Samsung 740N power problem

[bikkerss]

Hi,

I just tried my first reparation ever but it doesn't seem to work out as planned
I got a defective samsung 740N monitor that doesn't give any display.
So I opened it up and saw the the 820uf 16v capacitators are bulky.
Replaced those and tested the soldering with my multimeter.
The soldering seems to be ok.
I tested the pico fuse, it also seems to be ok.
But when I assembled the monitor I don't have any power.
No power even no led.
Anyone any suggestions what I did wrong ?

[Andrew F. Ali]

Re: Samsung 740N power problem

Did you put the e-caps polarity correct in the cct? Is there afuse that may be open? Is the monitor getting 120Vac? Is it plugged in? Did you have to pull out any connectors? Did you put them back in correctly?

[bikkerss]

Re: Samsung 740N power problem

hi andrew,

the condensators are installed with the correct polarity otherwise I think they would have blown when I tried to power the screen on.
Can you tell me how I can check the fuse ? (I only can find one fuse on the board)
I dismantled the screen again but I think it was correctly reassembled.

[budm]

Re: Samsung 740N power problem

Can you provide the straight shot pictures of your board top side and bottom side?

[bikkerss]

Re: Samsung 740N power problem

Front



Back

[budm]

Re: Samsung 740N power problem

Pictures of the guns?
Too blurr for me to read any designator, and It should be straight shot.

[bikkerss]

Re: Samsung 740N power problem

What do you mean with the guns ?
Do you want a photo of only the soldering ?

[budm]

Re: Samsung 740N power problem

It shows the picture of the guns before, but now it shows the pictures of the top side of the board.
But the pictures are still at an angle so hard to trace the connection and read the designators.

[bikkerss]

Re: Samsung 740N power problem

I will post new photo's tomorrow, but there should be 2 photo's.
One of the front of the board and one of the back.

[bikkerss]

Re: Samsung 740N power problem

Is this beter ?

[budm]

Re: Samsung 740N power problem

Since the the picture of the bottom side of the board is cutout, I can only see the fuse for the Inverter circuit.
Soldering works look bad.

[bikkerss]

Re: Samsung 740N power problem

I know, first time soldering but the continuity test with the multimeter are ok
Should I try again to solder it ?
I also did that test for f301 (I think the fuse you mean) it is also ok

[budm]

Re: Samsung 740N power problem

'I also did that test for f301 (I think the fuse you mean) it is also ok' It shows as <1 Ohms?
You should check the resistance between the two legs of the caps that you replace to see if they show low resistance reading, if the readings are on the K Ohms range then you can just running the power supply board by itself and check to see what DCV you get between the two legs of those caps.

[bikkerss]

Re: Samsung 740N power problem

The resistance On the fuse is 0.1 ohm i also massured the condensators they seem to charge. I reassembled the screen again but no display . I hear a soft high Pich sound when it is plugged in.

[budm]

Re: Samsung 740N power problem

How about the DCV between the two legs of the caps you replaced when running the power supply board by itself as asked in post 13?

[bikkerss]

Re: Samsung 740N power problem

No, beceause I'm not sure it's safe.
I'm just starting with elektronics.
but the caps are brand new.

[mikail13]

Re: Samsung 740N power problem

you must check the Transformers for any short circuit.. i suspect this..

It happened to me before.

[ehrja]

Re: Samsung 740N power problem

Did you check if there is any voltage in the white conector?

[bikkerss]

Re: Samsung 740N power problem

you must check the Transformers for any short circuit.. i suspect this..

It happened to me before.

Can I check this with a multimeter ? and how do I do this ?

[budm]

Re: Samsung 740N power problem

Sooner or later you MUST take the DCV readings when the board is powered up.
You should check the DCV on the connector that goes to the main/logic board without the main/logic board connected to see if the squealing is still there or not when you power it up.