way I can safely mimic a working lamp, e.g. by shorting
the leads with an appropriate resistor or something?

I will check the electrolytic caps... though I'm not sure how I can
actually measure their capacitance while they're connected to the
circuit, any trick to that?

Dan

 

[James Sweet]

Thanks James! From the healthy market for inverters, it does seem like
the inverter is a likely culprit.

Is there any way I can safely mimic a working lamp, e.g. by shorting
the leads with an appropriate resistor or something?

I will check the electrolytic caps... though I'm not sure how I can
actually measure their capacitance while they're connected to the
circuit, any trick to that?

Dan

You can get CCFL lamps from lots of places, they're not expensive and
would work for testing.

You don't measure the capacitance, what you need is an ESR meter, or you
can try just replacing them. Surface mount lytics are the most suspect.

 

[[email protected]]

You can get CCFL lamps from lots of places, they're not expensive and
would work for testing.

Right... I guess I'll need CCFL lamps anyway to replace the burnt-out
backlight lamps. Do you know of a source where I can get them cheap?
The cheapest I can find for 390 mm lamps like this monitor has is $13 /
each, at lcdparts.com ... would it be a terrible idea to use these
cheap 310 mm ones:
http://www.xoxide.com/dual-white-cold-cathode-kit.html

You don't measure the capacitance, what you need is an ESR meter, or you
can try just replacing them. Surface mount lytics are the most suspect.

Gotcha. Don't have an ESR meter, but there's only one electrolytic cap
on the board, AFAICT, so I'll figure out some way to test it.

Thanks!
Dan

 

[[email protected]]

I've just measured the 12 input pins to the inverter board with a
voltmeter. As far as I can tell, 8 of the pins are at ground, and the
other 4 are at 14V. Does this sound reasonable?

If so, I suppose I could just replace the whole inverter with the
inverter from the cheap kits at xoxide.com that I mentioned above.

Dan

 

[James Sweet]

Right... I guess I'll need CCFL lamps anyway to replace the burnt-out
backlight lamps. Do you know of a source where I can get them cheap?
The cheapest I can find for 390 mm lamps like this monitor has is $13 /
each, at lcdparts.com ... would it be a terrible idea to use these
cheap 310 mm ones:
http://www.xoxide.com/dual-white-cold-cathode-kit.html

How do you know the lamps are burned out? They can and do fail, but they
normally last quite a long time.

 

[[email protected]]

How do you know the lamps are burned out? They can and do fail, but they
normally last quite a long time.

The ends have what appear to be burn marks inside them... and when I
try to measure the resistance across them, it's infinite. Not sure if
this is a good way to check the lamps :-/

Dan

 

[Andy Cuffe]

Hi all,
I am trying to repair a 19" Samsung 191T monitor that appears to have
been damaged due to a storm-related power surge over the weekend.

The problem with the monitor is clearly related to the backlight: the
four backlight lamps appear "burnt" and if I shine a bright light into
the back of the panel, the image on the screen appears perfectly fine


The monitor includes two main circuit boards: a big one that the DVI
input and power connects to, and a smaller LCD inverter board. The LCD
inverter board is an Ambit BN44-00060A, which appears nearly identical
to this one: http://www.lcdpart.com/images/AMBIT_K021036.gif
There are no fuses on either board, as far as I can tell.

The LCD inverter board is connected to the main circuit board via a
12-pin connector. The voltages output by the LCD inverter board are
very close to zero, so obviously something is wrong with it :-( If I
understand correctly, the cold cathode backlight bulbs need something
like 1000V AC, is that right?

I would like to be able to verify that nothing is wrong with the power
supply part of the MAIN circuit board as well... does anyone know what
the voltages should be on the 12 input pins to the inverter board?
Since the image is apparently fine, there is no problem with the
display electronics on the main board, but there could be a problem
with the power supply on the main board... I would like to know if
there is a way to tell this.

Can anyone give me any advice on how to test the LCD inverter and main
power supply boards? There are no fuses. Thanks for any advice!!!

Dan Lenski

The back light inverter is the most common failure on LCD monitors.
Normally you should see +9 to +12 v going to the inverter. You will
also probably see +5 and one other voltage to control brightness.

