Hello to all, I need some help fixing the power supply of the mentioned monitor. I turn on the monitor, it is working fine for a second or two, and then the backlight goes off.

The problem is with the inverting part of the suply that powers the backlight of the monitor (650 volts AC).

I have come to the conclusion that the problem is somewhere around the IC SEM2005, which is the driver for the invertor. It gives oscilations to the gates of MOSFETs to convert DC to AC voltage, that is raised from 12V to 650V. My driver was NOT oscilating on the pins 8 and 9 that drive the MOSFETs.
I thought it was the driver, I changed it but the problem remained.

Now, somewhere I read that the pin 1 of the IC, which is Shut Down Timing Capacitor, (basicaly the pin is connected to GND with a SMD capacitor) is some kind of protection, and can be bridged with a 82K resistor to GND. I did that and I got the required voltage on the output pins.

Now, to anyone who has experience with this driver or with these power supplies:

Is this a permanent solution? If no, what is causing the driver to malfuction, is it the capacitor? I am stuck for days, I cannot realise the purpose of this pin and the capacitor itself.

Thank you in advantage,
Meth.

 

Read datasheet, pin 1is shut down..

 

Bro I read the datasheet 15 times, my question is, is the capacitor causing the malfunction, and can the supply work normally with a resistor parallel with the capacitor. I do not have experience with backlight power supply and I dont wanna burn em.

Thanks.

 

Can you see anything on the screen before it shuts down ?
And is it shutting down or back light shutting down without monitor going to stby ?

Post some pictures.

 

You probably have bad CCFLs. The shutdown is being expressed for a reason.

 

Yes I can see the monitor working fine for a second or two and then it shuts down.

I dont think the problem is with the CCFLs because I am measuring the voltage of the power supply with the lamps disconected. The part with 3,3V, 5V and 12V is working perfectly, the part with the inverting voltage is malfunctioning. The IC SEM2005 stops giving oscilations to the gates of the FETs after 1-2 seconds. When I connect the Pin 1 to GND with a resistor it is working fine.

I repeat, these malfunctions happen WITHOUT the CCFLs connected.

 

Your shutdown will also occur WITHOUT the CCFLs connected. This is interpeted as an OPEN CCFL and a failure mode is flagged.

 

Yes I thought that might be an option, because what I am doing is bypassing the shutdown mechanism. So what is your suggestion, that I change the CCFLs?

And I am also not sure about this, if I turn on the monitor with the Pin 1 bypassed with a resistor, can something go terribly wrong? Because the IC will not be able to shut-down itself.

 

You have a Inverter or CCFL issue. Most of the time it's the CCFL

 

Tear down your monitor and examine the connections to the CCFLs. I've seen quite a few monitors where the CCFLs had bad solder joints at the ends. You might be lucky and just have to reflow the ends (be careful, as too much heat will damage the tubes).

This is the equivalent to open heart surgury - a bad surgeon will kill the patient.

 

Or get a heart attack instead when you break the CCFL.

 

According to Samsung T220hd Manual:

• Remove Stand
• Remove All Screws That Attached the Back Casing of the TV
  
• Remove Wiring Harnesses From Circuit Board
  
• Remove Screws Holding Down Circuit Board
  
• Identifying the Problem Capacitors
  
• Remove Capacitors
  
• Time to Go Shopping
  
• Time to Install New Capacitors
  
• Reverse the Process

 

I suspect he ditched that monitor long ago.