Samsung 940N 2 Seconds fade to black

[Setsuna]

Re: Samsung 940N 2 Seconds fade to black

Hi guys

Long time lurker here, I don't even remember if I ever posted or not...

Anyway, I have two monitors, 940N and 943N, with this type of problem. They both turn on, then lose the backlight after about a second.

First I cracked open the 940N, and, predictably, there were bulging C(r)apXons on both the MCU and inverter supply rails. Replaced every cap with corresponding Rubies, ZLJ series (CapXons were GL and KM (I think) series). The problem is now downgraded to "backlight off". Measured the inverter transformers, primaries have negligible resistance, secondaries are at about 1.2 kOhm, they differ on the third digit, so it's all neat. CCFLs are good, tested with known good 650V source. Baffled.

Then I took the 943N, thinking I may have more luck with that one. At first the backlight didn't come on at all, but I could see the panel producing a picture without error. Elite caps inside, all good. Took out a couple, ESR is fine, capacity is fine, put them back in (against better judgement, but I figure they aren't the problem, just like the CrapXons weren't in the 940). So I measure the inverter transformer and find that one of the secondaries is open-circuit. So I take a spare one (with ~1.13 and ~1.15 kOhm on the secondaries, a couple of Ohms on the primary), solder it in and... Now I've upgraded the problem to "blank after a second". Baffled.

Then I took to the multimeter. It turns out that the 13V rail (on the 940N) and the 14V rail (on the 943N) are too high! On the 940N this rail reads ~17V, and on the 943N it's around 16.5-16.8V. The voltages are pretty stable, meaning no fluctuations greater than ~2%. I measure the same voltage across the whole path, from the Schottky rectifier to the control board connector.

So, where do I start?

[PlainBill]

Re: Samsung 940N 2 Seconds fade to black

Start by splitting the thread. Most of us will refuse to assist on two monitors simultaneously on a single thread - it is just too confusing. It doesn't cost anything to open a new thread for the 943N.

Pictures help us help you. I have a couple of questions about the layout and components on the 940N board, but I'm not going to spend my time searching for pictures that might not be the correct ones. Pictures should be taken from directly over the board, as close to 2000 pixels on a side (without going over), sharp focus, and well lit. I prefer one picture showing the overall layout of the monitor, and in these cases one of the top of the power supply and one of the bottom. Attach using 'Manage Attachments' - below the test entry area.

On the 940N monitor, there are several possibilities. If the problem went from 'two seconds to black' to 'always black', look for a solder splash or similar problem shorting something out, or a blown fuse - probably a surface mount fuse.

The voltages are normal - the design of the power supply results in a higher output voltage when the backlights are off.

PlainBill

[820uF]

Re: Samsung 940N 2 Seconds fade to black

Good job. The transformer was just on the edge of the 3%.

is there any way to fix this transformer ?
Tms91429ct

[Jasgriff]

Re: Samsung 940N 2 Seconds fade to black

Добро пожаловать на форум,

Nope when they are gone they are gone. You just need to order a new one.

Russian Link
http://search.eim.ebay.ru/?ev=&kw=Tm...&hidden=search

UK Link
http://www.ebay.co.uk/itm/TMS91429CT...item3cc55ec970

[820uF]

Re: Samsung 940N 2 Seconds fade to black

Data printed on broken transformer is:
TMS91429CT
●K 07TM12 F

What does the second line mean?
Can this transformer be replaced with
TMS91429CT
●M 09TM51 D

or

TMS91429CT
●M 09TM51 E

?

[Jasgriff]

Re: Samsung 940N 2 Seconds fade to black

The second line is normally a date stamp or a manufacturing reference number. As long as the actual model number is ok then you should be fine.

[820uF]

Re: Samsung 940N 2 Seconds fade to black

Спасибо

[Blaskoo]

Re: Samsung 940N 2 Seconds fade to black

Thank you very much. Who has a problem with remission after 2 seconds, a change in the electrolytic capacitors all, there are 4, and transformer. That is the biggest problem. I've exchanged a monitor and it works perfectly. Thanks again BADCAPS
Best rgds from Serbia,
Milos

[BoMarK]

Re: Samsung 940N 2 Seconds fade to black

[QUOTE=jack-pwc;139286]So my 2007 died last night after much trouble free service.
Hi, I had the same problem with Samsung 940N, picture starts and after 2 seconds, turned off. I checked with strong flashlight and see that the picture exists on display, but no back lights on, which means that the power supply works. I changed all caps (they are very cheap, and it is good to do that as a prevention), then I measured the primary and secondary coils on high voltage transformer TMS92515CT, and found some differences with resistance on secondary, one is 1125, and the second 1246ohm. Primary should be under 3 ohm. Then I extracted the defected high voltage transformer TMS92515CT out and soldered a new one, and everything works just perfect!
But, before that, I found on some Russian webs that there is a way to check if the CCFL lamps are okay, and for that you need to find diodes D10 and D11 on the board (CA2 - 3 pin double diode BAW56), and pull them out (there are in a protection circuit which, if something's wrong, send a feedback to IC STU407D to shut off the power supply,). Then try without diodes, but that test should be very short.., and if CCFL lamps works, let say for 7 seconds, turn monitor off, and exchange defected high voltage transformer TMS92515CT.