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From: dlenski on 5 Sep 2006 15:53
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

From: James Sweet on 5 Sep 2006 15:57
dlenski(a)gmail.com wrote:
> 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.
From: dlenski on 5 Sep 2006 16:02
James Sweet wrote:
> 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

From: James Sweet on 5 Sep 2006 16:06
dlenski(a)gmail.com wrote:
> James Sweet wrote:
>
>>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
>


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.
From: dlenski on 5 Sep 2006 16:55
James Sweet wrote:
> 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

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