LG L1710S 2 seconds to black? (pics)

[TransAm84]

Hello everyone, this is my first post as i am new to the electronics repair hobby. I have re-capped my 24" monitor and replace the IC's in my projection TV but that is the extent of my electronics knowledge. This is a great site! I have read a lot of posts on this particular issue but I am still unsure which direction to go. I have an LG Flatron L1710S that when powered up, flashes a picture then goes black. I know it has some type of backlight issue because i can still see the picture with a flash light. All the caps look fine but i do not have a capacitence meter to test them. I have tried a known-good screen (ccfls) from a matching monitor to no avail. On both monitors I have compared resistance readings from, what i think are, the mosfets or transistor (black bolt-on squares with 3 prongs). The readings are very similar. That is about as far as i can go with out assistance. Any help is greatly appreciated!
Thanks in advance.

[selldoor]

Re: LG L1710S 2 seconds to black? (pics)

Hi - Did you try the panel from this monitor on the other one?

Picture of the bottom of the board? There is some browning on this side
what is below? One of the Inverter Transformers has some browning.
If the panel from this set works on the other then I would check the
secondary windings on the inverter transformers - the joints may be lacquered and may need scraping to test.
Did you read this: https://www.badcaps.net/forum/showthread.php?t=10419

there are a few revisions I think the last starts at post 18/19

Does the Led stay blue when the screen goes off

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

Here are some pics of the bottem of the board. The brown spot on the top of the board is from the small black square thing in the middle of the second pic (the one with no numbers on it). I have read the other posts on repairing this problem, however i do not yet have a total understanding of how these work. I am having trouble identifying some of the components. Are the inverter transformers the yellow squares with the PWI writing on them? After the screen shuts off, the power light remains on and i can see the picture if i hold a flashlight up to the screen.

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

*UP DATE* I installed a known good power supply board into the monitor and everything work good. This tells me that the problem is definately in the power supply board and not the ccfls or the video board. I dont know if i am testing them correctly, but on one side of each transformer i am reading 839 ohms, and on the other side i am reading 0.9ohms. Both transformers read the same when i test them the same way. Is it possible I have bad caps even though they look fine? The last monitor i repaired had visably bulged caps. The caps on this board are SamYoung.

[selldoor]

Re: LG L1710S 2 seconds to black? (pics)

Just for info if you post any more pics can you have them the other way up so
we can read the part numbers more easily
U3 and D3 look a bit burnt - check with a magnifier and advise.
Does U3 have any identification numbers left on it?

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

I can't read any number numbers on U3 because there is some type of clear coating on it (like glue). U2 says Z317 & STC4F30 I think (very hard to read). I dont think D3 is burnt, just some varnish on there I think. I am not really sure about U3, I am not really sure what to look for. The solder and circuit board do look different from U2 (U3 solder is dull and circuit board is brownish). Thank you for the help so far.

[selldoor]

Re: LG L1710S 2 seconds to black? (pics)

Sam Young are not great - but not a lot of reports of them being bad.
The caps can be faulty but still not show leaks or bulges.

I can't read any number numbers on U3 because there is some type of clear coating on it (like glue). U2 says Z317 & STC4F30 I think (very hard to read). I dont think D3 is burnt, just some varnish on there I think. I am not really sure about U3, I am not really sure what to look for. The solder and circuit board do look different from U2 (U3 solder is dull and circuit board is brownish). Thank you for the help so far.

The ST part is the manufacturer - according to the schematic there are 4 of these so yours must be a different revision. I think all 3 (U2-4) should be the same part if you can try to check the rest of the numbers and you have the advantage of 2 boards to look at. The "glue" might come off with isopropyl alcohol if rubbed gently to remove the glue and leave the letters visible. If we can find out what they are someone will be able to advise what voltages should be on each pin.

Can you check that D3 if you have a diode check setting on your meter use that. It tells you how in the 2 sec to black post.
Yes the yellow blocks are the inverter transformers and those resistances suggest they are ok.

Next thing to check is (with it all connected and switched on )the voltages at the connector - it may not be getting a signal from the logic board to turn on the back lights. It is CN1 and is in the corner next to the heatsink. You can use the metal hole in the same corner as ground (Black Probe) then check the DC voltage on each pin.
Then disconnect the logic board and repeat the test and post the results.

You should also post a picture of the front and back of the logic board.

