Samsung 731BF no light

[xifias]

hello
i have an lcd monitor,samsung 731bf that goes off from illuminocity almost imediately.i changed the bulged caps from the power supply board,yet the problem insists.i found accidentally that if the voltmeter is in paralel on the high voltage output,it manages to stay on and show picture normally.
i tried a 47nF/100V capacitor that laid around instead of the voltmeter and it worked also.

but i want a more confident solution.why is that happening?will it be dangerous or so?its not mine you see,its for a friend,and i wont be able to monitor it after i give it back.

here are some info
http://monitory-lcd.magazyn.pl/pic_m...sung-731bf.jpg

http://www.gzskyhorse.com/images/FGH001.jpg

and the schematics

http://dc435.4shared.com/doc/NYMefbxe/preview001.png

http://dc435.4shared.com/doc/NYMefbxe/preview002.png


the model of the power supply board is IP-35155A

many thanks in advance!!

PS:the voltmeter is a typical multimeter,no high voltage.it reads no voltage,but the picture is on.i placed it on the upper end of the last resistor on the left and to the ground.typically the output is 650 volt there.the cap didnt even warmed up even its rated 100V.

[retiredcaps]

Re: Samsung 731BF no light

hello
i have an lcd monitor,samsung 731bf that goes off from illuminocity almost imediately.

If I understand you correctly, it sounds like you are describing 2 seconds to black. If yes,

2 seconds to black can be

a) bad caps
b) bad ccfls
c) bad ccfl wiring (especially the return wiring)
d) bad secondary windings on the inverter transformer

More details starting with post #19 at

https://www.badcaps.net/forum/showthread.php?t=10419

Also, see budm's Inverter tester using old CFL

https://www.badcaps.net/forum/showthread.php?t=19987

here are some info

Post your pics and schematics following my instructions below exactly.

i placed it on the upper end of the last resistor on the left and to the ground.typically the output is 650 volt there.the cap didnt even warmed up even its rated 100V.

Indicate that in your picture. Right now, I have no idea what area you are talking about.

[xifias]

Re: Samsung 731BF no light

thank you for your reply.

yes it is the "2 second to black" problem yet now it became almost imediately.0,2 sec max. i suppose.so,i attached a picture that is taken right after i opened the screen.

the caps marked with red x are all changed with equals and higher volts some of them.

the childish draw with green on the top of the picture is where i connect the cap (and the voltmeter also) and it works.one end here other on the ground.

i checked diodes,resistors,smd diodes on the other side and all seem ok.also checked the parallelogram black transformer on the top.3 windings one primary 0,6 ohms and two secondaries at 1K each.

the lamps that the monitor has light equally,with good brightness.visualy also the cables seem ok with no burn markings,and the board is also clean from any mini explosion

some idea will be more than grateful.its very important for me to make it work.
thanks in advance once again.

[budm]

Re: Samsung 731BF no light

So you hook up the lamps of this monitor to another board and all four lamps light up and stay on?
"i found accidentally that if the voltmeter is in paralel on the high voltage output,it manages to stay on and show picture normally." This make it sounds like it is not seeing one of the lamp and you meter becomes load and cause the inverter to stay on.
There are 4 High voltage (3000V) at the output of the transformer, do they show any resistance reading at all?

[retiredcaps]

Re: Samsung 731BF no light

There are 4 High voltage (3000V) at the output of the transformer, do they show any resistance reading at all?

edit for budm.

There are 4 high voltage (3000V) blue ceramic tuning (C311, C312, C313 and C314) caps at the output of the transformer, do they show any resistance reading at all?

They should all read open circuit. If you get any resistance reading, it is likely bad.

See budm, I pay attention!

[xifias]

Re: Samsung 731BF no light

well hello again.
the blue 3kv caps on the end all seem ok.i've checked them with the ohm meter.also and some others that are smd have checked.

the lamps arent tested to another board.but with the "cap trick" stay on for at least 20 minutes.no heat no something weird,even though the cap was 100V rated,at a place that has 650V theoretically.

your interesting is very important to me.thank you again.

[xifias]

Re: Samsung 731BF no light

some info is that in the input of the inverter is 14,29V (the schematic says 15) measured now that works (with the cap hooked on) and in off state (with the lamps off) goes up to 16,82V.

some guy also told me that with the cap trick i bypass some kind of built-in leakage detection of the inverter so it doesnt shuts down.

anyway can be the problem before the inverter on the smps part of the power supply?

i attach a much more cleaner schematic i found on pdf.

