Samsung 193v backlight doesn't stay on

[sceva]

When first applying power the monitor works fine for one or two seconds, then the backlight goes out. With a flashlight I can see the computer image, so the display part seems to be ok. I don't see any bulging caps or hot spots on the board... I have seen another one of these where the two larger caps in the upper right corner were bulging.

Anyone have an idea of where to start looking?

[PlainBill]

Re: Samsung 193v backlight doesn't stay on

When first applying power the monitor works fine for one or two seconds, then the backlight goes out. With a flashlight I can see the computer image, so the display part seems to be ok. I don't see any bulging caps or hot spots on the board... I have seen another one of these where the two larger caps in the upper right corner were bulging.

Anyone have an idea of where to start looking?

This is referred to as the 'Two seconds to black' problem. Basically, the inverter controller detects a fault condition and shuts down. Possible causes are a defective CCFL, wiring problems to the CCFL, a bad transformer, or a flaw in the protection circuit that falsely indicates a problem.

The first step is to measure the resistance on the transformer secondaries. If one is different, that is most likely the problem. If you need any more help I will need better (larger) pictures of both the front and back of the power supply / inverter.

PlainBill

[sceva]

Re: Samsung 193v backlight doesn't stay on

Thanks Bill, I checked the transformers and they all read the same. Taking your suggestion I plugged in some other working ccfl's and they came on and stayed on - although I only left them on for about 10 seconds each time I tested as I could hear some noise coming from the board when they were on. Here are some better pics.

[PlainBill]

Re: Samsung 193v backlight doesn't stay on

Thanks Bill, I checked the transformers and they all read the same. Taking your suggestion I plugged in some other working ccfl's and they came on and stayed on - although I only left them on for about 10 seconds each time I tested as I could hear some noise coming from the board when they were on. Here are some better pics.

Those pictures are better, but still pretty marginal. There are two things to try.

First, try substituting only one working CCFL for one of the originals. If three of the original CCFLs stay on for 10 seconds when the fourth is replaced with a 'working' CCFL you have identified the culprit. I fully agree, don't leave the setup on too long, particularly if you are hearing strange sounds.

The other approach is a little more tricky. Look at the back of the card. Note that one of the two pins of the CCFL sockets are covered in a white insulation. Those are the drive pins. The other pins are the return pins. Try to measure the voltage from each return pin to ground while the original CCFLs are hooked up. Again, we are looking for one voltage that is different than the rest.

PlainBill

[sceva]

Re: Samsung 193v backlight doesn't stay on

I tried again today and had a little different results - First I connected two pairs of working CCFL's and they worked for several minutes, and no noise this time. I then reconnected the original CCFL's for the bottom of the screen, leaving the working CCFL's plugged in where the top ones go, and again the monitor worked well. Then I tried swapping either of the top CCFL's to see if one or the other of the pair was causing the trouble - and found that if either of the original top CCFL's are connected it is 2 seconds to black.

But what was unusual is when I have the working CCFL connected to the top and I touch the metal 'skin' that wraps around it on three sides (it came out of another panel) it immediately goes to black. It only happens if I touch it with my finger. Touching it with an insulated metal tool has no effect. I tried another working CCFL and got the same result. When I have a working CCFL on the bottom and touch it there is no problem.

Anyone ever see this before? Thanks.

[sceva]

Re: Samsung 193v backlight doesn't stay on

I finally figured out that one of the two transformers powering the upper ccfl had no output. When I connected the external ccfl it would only light one of the bulbs, but possibly because it was not grounded to the lcd it did not cause the feedback circuit to shut it off?

I am repairing several of these monitors, and so far 3 of them have had the transformer in location T3 on the board that did not work. So far I have replaced two of them and both monitors are working well. My question is, what would cause this particular transformer to fail and not one of the other three? Should I possibly need to look for something else? The primary voltages on the transformers range from 3.5 to 4 volts on the different monitors, and the shorted transformer doesn't have higher voltage than the working ones.

[sceva]

Re: Samsung 193v backlight doesn't stay on

I might have spoken too soon. LCD #4 was at location T4. On the first one I tried resoldering it without success. The rest I have just been replacing.

[PlainBill]

Re: Samsung 193v backlight doesn't stay on

I finally figured out that one of the two transformers powering the upper ccfl had no output. When I connected the external ccfl it would only light one of the bulbs, but possibly because it was not grounded to the lcd it did not cause the feedback circuit to shut it off?

I am repairing several of these monitors, and so far 3 of them have had the transformer in location T3 on the board that did not work. So far I have replaced two of them and both monitors are working well. My question is, what would cause this particular transformer to fail and not one of the other three? Should I possibly need to look for something else? The primary voltages on the transformers range from 3.5 to 4 volts on the different monitors, and the shorted transformer doesn't have higher voltage than the working ones.

Winding transformers with fine gauge wire requires very good technique. When they are working with 18 ga. wires there are few problems. But when you are working with very fine wires it is easy to damage it where it goes into the bobbin or around the post on the core. If there isn't enough slack the normal expansion and contraction with temperature changes may be enough to break the wire.