[retiredcaps]

Re: LG L1710S 2 seconds to black? (pics)

*UP DATE* I installed a known good power supply board into the monitor and everything work good. This tells me that the problem is definately in the power supply board and not the ccfls or the video board.

Good job in narrowing down the problem.

I dont know if i am testing them correctly, but on one side of each transformer i am reading 839 ohms, and on the other side i am reading 0.9ohms. Both transformers read the same when i test them the same way.

I circled what I think are your secondary pins. Measure the resistance between the red dots. Do the same for yellow.

Report both readings. They should be within 3% of each other.

PS. Those are great focused pictures.

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

Good job in narrowing down the problem.



I circled what I think are your secondary pins. Measure the resistance between the red dots. Do the same for yellow.

Report both readings. They should be within 3% of each other.

PS. Those are great focused pictures.

The resistance from red to red = 834ohms
The resistance from yellow to yellow = 835ohms

I tested the diode and i got 820ohms one way and open (1.) the other way (I believe this is how a diode should operate?)

Next thing to check is (with it all connected and switched on )the voltages at the connector - it may not be getting a signal from the logic board to turn on the back lights. It is CN1 and is in the corner next to the heatsink. You can use the metal hole in the same corner as ground (Black Probe) then check the DC voltage on each pin.
Then disconnect the logic board and repeat the test and post the results.

You should also post a picture of the front and back of the logic board.

I tried a different power board from another monitor and everything worked good. Should that prove this power board is the problem? Or should I still do the logic board testing?

The other power board I used was from the same make and model but it was definately a different board (different manufacturer) All the parts were similar but different. (different colours, arrangement ect)

U3 & U2 have the same part numbers ST Z317 & STC4F30 (S7C4F30?) These are very hard to read. Also there is another similar looking thing that says IC2 and has part number 200D6 & PXDB.

I hope this helps!

[jetadm123]

Re: LG L1710S 2 seconds to black? (pics)

Try checking the voltages of Q7 (top of power board next to yellow inverter transformer T2). It 3 pin labeled 1,2 and 3. Use a mounting screw for your ground and set your meter to 20V DC. Also, what is the part number of it?


As for checking U2 and U3, try the following:

With power OFF and your meter set to 2kohms resistance setting, measure across the following pins of U2:

pins 1 and 8
2 and 8
1 and 2
3 and 8
4 and 8
3 and 4

do the same for U3 and post your results.

[retiredcaps]

Re: LG L1710S 2 seconds to black? (pics)

The resistance from red to red = 834ohms
The resistance from yellow to yellow = 835ohms

The above readings suggest the inverter transformers are good.

[selldoor]

Re: LG L1710S 2 seconds to black? (pics)

Hi

I tested the diode and i got 820ohms one way and open (1.) the other way (I believe this is how a diode should operate?)

Yes - that looks ok.

I tried a different power board from another monitor and everything worked good. Should that prove this power board is the problem? Or should I still do the logic board testing?

No- i missed that - probably dont need to retest.

The other power board I used was from the same make and model but it was definately a different board (different manufacturer) All the parts were similar but different. (different colours, arrangement ect)

Yes I have seen lots of different versions looking for schematics. ( Not one for this one that I can find)

ST Z317 S7C4F30 IS the part number - it is obsolete but there is enough info if we need to find a substitute.

If the test you are doing for Jetadmin123 is ok then there is not much left. That being the case I would replace the cap C3 .

Realistically as there are so few caps on the board you might as well replace them all except for the big one.

(Aside)
I recall having a monitor that worked intermittantly I got another, the same model and swapped parts like you did. They had a separate inverter boards and I resoldered some of the joints and played about and eventually they both worked. Next day the one that worked fine didnt!! This went on for a few days worked then didnt then did etc. It turned out to be one of the two caps (looked perfect) on one of the inverter boards would work when hot ie after soldering the joints, and would stay working while testing but would not work from cold.

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

I just want to thank everyone again for all the help. This is awsome! I will do the testing today on Q7, U2 & U3 and post my results. What are U2 & U3? I think I can name most of the components on this board but i dont know what those are called.

Try checking the voltages of Q7 (top of power board next to yellow inverter transformer T2). It 3 pin labeled 1,2 and 3. Use a mounting screw for your ground and set your meter to 20V DC. Also, what is the part number of it?

When doing this test, do i need everything hooked up & plugged in? Or can i just plug in the logic board and front power switch board and leave the screen unhooked?