PS:i just saw that the cap also does the same effect even in paralel on the resistor we said so (R341) and with another value i had,22pF at 6kV.

[budm]

Re: Samsung 731BF no light

""cap trick" Can you please explain more about the cap trick?
"some guy also told me that with the cap trick i bypass some kind of built-in leakage detection of the inverter so it doesnt shuts down" The circuit had detected fault some where either in the lamp assembly or in Inverter circuit so it will shutdown, it does not mean the lamps or the circuits are OK.
Bad lamps:
http://s807.photobucket.com/albums/y...klight%20lamp/

[xifias]

Re: Samsung 731BF no light

omg you have a serius collection there..and experience i suppose.

by the term "cap trick" i mean shortly what i described and above,the connection of a capacitor from the resistor to earth.recently i found that and onto the resistor in parallel does the same.according to the schem,that resistor (R341) looks like it "connects" the two secondaries of the transformer.can a resistor have and a capacitive behavior that is lost due to stress?

from a close look to the cables they seem good,but i was unable to open the panel more deeply to get to the lamps,it seems to fragile,and im afraid not to make more damage.

[budm]

Re: Samsung 731BF no light

OK, I will look at the SCH and see what the R341 is for. You may want to make a test lamp to see if you are having inverter board or the lamp problem.
https://www.badcaps.net/forum/showthread.php?t=19987

http://s807.photobucket.com/albums/y...g%20old%20CFL/

[xifias]

Re: Samsung 731BF no light

thank you very much for your time.
check the pdf.the schem makes it little complicated with the numbering on the transformer,but i desoldered him from the board,and measured him,and visually from the bottom (its open) it is exactly as i describe it to you.2 secondaries that are connected through the R341.

thanks once again.

[xifias]

Re: Samsung 731BF no light

so..i bought a small ccfl lamp.F6T5/D says.
i tried all the four outputs of the board.in the first position (cn302) the monitor stayed lit.in the second one (cn301) shut down.so did to the other two (cn303 and cn304) but with a little "hesitation" after 2-3 secs did fade away.

now?i need a new lamp?or 3 of them? :P

[retiredcaps]

Re: Samsung 731BF no light

so..i bought a small ccfl lamp.F6T5/D says.
i tried all the four outputs of the board.in the first position (cn302) the monitor stayed lit.in the second one (cn301) shut down.so did to the other two (cn303 and cn304) but with a little "hesitation" after 2-3 secs did fade away.

now?i need a new lamp?or 3 of them? :P

The ccfl connected to CN302 is bad.

My guide explains why and budm as provided pictures of either a bad ccfl or bad wiring.

[xifias]

Re: Samsung 731BF no light

thank you once again.your help was priceless.do you know where to find one,or is it very pricey to repair.i live in greece.

[retiredcaps]

Re: Samsung 731BF no light

do you know where to find one,or is it very pricey to repair.i live in greece.

You can try ebay. Or you can take it apart and hope it is just bad wiring as shown in the pictures. If it is bad wiring, you can resolder the ends. See

http://www.ccflwarehouse.com/lasotu.html

[retiredcaps]

Re: Samsung 731BF no light

Alexanna is the king of these types of repairs. I tried it once.

[xifias]

Re: Samsung 731BF no light

Alexanna is ebay dealer or so?

[Rtech]

Re: Samsung 731BF no light

Once you have done 9-10 its usually not a problem,BUT then you mess the next one up, and you need a new screen. NOT for the faint hearted,by any stretch of the imagination.

[retiredcaps]

Re: Samsung 731BF no light

Alexanna is ebay dealer or so?

Alexanna is an USA member here who has done a lot of these repairs. If you are wondering, he does not have a service to fix for other people and he does not sell parts. He just fixes it for his own use.

[xifias]

Re: Samsung 731BF no light

Once you have done 9-10 its usually not a problem,BUT then you mess the next one up, and you need a new screen. NOT for the faint hearted,by any stretch of the imagination.

excuse me but could you please repeat your answer in lets say, basic english?


@retiredcaps

oh got it,but of course even if he had a service or smtg it would be very expensive such type of repair (post the screen,etc..)


just to mention,i oppened the panel and it has 2 lamps down on the bottom and 2 up.one lamp of the 2 on the bottom is black on the end,(usual for you'll say!).bad luck.i was hope to see some bad cable as you said.

now the screen works with the capacitor that we were saying,but i dont trust it AT ALL.i dont care if it darks out again but for the risk of fire or so.or im just worrying too much?the luminence is set to 30% just to not stress the circuit.

from your experience what you have to say?