Chances are, the inverter is the problem since it usually shares its
power supplies with the LCD. Look for any fuses on the inverter
board. They will probably be small surface mount components near the
12 pin connector. That orange component to the left of the big green
capacitor could be a fuse. There may be some fuses on the bottom of
the board too. If you find any open fuses, there's a good chance that
a new fuse will fix it. About 3/4 of the inverters I see with blown
fuses have no other problems. If a new fuse doesn't fix it, you will
probably need to replace the inverter board. Schematics are rarely
available and parts are impossible to find.
Andy Cuffe

[email protected]

 

[Homer J Simpson]

The ends have what appear to be burn marks inside them

Could be getter?

and when I
try to measure the resistance across them, it's infinite.

I'd expect that.

 

[[email protected]]

The back light inverter is the most common failure on LCD monitors.
Normally you should see +9 to +12 v going to the inverter. You will
also probably see +5 and one other voltage to control brightness.

Thank you, Andy! This is exactly the info I'm looking for. On this
board, the input is most definitely around 14V. The power brick is
labeled 14V/3A as well. Perhaps the 14V is to add a couple diode drops
to the 12V?

Chances are, the inverter is the problem since it usually shares its
power supplies with the LCD. Look for any fuses on the inverter
board. They will probably be small surface mount components near the
12 pin connector. That orange component to the left of the big green
capacitor could be a fuse. There may be some fuses on the bottom of
the board too. If you find any open fuses, there's a good chance that
a new fuse will fix it. About 3/4 of the inverters I see with blown
fuses have no other problems. If a new fuse doesn't fix it, you will
probably need to replace the inverter board. Schematics are rarely
available and parts are impossible to find.
Andy Cuffe

I will look more closely for fuses. Is there any particular label used
for fuses? As in "C3"=capacitor, "R24"=resistor, "U12"=IC?

If I have to replace the inverter, is there anything wrong with using
the ones that come with the cheap case-mod kits? The backlight lamps
certainly do appear to be fried, since they have a dark gray haze
around one electrode that I don't see in any of the photos at
lcdpart.com.

Thanks a lot,
Dan

 

[Bob Shuman]

Andy's advice is right on. I'd bet that it is your inverter that went South
.... There is usually at least one surface mount fuse on the inverter board
and this can be tested with a simple ohmmeter. But, if the fuse blew, it is
usually due to a fault elsewhere on the inverter board so simply replacing
it will not likely solve your problem.

Bob

 

[ampdoc]

I will look more closely for fuses. Is there any particular label used
for fuses? As in "C3"=capacitor, "R24"=resistor, "U12"=IC?

Fuses are commonly marked with F or FU.

 

[[email protected]]

There are usually 4 different lines on the power connector:
1. 12 or 14 volt main power source.
2. Ground (usually 2 or more wires)
3. On/Off control line (zero or 5 volts)
4. Brightness control (zero to 5 volts).

Every inverter board I have worked on had a fuse on the main
power source. Usually located right near the input connector
and the larger electrolytic capacitor. Trace your 14 volt line.
The fuse us normally marked F1 or something like that.
They are surface mounted and may look like a black resistor.
Usually rated about 2-1/2 amps.

If the fuse is open, check the driver transistors (or FETs) that
drive the primary winding of the step-up transformers. They
usually become shorted.

Hope this helps....
John

 

[[email protected]]

There are usually 4 different lines on the power connector:
1. 12 or 14 volt main power source.
2. Ground (usually 2 or more wires)
3. On/Off control line (zero or 5 volts)
4. Brightness control (zero to 5 volts).

Thanks! As far as I can tell, all the lines on the inverter connector
are at 0 or 14 V :-(

Every inverter board I have worked on had a fuse on the main
power source. Usually located right near the input connector
and the larger electrolytic capacitor. Trace your 14 volt line.
The fuse us normally marked F1 or something like that.
They are surface mounted and may look like a black resistor.
Usually rated about 2-1/2 amps.

There was indeed a (tiny) fuse, I finally found it. It is not blown,
so that wasn't the problem.

After messing around with this thing some more, and accidentally
shorting 14 V to ground in the power supply, I've made the whole thing
worse. Now I can't even get an image to show up by shining a bright
light into the backlight tubes. Ugh. I think I'd better cut my losses
and stop messing with this thing. Oh well. It's been a good learning
experience!

Dan

 

[[email protected]]

Are the inverter fuses quick action or slo blo?

Thanks,
--E