Problems with Samsung 740N monitors

[Behemot]

Re: Problems with Samsung 740N monitors

Seems like open-circuit voltage is high, should be OK under load. Replace all caps, usually all of them go bad even though it may not be visible. Check inverter transistors and transformer(s), everything is coverd many times in this thread.

[tvl_svl06]

Re: Problems with Samsung 740N monitors

Problems with Samsung 740N monitors

samsung 740n monitor display in 2 sec.. go to screen block mode

display time inverter input voltage 14.0v
inverter power input 18.5v this voltage is normal?
change 470mf25v cap.3no,1000mf25cap no1
send more information

[skydec]

Re: Problems with Samsung 740N monitors

Just repaired 2 samsung 740N LCDs.
On the first one I just replaced the 3 capacitors on 13V rail and that was it, worked after that.
Second one was a bit tough. It was completely dead, LED also dead. Fuse, thermistor and varistors were OK. ZD102 was found dead, replaced it but it didn't help. After I powered the LCD still nothing, replaced zener didn't burn. Comparing ohm readings form first LCDs psu I saw lower ohm readings across several components in primary side of the second LCDs psu (Zener diodes were not reading as (600,infinity) but (200,200). So I decided to swap the DM0565R and now the second psu is also working. The faulty DM0565R was not in short circuit but still is not working properly.

[Behemot]

Re: Problems with Samsung 740N monitors

I can supply you with quality caps, maximum capacity in each dimension and too extremely low ESR. PM me for more information…

[qbicdesign]

Re: Problems with Samsung 740N monitors

seems I have answered my own question.

I replaced the bulging 1000uf 10v with a 16v which I had salvaged from an old mobo.
There's a single 470uf 25v which lays horizontally next to the mosfet board. I didn't have a same replacement, but figuring that the board printing states 13v, i put a 470uf 16v there (also salvaged from an old mobo).

The 3 monitors are now working!

There are some other still bulging CapXom capacitors rated at 470uf 25v, which I'll replace as soon as I can get to the store. But as proof of concept, I know that my monitors are OK.

The cool thing is that I have about 40 740n monitors, which seem to be dying on a fairly regular basis with all the same symptoms (flashing blue light, no image on screen), and now I know how to fix them all

All of those monitors shared the same symptoms, same swollen caps, in all the same locations. It should be enough to switch out (per monitor) 3x 470uf 25v, 1x 1000uf 10v.

Again, thanks to all posters in this forum for your contributions!

[Behemot]

Re: Problems with Samsung 740N monitors

Can't see anything, the picture is terrible. It may very well be under the second board, just have a look yourself, fuse is F.

[qbicdesign]

Re: Problems with Samsung 740N monitors

To everyone posting here, this thread is gold dust!
I have one 740n with the 3 blown caps (which i already replaced) and the blown 3A picofuse, which I'm waiting on.

I also have 3 of the 740n monitors with the circuit board from pic 1 referenced here on the very first page in this thread:
https://www.badcaps.net/forum/showpo...18&postcount=4

I'd like to know if (beyond replacing the bulging caps), is there any fuse replacement on this board?

[selldoor]

Re: Problems with Samsung 740N monitors

I think we usually suggest replacing both mosfets as a pair

[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)

[Behemot]

Re: Problems with Samsung 740N monitors

That's one of those transistor IIRC.

[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

Have you checked inverter transistors and transformers?

[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

Are those Panasonic or Hermei?

[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!

[Pentium4]

Re: Problems with Samsung 740N monitors

GF are bad, KM are mediocre

[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.

[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 .

[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

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?