Samsung 912T click then off

[Th3_uN1Qu3]

Re: Samsung 912T click then off

No one has commented on the clicking noise. Any idea which component would click?

Transformer most likely. Because the power supply detects that those mosfets are shorted and enters protection mode.

[jetadm123]

Re: Samsung 912T click then off

Sorry. In my previous post I mentioned pin 6 as the voltage supply for the OZ960, it's actually pin 5. I also didn't realize the board was soldered in place. Ouch!! If you decide to unsolder it again, using your DMM, maybe you can trace back to one of the soldered pins for your measurement. Or, maybe you can trace back from pin 5 to the nearest capacitor or resistor and solder a small wire at that point and route the other end of the wire off to the side where you can take your measurement.

As for the fds4935 chip, it consists of two p-channel mosfets. The chip above it should have 2 n-channel mosfets. These two chips work together with the OZ960 to generate the square wave for amplification in the transformer(s).

I believe the clicking sound you hear is the power supply trying to start up. The sound might coming from a transformer. Try this the next time you turn on the display: place a cardboard tube taken from a paper towel roll and place it over the transformer (large yellow one T601) while listening to the other end. Move to other areas if that wasn't the source of the click. Also, try looking at the power supply in a darkened room to see if there's any arcing during the power on process.

[Dgtech]

Re: Samsung 912T click then off

FH602 is right near that little board. Have you checked this fuse yet? Just checking.

[sceva]

Re: Samsung 912T click then off

FH602 is right near that little board. Have you checked this fuse yet? Just checking.

It is good, although I went ahead and tested it with power on and it read the same on both sides.

Maybe try testing ZD1 and ZD2 next to the burnt mosfet

OK, I hope we are onto something here. With the dmm in diode mode, ZD1 reads .22, and reads the same with I swap the probes. ZD1 reads .72 with the red probe on the mosfet end, and open circuit with probes swapped. I think this means ZD1 is bad? If so, any ideas about what part is needed? Thanks

[sceva]

Re: Samsung 912T click then off

About the clicking noise. I checked and could not see any arcing. The noise sounds loudest over the raised circuit board, not the transformers, but it is difficult to be sure.

[retiredcaps]

Re: Samsung 912T click then off

It is good, although I went ahead and tested it with power on and it read the same on both sides.

Always do resistance tests with power OFF and unplugged!

A good fuse is 0.3 ohms or less. Some multimeters like mine will beep "good" if I use the continuity check feature. "good" is 25 ohms +/- 10 ohms.

So when I measured a picofuse and it was "good" with the continuity feature, but it was actually bad because it was 19 ohms.

With the dmm in diode mode, ZD1 reads .22, and reads the same with I swap the probes. ZD1 reads .72 with the red probe on the mosfet end, and open circuit with probes swapped. I think this means ZD1 is bad?

In order to verify if ZD1 is bad or not, you need to test it out of circuit. In circuit may not give accurate readings because other components or board design may give false readings.

edit: If ZD1 is bad, it may have been the reason that caused the IC to go bad.

[sceva]

Re: Samsung 912T click then off

Always do resistance tests with power OFF and unplugged!

A good fuse is 0.3 ohms or less. Some multimeters like mine will beep "good" if I use the continuity check feature. "good" is 25 ohms +/- 10 ohms.

So when I measured a picofuse and it was "good" with the continuity feature, but it was actually bad because it was 19 ohms.

I had checked one in the past that showed 'good' with continuity, but bad under load, so if I am in doubt, I had been checking voltage coming in and out of the fuse when power is flowing. Good to know to about the 0.3 ohms rule.

In order to verify if ZD1 is bad or not, you need to test it out of circuit. In circuit may not give accurate readings because other components or board design may give false readings.

edit: If ZD1 is bad, it may have been the reason that caused the IC to go bad.

I removed ZD1 and it tested the same as when it was in the board, about .22 both ways. It has a line or groove across it and then it has the letters BC. Is that a part number?

[PlainBill]

Re: Samsung 912T click then off

I had checked one in the past that showed 'good' with continuity, but bad under load, so if I am in doubt, I had been checking voltage coming in and out of the fuse when power is flowing. Good to know to about the 0.3 ohms rule.

I removed ZD1 and it tested the same as when it was in the board, about .22 both ways. It has a line or groove across it and then it has the letters BC. Is that a part number?

As retiredcaps has said, often the threshold for the continuity test is rather high. .3 ohms is a good number to use.

I believe BC is the 'smt package marking'. It corresponds to a 5.6V 0.3W zener diode.

PlainBill

[retiredcaps]

Re: Samsung 912T click then off

I removed ZD1 and it tested the same as when it was in the board, about .22 both ways. It has a line or groove across it and then it has the letters BC. Is that a part number?

ZD1 is defective if it tests 0.22 both ways out of circuit.

I don't know how to read that code. Someone else will have to help out.

[sceva]

Re: Samsung 912T click then off

I looked up the 5.6V 0.3W zener diode at digikey and it looks like the right part.
http://search.digikey.com/scripts/Dk...client_id=5042

I will get one ordered and let you know. Thanks again for the help and education. I am getting the hang of the chip quick and soldering these smaller devices.

[PlainBill]

Re: Samsung 912T click then off

ZD1 is defective if it tests 0.22 both ways out of circuit.

I don't know how to read that code. Someone else will have to help out.

Google smd zener package marking bc.

PlainBill

[retiredcaps]

Re: Samsung 912T click then off

I will get one ordered and let you know.

I wonder if you need another FDS4935A now that we know ZD1 is bad? I know you already replaced it, but if you tested the new FDS4935A with the bad ZD1, I wonder ZD1 had destroyed the new FDS4935A?

[sceva]

Re: Samsung 912T click then off

I wonder if you need another FDS4935A now that we know ZD1 is bad? I know you already replaced it, but if you tested the new FDS4935A with the bad ZD1, I wonder ZD1 had destroyed the new FDS4935A?

Thanks for the heads up. I have a spare, usually buy a couple of extra when the parts are inexpensive. I also have a couple of the small resistors that are inline with these parts. Sometimes I do more damage than good with the soldering axe.

[sceva]

Re: Samsung 912T click then off

Another LCD temporarily saved from the recycle bin. (I hope, still testing...)

The parts came in the mail, and when I got out the zener diode, I realized that it was almost 1/2 the size as the original... I was concerned, but thought I would try it since the specs should be the same. It was a little difficult but I managed to get it soldered in good. So a reminder to folks - always check the dimensions before ordering!! Fired it up and it works very well.

One other question if anyone has time. Right above and below ZD1 and ZD2 are two smd's with the numbers 103 on them. I thought they were resistors or diodes. When I test them on the board, they read like diodes. About .6 one way, and open circuit with probes reversed. I removed the one near ZD1 to check it as it was in line with ZD1 and the blown mosfet. When it was off the board it reads an open circuit either way the probes are connected. I thought I had damaged it while removing it from the board, so I took another one off an old board and it did the same thing. When I replaced it on the board it tested about .6 again. What is this, and is it possible to test it off the board??

Thanks again to everyone who helped with this one.

[retiredcaps]

Re: Samsung 912T click then off

One other question if anyone has time. Right above and below ZD1 and ZD2 are two smd's with the numbers 103 on them. I thought they were resistors or diodes.

I believe 103 is a resistor.

It is 10 (with 3 zeros) or 10000ohm 10k ohm resistor. So it should read about that.

Glad you got the monitor working.