PlainBill

[sceva]

Re: Samsung 193v backlight doesn't stay on

Thanks for the help with the transformers.

I would like to fix one more of these monitors, but it has a different problem. The fuse one the inverter side keeps blowing. I haven't found any dead shorts in the transformers, which from the other boards I suspected right away. I did locate a shorted surface mount component and replaced it, but it didn't clear up the problem completely. The component is marked with a red x in the pick. I am running out of ideas on how to test. I know some folks recommend using a regulated ps to trace these down but I don't have one.

I didn't want to keep blowing fuses, so to test to see if the short has cleared I am comparing ohm/esr readings from a working board. Before I replaced anything all the caps in this area were showing 10 on the esr meter. after replacing the component they have dropped to around 8. So I am assuming there is still a circuit or component shorted somewhere. I just don't know where to look.

[sceva]

Re: Samsung 193v backlight doesn't stay on

I took a break, and when I came back to it, I didn't see any other posts, so I thought, what most likely is wrong? Then I thought of the name of this website, and decided to drop a few caps in this area and test them with my new Anatek Blue ESR meter, and incredibly (for me at least) the first cap I removed from the board and tested was bad. I replaced it and everything is awesome. It was the larger cap to the left of the fuse.

[FIXITNOW]

Re: Samsung 193v backlight doesn't stay on

whats it value is the pcb marked with location C**
thanks

[sceva]

Re: Samsung 193v backlight doesn't stay on

I am not sure what you mean. Can you tell me about where on the board I should be looking?

[FIXITNOW]

Re: Samsung 193v backlight doesn't stay on

the capacitor you changed must of had a value and a voltage and the pcb where you fitted it some sort of marking to show

[FIXITNOW]

Re: Samsung 193v backlight doesn't stay on

like this 220u 25volt c12

giveing full data helps other that find this web site in the future
know what you did just say changed cap near fuse is not enought to help others
and this site is all about helping others


ps picture example from other posting

[sceva]

Re: Samsung 193v backlight doesn't stay on

Sorry about the mis-communication, I was kind of rushed when I made the last post. The cap in question was at C17, and is 220uf, 25v.

As for others looking for help with this model, I have had about 15 of them and 13 have had a bad transformer at either T1, T2, T3, or T4 on the inverter part of the power board, and the only symptom was the 2 seconds to black (the backlights and osd come on for about 2 seconds, then the backlights go out but the osd remains)

To quickly find which transformer is faulty:
open up the back cover
disconnect the 4 connectors to the ccfl's
attach two working ccfl's to the board
turn it on and note which tube does not come on - then replace the faulty transformer. When connected to the external ccfl's the lcd will not shut down for some reason so you have plenty of time to see which one is bad.

A couple have had bad components on the bottom of the board, labeled U2, U3, U4, or U5, although it takes more skill to replace them.

Two have had shorts in the inverter area that I was unable to locate, so I used those boards as spare parts for the others.

And two or three have had faulty main or logic boards - when the lcd was on for a couple of hours vertical lines would appear on the screen and the rest would go black. I had spare boards so I just replaced them and did not try to diagnose or repair them.

Of course there were bulging caps in about 1/2 of them, usually at C551 and C661.

Hope this helps.

[FIXITNOW]

Re: Samsung 193v backlight doesn't stay on

nice work thanks for the info good detail
cheers

[canuckbc]

Re: Samsung 193v backlight doesn't stay on

Tried your ideas and it looks like a transformer is faulty.

Can you tell me who supplies replacements?

[sceva]

Re: Samsung 193v backlight doesn't stay on

I never had to order any, just salvaged parts off of other boards, but I did notice that they were available here:
http://www.lcdparts.net/TransformerD...ProductID=2797

[sceva]

Re: Samsung 193v backlight doesn't stay on

I have just ran across several monitors (a different make and model) that had bad ccfl's but good transformers. While looking for ccfl replacements I saw some comments that a faulty ccfl can possibly cause a transformer to fail. Can anyone confirm/deny this?

Also, is it possible to test a ccfl and see if it is working without removing it and looking at it? Most of the bad ones I have ran across one of the two bulbs is working, and when looking at the display it is hard to tell that one is out... thanks.

[canuckbc]

Re: Samsung 193v backlight doesn't stay on

Thanks for the quick reply. I decide to try your transformer test:

To quickly find which transformer is faulty:
open up the back cover
disconnect the 4 connectors to the ccfl's
attach two working ccfl's to the board
turn it on and note which tube does not come on - then replace the faulty transformer. When connected to the external ccfl's the lcd will not shut down for some reason so you have plenty of time to see which one is bad.

The two external ccfl's come on and flood the alternative external screen (not physically grounded to the PCB of the faulty unit) using two transformers but only slightly come on when attached to the other two transformers. However, in either case, they still turn off after two seconds. Would you suggest replacing the two tranformers or due to the fact that they still cut out in two seconds that the issue is elsewhere. By the way, I have replaced caps, 551, 660, 552, 662, 665.