Thanks again

[PlainBill]

Re: LG L1710S 2 seconds to black? (pics)

I just want to thank everyone again for all the help. This is awsome! I will do the testing today on Q7, U2 & U3 and post my results. What are U2 & U3? I think I can name most of the components on this board but i dont know what those are called.



When doing this test, do i need everything hooked up & plugged in? Or can i just plug in the logic board and front power switch board and leave the screen unhooked?

Thanks again

U2 and U3 are FET transistors in a dip package, and drive the ends of the transformer primaries.

No need to hook up the LCD panel at this point.

PlainBill

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

I have tested Q7 and here are the results:

Pin 1 - 5.04v
Pin 2 - 0.00v
Pin 3 - 4.96v

The part numbers on Q7 are: S, 7142, Y2

Here are the resistance readings on U2 & U3:

*I could not see any pin numbers on U2 & U3 so please see picture for pin numbering* *Also U3 is mounted upside down compared to U2 and that is why the pin numbers are reversed*

U2
1-2 = .001
1-3 = .001
1-4 = .001
1-5,6,7,8 = open

2-3 = .001
2-4 = .001
2-5,6,7,8 = open

3-4 = .001
3-5,6,7,8 = open

4-5,6,7,8 =open

5-6 = 1.911
5-7 = starts @ .200 and climbs to almost 2.000 then goes open
5-8 = open

6-7,8 = open

7-8 = open


U3
1-2 = .001
1-3 = .001
1-4 = .001
1-5,6,7,8 = open

2-3 = .001
2-4 = .001
2-5,6,7,8 = open

3-4 = .001
3-5,6,7,8 = open

4-5,6,7,8 =open

5-6 = .486
5-7 = starts @ .200 and climbs to almost 2.000 then goes open
5-8 = starts @ .200 and climbs to almost 2.000 then goes open

6-7 = starts @ .200 and climbs to almost 2.000 then goes open
6-8 = starts @ .200 and climbs to almost 2.000 then goes open

7-8 = .713

I hope this is understandable!

[jetadm123]

Re: LG L1710S 2 seconds to black? (pics)

Your numbering scheme is incorrect. Looking at U2, there appears to be a white dot printed on the board. That dot indicates pin 1. Going in a "U" pattern you have pins 1-4 on one side and pins 5-8 on the opposite side. Pins 5-8 all connected together.

From your photo U3 looks "bulged" at the top. Is that just some residue from the soldering process?

[TransAm84]

Re: LG L1710S 2 seconds to black? (pics)

Looking at U2, there appears to be a white dot printed on the board. That dot indicates pin 1. Going in a "U" pattern you have pins 1-4 on one side and pins 5-8 on the opposite side. Pins 5-8 all connected together.

Using the proper method, here are my readings

With power OFF and your meter set to 2kohms resistance setting, measure across the following pins of U2 & U3:

U2
1 and 8 = open
2 and 8 = open
1 and 2 = 1.910
3 and 8 = open
4 and 8 = open
3 and 4 = open

U3
1 and 8 = open
2 and 8 = open
1 and 2 = .485
3 and 8 = open
4 and 8 = open
3 and 4 = .713

From your photo U3 looks "bulged" at the top. Is that just some residue from the soldering process?

That bulge was some type of clear 'glue'. I have since cleaned it off so i could rear the part number.

[selldoor]

Re: LG L1710S 2 seconds to black? (pics)

Hi - Unfortunately I dont know what those results mean but the fact that they differ suggests to me they are not good.
I have looked up the data for the ST Z317 S7C4F30 and a find similar item came up with ZXMC3A17DN8 and it does have some of the same attributes I also checked the item in the schematic and it has these items as IRF 7389 - I do not know which is nearest and will let more experienced
members advise.

[retiredcaps]

Re: LG L1710S 2 seconds to black? (pics)

Hi - Unfortunately I dont know what those results mean but the fact that they differ suggests to me they are not good.

When testing for shorts, we are looking for anything less than 30 ohms. 30 ohms is a figure I came up and not some industry standard.

Since all the readings are well above 485 ohms, there are no shorts. It doesn't matter if the readings are the same or not in this case.

The readings suggest the mosfet is likely good.

[selldoor]

Re: LG L1710S 2 seconds to black? (pics)

@retired caps - ok -thanks - another free lesson.
As we have tested most things on the inverter section is it time to change the caps.
Having had a previous situation with caps on an inverter board I would have swapped at least C3 out before now, if only to get it out of the equation.