Problems with Samsung 740N monitors

[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

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

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

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 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

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

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

I don't see anything on the screen when I switch on the power. It stays black with power led continously working. I'm attaching the picture. Thanks :-)

[selldoor]

Re: Problems with Samsung 740N monitors

There are different boards in these machines good clear pictures would help us to help you.
Does the screen flash on for a second when you first switch on?

[mawais15]

Re: Problems with Samsung 740N monitors

Hello friends, I joined this forum after my syncmaster 940n stopped working the other day, dark screen with no display and no flashing led. Led continuously working without flashes.

I've check the fuses, they're Ok, I've replaced the capacitors as well 1000uf 25V. Any idea of please

[lexwalker]

Re: Problems with Samsung 740N monitors

Tnx for info about findig fuses. That kind of garbage we have a lot. I agree that a monitor is possibile fault, it was a toy practicly. Best is that i learned something new. Tnx again.

If you had opened up and fixed/repaired enough of those ATX PSUs, then should be familiar with that 3A fuse. Those two I've mentioned (with Dscaler board problems) was a Samsung SyncMaster 153V and Samsung SyncMaster 931BW. Just waiting for spare boards preferably for free and/or cheap bargains from other faulty units (with panel problems)...

Recently revived a Samsung B2030 which was struck by lightning (pretty badly) but luckily the Dscaler board was not toasted. Every time a new different monitor came in for repair, its always a new learning experience in getting to know the monitor better (whether its disassembling it, or examining the boards inside). Now two more just came in, a BenQ T221W and a BenQ G920HD, both of which I had not laid my hands on before...

[mshone]

Re: Problems with Samsung 740N monitors

Stranded wire I guess. But this is not really good…if it goes bad again, the problem would propagate further, destroying other components on the way. You can use more or less any type/size of fuse as long as you are able to solder it in somehow

I agree, it was just experiment. Ill try to find original replacement, tnx for advice.

[mshone]

Re: Problems with Samsung 740N monitors

If the Dscaler board ever fail then its always going to be very difficult (searching for a spare board from the same monitor brand and model somewhere). As mentioned earlier, was hoping it wasn't the Dscaler board but unfortunately it was (as you've had found out). Myself also have at least two monitors collecting dust due to Dscaler board problems...

You can find 3A fuse easily in ATX power supplies used for PCs. Thus can look for them in scrap/faulty ATX power supplies. If those vertical lines stay in the same place all the time, then very likely the LCD panel is bad. Usually either the Tcon board and/or bonding tabs.

Tnx for info about findig fuses. That kind of garbage we have a lot. I agree that a monitor is possibile fault, it was a toy practicly. Best is that i learned something new. Tnx again.

[mshone]

Re: Problems with Samsung 740N monitors

There are 10 kind of people in this world: those that understand binary, and those who don't.

Like ! i am "1".

[Behemot]

Re: Problems with Samsung 740N monitors

i just fix my samsung 740N. Didn‚t have original 3A fuse and replaced it /bridged from other side with isolation/ with -dont know in english, a thin wire, one form cable- one "licna" (here in Serbia, it is very common way ). I know that 1 "licna" is approx about 3,5A-4,5A and my monitor still working. I did it just because of curiosity. The picture is clear, but a have 2 thin vertical lines. I thing that the reason is bad connection or simmilar.

Stranded wire I guess. But this is not really good…if it goes bad again, the problem would propagate further, destroying other components on the way. You can use more or less any type/size of fuse as long as you are able to solder it in somehow

[lexwalker]

Re: Problems with Samsung 740N monitors

Hello, thanks all for your support.

I've installed third party inverter and the unit was able to lit, but it won't receive VGA signal CPU when attached to CPU.
Then I was able to get the exact same scalar board from a friend, turns out it was the MCU. I've swap the memory (A81SC) between two boards, and the old scalar board is still dead.
So, in conclusion, this particular unit got scalar board problem, and case closed, since it is difficult to find MCU replacement in my country, and changing new scalar board just too expensive.

If the Dscaler board ever fail then its always going to be very difficult (searching for a spare board from the same monitor brand and model somewhere). As mentioned earlier, was hoping it wasn't the Dscaler board but unfortunately it was (as you've had found out). Myself also have at least two monitors collecting dust due to Dscaler board problems...

i just fix my samsung 740N. Didn‚t have original 3A fuse and replaced it /bridged from other side with isolation/ with -dont know in english, a thin wire, one form cable- one "licna" (here in Serbia, it is very common way ). I know that 1 "licna" is approx about 3,5A-4,5A and my monitor still working. I did it just because of curiosity. The picture is clear, but a have 2 thin vertical lines. I thing that the reason is bad connection or simmilar.

You can find 3A fuse easily in ATX power supplies used for PCs. Thus can look for them in scrap/faulty ATX power supplies. If those vertical lines stay in the same place all the time, then very likely the LCD panel is bad. Usually either the Tcon board and/or bonding tabs.

[mshone]

Re: Problems with Samsung 740N monitors

No problem,
i repairing lcd monitors
this 740N board is replaced with new psu/inverter board (other model - better one) that works with same monitor, so CCFL are ok and in most such cases CCFL are ok

also i have many monitors that have only bad caps in secondary side, they are bad quality but this is not always main cause of their failing

you can check this Panasonic Reliability chart for Aluminium electrolyte capacitors to see why are all possible causes of failing



Some thing i miss to say about original 3A fuse is that may be an fast reacting fuse, also current consumption for inverter part with ccfl is about 1,75-2A, can't be more for this 17'' monitor, so that current can't blow fuse even fuse is fast

i just fix my samsung 740N. Didn‚t have original 3A fuse and replaced it /bridged from other side with isolation/ with -dont know in english, a thin wire, one form cable- one "licna" (here in Serbia, it is very common way ). I know that 1 "licna" is approx about 3,5A-4,5A and my monitor still working. I did it just because of curiosity. The picture is clear, but a have 2 thin vertical lines. I thing that the reason is bad connection or simmilar.

[agun82]

Re: Problems with Samsung 740N monitors

Hello, thanks all for your support.

I've installed third party inverter and the unit was able to lit, but it won't receive VGA signal CPU when attached to CPU.
Then I was able to get the exact same scalar board from a friend, turns out it was the MCU. I've swap the memory (A81SC) between two boards, and the old scalar board is still dead.
So, in conclusion, this particular unit got scalar board problem, and case closed, since it is difficult to find MCU replacement in my country, and changing new scalar board just too expensive.

[budm]

Re: Problems with Samsung 740N monitors

Service manuals: Notes, there are more than one version of 740N.
http://elektrotanya.com/?q=showresul...oria=&kat2=all

[lexwalker]

Re: Problems with Samsung 740N monitors

I'm new here, so nice to meet you all.
I'm facing problems for 740N as well:

1) Power Led is not on at all, even if I push the power button several times.
2) I changed all three 820uf bulged caps with 1000uf/25v, and changed the 3A pico fuse (broken) , but the power led still not on.
3) I already changed the TL431, power led still not on.
4) All rectifiers are ok.
5) 3.15 fuse is ok
6) I have checked all CCFL with CCFL testers, all lamps light normally.
7) Although the power led is not on, but when I measure the voltage of the output pins (as shown in the pic below, circled.. I borrowed photos from kharr)
the readings are 5,1V and 14,8V. I know this should be 13V. Does 14,8V normal?



Any help please? Thanks a lot.

Sounds like no response from the Dscaler board. Have you tried changing the capacitors for the +5V section? That one is the power supply for the Dscaler board. And just cross your fingers (hoping) that it wasn't a Dscaler problem...