Problems with Samsung 740N monitors

[selldoor]

Re: Problems with Samsung 740N monitors

You found the little green fuse under the the left hand side halfway down under the glue between the heatsink and the capacitor.? If it is back in the set what is the voltage on each side with respect to a ground screw. If it is out of the set can we have a picture of the back. Also can you test the secondary windings on the inverter transformers.
Meter on Ohms 2000 give answer in Ohms

[mawais15]

Re: Problems with Samsung 740N monitors

Yes I have checked the fuses, both near the heats ink and near the power plug (cylindrical one) both are short. I could not understand where to measure the voltage on board. Cantilever please highlight it

[selldoor]

Re: Problems with Samsung 740N monitors

OK I have marked for testing S1 to S2 and S3 to S4 But as you are getting NO flash I am more concerned with other items.
In the pink Square that looks a bit burnt on the picture - is it ok
In the Yellow squares the items are not soldered???
In the Red Squares check that these are not bridged

[mawais15]

Re: Problems with Samsung 740N monitors

OK I have marked for testing S1 to S2 and S3 to S4 But as you are getting NO flash I am more concerned with other items.
In the pink Square that looks a bit burnt on the picture - is it ok
In the Yellow squares the items are not soldered???
In the Red Squares check that these are not bridged

The voltages between s1 and s2 as well s3 and s4 are zero when testes the board alone. The pink 8 pin component is 4502C and it looks in bad condition, I'm not sure if it's fine ( how do I check it). Those capacitors in yellows squares are soldered. In one of the red square that seems bridged is the fuse. And the 2nd red square is not bridge.

[selldoor]

Re: Problems with Samsung 740N monitors

OK you were not checking VOLTAGE --it is 2000 Volt
s on those joints!!!!!
NO POWER SWITCHED ON
You are checking resistance - Meter on OHMS 2000.

[mawais15]

Re: Problems with Samsung 740N monitors

Sorry I should have measured resistance across secondary windings, the values are 1335 and 1360 ohms across secondary windings

[selldoor]

Re: Problems with Samsung 740N monitors

They are ok then
4502C




If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.

[mawais15]

Re: Problems with Samsung 740N monitors

They are ok then
4502C




If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.

Thanks for your help :-) . I will check the mosfets once I find the datasheet.
One more thing I've noticed is the voltage across the DC pins of bridge rectifier is above 300V. Is this OK?

[selldoor]

Re: Problems with Samsung 740N monitors

Datasheet is at top of my post
DC voltage at Bridge should be 1.41 x your mains AC voltage - you can also check that at the BIG CAP - carefully test to see if it holds steady for 7-10 seconds.

[mawais15]

Re: Problems with Samsung 740N monitors

I just read the part number on mosfet and it's 4502C AA5NP if my eyes caught it right. My main AC is 240V that means bridge rectifier is ok same voltage is across big Capacitors around 328V. NOW I'll be checking mosfet .

[RJARRRPCGP]

Re: Problems with Samsung 740N monitors

those are samxon gl.
known junk.
they are a replace on sight part.

I dunno if even GLs are that bad. (And possibly the same for GF)

But, definitely real worried of the KMs!

There was a report of KMs being real bad, even without bulging. At least many, if not most caps, usually bulge or leak at the bottom when bad.

[Pentium4]

Re: Problems with Samsung 740N monitors

GF are bad, KM are mediocre

[Crauler]

Re: Problems with Samsung 740N monitors

I just replaced all these caps, even those that are not inflated and my monitor "Samsung 740N D" works!

[Behemot]

Re: Problems with Samsung 740N monitors

Are those Panasonic or Hermei?

[me.just88]

Re: Problems with Samsung 740N monitors

hi ,i'm new to this forum.

I also have the same problem like "ethernet","lit","selldor"
I chenged all the caps and fuse,after that the monitor is restarting.
has anyone solve that problem?

[Behemot]

Re: Problems with Samsung 740N monitors

Have you checked inverter transistors and transformers?

[Gemat]

Re: Problems with Samsung 740N monitors

Hi,

I finaly managed to fix my 740 Samsung with the PSU like the one pictured in the post above!

I had to replace the three 820uF capacitors, the 3amp micro fuse and one of the 8 pin ICs on the solder side of the board.

Thanks for all the input in the above posts...

[Behemot]

Re: Problems with Samsung 740N monitors

That's one of those transistor IIRC.

[Gemat]

Re: Problems with Samsung 740N monitors

That's one of those transistor IIRC.

Its a surface mount mosfet (AF4502C). I replaced it with a SP8M3.

It also works with 4620 (I temporarily stole one from another working PSU)

[selldoor]

Re: Problems with Samsung 740N monitors

I think we usually suggest replacing both mosfets as a pair