Another Samsung Syncmaster 710N

[robcar]

Dear badcappers!
As previously said in this thread
https://www.badcaps.net/forum/showth...highlight=710n
I have a bunch of Samsung 710n that my colleagues are bringing to me as 'not working'. So I thought it was a nice opportunity for me to try to repair them, even if it's not my job, and learn new things.
Please remember, I'm new to this field and so far my main tasks so far are:

1. Not to get shocked by current and to stay alive (safety first: I got family...)

1. To learn by doing

Speaking of the former point: I bought myself an isolation transformer, I know what it does, but I got a question.
Suppose I connect a monitor to it AND I connect a PC to its video port; now the video has got a ground reference and so the extra safety that the isolation trasformer should provide is lost, isn't it?

Now the interesting part; right now I have two Samsung 710N. One was easy, since it got the primary filter cap defective and just by substituting it the video works. The other one got two 820uF bulging caps in the secondary and a 330uF near the inverter with a high ESR (I do have also a ESR meter); I replaced all of them.
The symptoms: LED lights on and always blinking, even with a PC connected; picture is faintly visible on screen. From what I've learned this should point to a backlight problem.

So another question: if ESR values are fine can I conclude that the cap is ok? I know that bad quality caps will eventually fail, but if ESR is inside limit, can I exclude the cap as possible culprit for the moment?
Just for the sake of completeness I list the ESR values for the caps that I have now on the board:

C107 0.69
C105 0.48
C109 0.08
C110 0.08
C111 0.01
C112 0.01
C113 0.40
C108 0.84
C302 0.01
C303 0.04
C304 0.04

I tested the resistance of the secondary pins of the inverter's trasformer:

1.103K
1.100K

So far everything's seem fine to me; now a strange thing that I saw was that the voltages on the connectors that I measured, with a PC connected to the video port, are (starting from the blue wire - 13V):

16V
16V
0V
0V
0V
5.11V
5.09V
0V
0V

16V seems like a wrong value to me; a working 710N unit shows the following values (no PC connected to video port):

13.6V
13.6V
0V
0V
0V
5.12V
5.12V
0V
4.68V

I have to mention that this working SMPS board, even if the monitor model is the same, got a different rev. number.

So could you please tell me what to check next?
Thanks

--
rob

[Joe Black]

Re: Another Samsung Syncmaster 710N

I'd check the Blight drivers-those 2 IC-looking like 8-pin thingies...

There are many examples on how to check on this forum.

[robcar]

Re: Another Samsung Syncmaster 710N

I'd check the Blight drivers-those 2 IC-looking like 8-pin thingies...

There are many examples on how to check on this forum.

Dual N e P Channel FETs? Ok, I guess I'll have to check them o.o.c., right?

Btw I just bought a blue ring tester (yep!) and both inverter's transformers' secondary windings measure 5 LED's (3 red and 2 yellow), which should be fine; I checked the working SMPS and the LED's lit are the same.

[robcar]

Re: Another Samsung Syncmaster 710N

No luck with this display. I tried to remove the FETs with a hot air desoldering station, but I didn't manage to. I tried with different temperatures, but the damned 8 pin IC's didn't move. After many minutes and with a very hight temp (400° C) I saw the PCB beneath one IC to lift! So I ruined the board and I can say bye bye to this video...
Damn, I don't know, maybe it's the cheap (useless?) hot air station that I bought or maybe it's me that doesn't know how to operate it.
Btw I ordered a Chip Quik kit, hope it's simpler to desolder SMD IC's that way.