Samsung SyncMaster 953BW psu fault?

[joshnz]

Hi I obtained this monitor from my dad.
This monitor started turning it's inverter off after 1 -2 seconds, I found that by using a torch I could the desktop was still been displayed.
so i brought up the menu and adjusted the brightness from 100 to 90 pushed the power button then turned it back on it worked like that for a few weeks then done the same so turned the brightness down to 60 worked then done the same in till once set to 0 still won't stay on.
So once they got a new monitor I got this one.

So I have replaced all the caps on the secondary side still same issue.
I measured the voltage on the 14.5 rail it was high once the inverter try's to start and then stops the rail jumps 18 odd volts?? then slowly lowers.
I disconnected the diodes on the 14.5 rail and connected a 16v power supply I figured what harm can i do the rail goes up to 18 volts with that setup.
The back lights stay on and the monitor runs as usual.
Here is some other information the 5v rail is 5.16.
to me it seems the psu is having trouble providing the current needed to start the inverter also the 14v rail dips down to 12 volts then overshoots to 18.
Any help would be much appreciated.

[Rtech]

Re: Samsung SyncMaster 953BW psu fault?

Have you substituted a 'test backlight',in place of the originals( does not need to be inside the Screen,) just alternatively plugged into each of the lamp sockets ?????, that would seem to be your next step.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

I don't have any to test with.
I think you have may miss understood i was using the inverter built into the power supply module I just fed the inverter from a power supply I had and it worked fine. just won't using the monitors psu.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

The inverter control ic is OZ9939 using STU309DH to drive the transformer.

[Rtech]

Re: Samsung SyncMaster 953BW psu fault?

Did not misunderstand.When you fed the inverter using the external Power Supply, this was a higher voltage than than the original, therefore after going through the output transistors, AND then being greatly increased by the Step up Inverter trannsformers, you actually applied a larger 'Strike' voltage to the lamps, than you were getting from the original...this would signify that one or more of the Lamps are faulty as they are not striking as they should...the only way to test is via a spare good Backlight.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

OK thanks
hmmm now I think should I modify the vsense to the ic to get more life from the tubes, or try and source compatible tube/tubes for testing or replace.

Another question is why is the psu overshooting when the load is removed.

That explains why it would not run with the psu set to 15volts.

anyone know why they shut down the inverter is a tube didn't start?

[alexanna]

Re: Samsung SyncMaster 953BW psu fault?

OK thanks
Another question is why is the psu overshooting when the load is removed.
anyone know why they shut down the inverter is a tube didn't start?

That voltage rail is not regulated, it kind of floats around until a load is applied. If it was not correct when a load is applied that would be the time to investigate why.
In the case of a defective CCFL causing shutdown of all the CCFLs ,it's a safety feature.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

oh ok I would have thought it is meant to be due to the 5.1v and 14.5 rail are both been used in a voltage divider in the feedback circuit.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

Good news I believe I have identified the fault.
I took the lcd apart and removed the CCFLs I proceeded to then attach them to the inverter/psu and the CCFLs connected to cn605 started but the ones connected to cn604 did not. So I swapped them same problem.
I then measured he resistance on the transformer secondary for cn605 that was 1266 Ω
and secondary for cn604 was 1162 Ω

So I guess it is safe to order a new xfmr.

It is a TMS92903CT

[PCBONEZ]

Re: Samsung SyncMaster 953BW psu fault?

Try resoldering the pins first. - Or at least look at it.
Not unusual for the wave soldering they use to leave cold joints on larger connections.
.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

Ok will resolder but there is signs of the xfmr been hot.
Will let you know tomorrow.
So I take from that the resistances seem fine.

[PCBONEZ]

Re: Samsung SyncMaster 953BW psu fault?

So I take from that the resistances seem fine.

No, but a cold joint can screw up the reading - and create heat.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

The heat mark was on the label.
I re-soldered the pins on the xfmr and checked the resistance after and was the same.
I removed the xfmr and was the same.
I then took the sticker off and found this look at pics to see.
Left pic is the side that starts the CCFLs right side is the one that doesn't.
1266 Ω 1162 Ω

[jetadm123]

Re: Samsung SyncMaster 953BW psu fault?

The heat mark was on the label.
I re-soldered the pins on the xfmr and checked the resistance after and was the same.
I removed the xfmr and was the same.
I then took the sticker off and found this look at pics to see.
Left pic is the side that starts the CCFLs right side is the one that doesn't.
1266 Ω 1162 Ω

There's more than a 3% difference in the resistance readings between the two transformers. Your photo is a little blurry, but it looks like one of the windings is burnt. These transformers look to be readily available (and low cost too) on Ebay, which probably tells you something.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

Those were taken from my mobile phone my digital camera is worse.
Well in my opinion is that is the fault shorted/burnt turns.
So far I have located a replacement for 9 USD which is 12nzd
and a ebay one for 6.16NZD

[PlainBill]

Re: Samsung SyncMaster 953BW psu fault?

Those were taken from my mobile phone my digital camera is worse.
Well in my opinion is that is the fault shorted/burnt turns.
So far I have located a replacement for 9 USD which is 12nzd
and a ebay one for 6.16NZD

Definitely bad. The entire group of windings are scorched.

PlainBill

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

righto off I go to order/buy a new one.
Thanks for everyone's help and opinions.
let you guys know when the new xfmr arrives in about four weeks lol.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

Soldered in new xfmr all go
The new Resistances are 1316Ω and 1323Ω
Thank you to all of your input.

[joshnz]

Re: Samsung SyncMaster 953BW psu fault?

Well looks like It needs a new transformer again can't run it at 100% brightness.

[Rtech]

Re: Samsung SyncMaster 953BW psu fault?

Maybe the lamps this time ??? you will have to measure the transformer again.