Samsung 743BM: "2 seconds to black" - transformer failure?

[d.free]

Hi,

I have a Samsung 743BM that's facing the (famous) "2 seconds to black" problem. Like many others, the backlight operates normally for 2 seconds after poweron, then turns itself off. The LCD itself works fine as in that I can see the image if I shine a flashlight onto the screen after it blacks out, and the monitor menus and GUI is operational.

I've checked the fuses and they appear in order. The caps look fine, and while they arent good brands I do not intend to replace them unless they are isolated as the problem.

Then, I measured the transformer (TMS92515CT) in-circuit and came up with the following measurements:

Primary Coil: 0.7 ohms
Secondary Coil 1: 1392 ohms
Secondary Coil 2: 1112 ohms

It appears that there is a clear mismatch between the two coils, so should the transformer be replaced? Are there any other parts that could also contribute to the problem, and should be tested as well?

I have a multimeter at home, but no ESR meter or scope for any additional tests that may be required.

Board pics are located HERE.

Thanks!

[stj]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

how old is it?

[retiredcaps]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

Then, I measured the transformer (TMS92515CT) in-circuit and came up with the following measurements:

Secondary Coil 1: 1392 ohms
Secondary Coil 2: 1112 ohms

It appears that there is a clear mismatch between the two coils, so should the transformer be replaced? Are there any other parts that could also contribute to the problem, and should be tested as well?

Yes, the transformer is bad and needs to be replaced. Obviously, you should desolder it and verify the readings out of circuit.

$2.80USD or $3.55 CDN (thanks to the falling CDN$) + free shipping will get you a replacement.

http://www.ebay.ca/itm/261656052934

I have no affiliation with the seller so caveat emptor. I just sorted by lowest price + shipping on ebay.ca.

As for other causes, see the 2 seconds to black sticky/faq.

edit: 1.1K ohm is likely the correct reading as per

http://www.lcdparts.net/TransformerD...ProductID=3717

lcdparts.net wants $21.99 USD + shipping for the part which makes it not economical to repair especially for a 17 inch monitor. On the other hand, $3.55 CDN ...

And yes, I have ordered transformers from ebay and they work.

[d.free]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

how old is it?

The sticker on the back says 2008. I got it for free and am unsure of how much the original owner used it.

Yes, the transformer is bad and needs to be replaced. Obviously, you should desolder it and verify the readings out of circuit.

$2.80USD or $3.55 CDN (thanks to the falling CDN$) + free shipping will get you a replacement.

http://www.ebay.ca/itm/261656052934

I have no affiliation with the seller so caveat emptor. I just sorted by lowest price + shipping on ebay.ca.

As for other causes, see the 2 seconds to black sticky/faq.

edit: 1.1K ohm is likely the correct reading as per

http://www.lcdparts.net/TransformerD...ProductID=3717

lcdparts.net wants $21.99 USD + shipping for the part which makes it not economical to repair especially for a 17 inch monitor. On the other hand, $3.55 CDN ...

And yes, I have ordered transformers from ebay and they work.

I'll desolder and report back.

[stj]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

personally i'd just replace the invertors and tubes with an led kit for about $11 (usd)

http://stores.ebay.com/lcd-led-parts...sub=5733816011

[d.free]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

I'll try replacing the transformer first to see how it goes, if not I might try the led kit

Ok, so here are my measurements (desoldered)

Primary Coil: 0.8 ohms
Secondary Coil 1: 1415 ohms
Secondary Coil 2: 1111 ohms

I also tested for shorts, of which there were none. Looks like I'll be buying a new transformer.

Thanks!

[Agent24]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

Transformer sounds dodgy alright but the backlights\backlight wiring and\or capacitors could be bad too.

If you need new lamps, you might as well go CCFL if you're buying a new transformer anyway - you'll have to tear the panel down in either case, and personally I wouldn't trust cheap LED backlights.

There seems to be a growing number of cheap LED-backlit TVs popping up on the forum here with bad LED strings within a couple of years of purchase.

[d.free]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

How would a bad backlight/backlight wiring be the problem? After all, the light does turn on every time, except its only for 2 seconds. If the backlight or wiring is broken, then the light would not turn on at all.

[budm]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

The lamp will still flash due to the fact that high Voltage can easily jumps the small air gap.
Here are some pictures of bad lamps wiring.
http://s807.photobucket.com/user/bud...?sort=3&page=1

[Agent24]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

How would a bad backlight/backlight wiring be the problem? After all, the light does turn on every time, except its only for 2 seconds. If the backlight or wiring is broken, then the light would not turn on at all.

Most (if not all) CCFL inverters power on the lamps for a second or two, measure current draw etc and then shut down if there is a problem.

A 17" LCD like yours will typically have 4 lamps (2 top, 2 bottom). Only one needs to be bad for a 2-second shutdown problem.

If the wire\lamp is open - inverter senses a lamp is not drawing current - it shuts down.
If the wire\lamp is shorted to ground - inverter senses overcurrent draw - it shuts down.
If the lamp is EOL (end of life) the current draw will be too high on startup - inverter senses overcurrent draw - shuts down.

Since all lamps do not typically fail simultaneously, you get light from all the lamps except the bad one, for 2 seconds or so, until the inverter shuts down due to the bad one.


Only if all the lamps were bad (or more likely, the inverter is faulty) - would you get no brief flash of light at all.

[d.free]

Re: Samsung 743BM: "2 seconds to black" - transformer failure?

The transformer arrived today; I installed it and the monitor works fine. Thanks!