Samsung 913V blinking LED

[Half-Saint]

I was given the monitor for free, the guy said that it wouldn't turn on. So I opened it up and found two bad caps 820uF 25V and the fuse was blown. I replaced the fuse as well as the caps. The monitor now powers up but doesn't display an image. All I get is the blinking power LED and a high frequency sound which goes louder then stops with a click. The sound and LED are synced. The sound is coming from the PSU.

Should I replace the rest of the caps? Visually they seem fine.

I measured the big cap and it shows 320V on the multimeter. As soon as I pull the plug, the voltage drops rapidly to almost zero. I remember getting a shock from a big 450V cap which was just sitting there for a week. Is this a sign of a bad cap?

EDIT: just desoldered the big cap and it reads 100uF 450V. Must be bad, if it only reads 320V while the monitor is powered up.

Cheers

[R_J]

Re: Samsung 913V blinking LED

No its not bad, they usually rate capacitors to be a higher voltage than what they are working at. Popst some pictures of the board.

[Half-Saint]

Re: Samsung 913V blinking LED

I don't understand why it loses the charge so fast. That's what got me thinking it must be bad... normally this kind of caps hold charge for a long time. I'll take a picture and post it tomorrow.

[Half-Saint]

Re: Samsung 913V blinking LED

OK here's a pic:

[Agent24]

Re: Samsung 913V blinking LED

If it has reduced capacitance or increased ESR it may not hold a charge. Test it.

[Half-Saint]

Re: Samsung 913V blinking LED

Haven't got a tester.. I'll see if I can find someone to borrow one.

EDIT: I thought the discharge could be due to a bleeder resistor. The only resistor nearby is a 37kOhm right next to the cap and it's in series with the cap. Shouldn't a bleeder resistor be in parallel with the cap?

[Half-Saint]

Re: Samsung 913V blinking LED

I replaced the big cap but the symptoms remain.

[bammbammfran]

Re: Samsung 913V blinking LED

BLinking light could be low power or a short on the inverter side

DO you have 5 volts at the wires that lead to the main board with the main board disconnected ?
If you do, plug the main board in and see if it is still there.
IF the 5 volts is low, replace the small startup capacitor that is at the end of the large capacitor and near the one heatsink

[Half-Saint]

Re: Samsung 913V blinking LED

DO you have 5 volts at the wires that lead to the main board with the main board disconnected ?

Yes.

If you do, plug the main board in and see if it is still there.
IF the 5 volts is low, replace the small startup capacitor that is at the end of the large capacitor and near the one heatsink

Yes again. As soon as the LED turns off, the voltage goes to 0V for a second then as the LED turns back on, the voltage follows.

[bammbammfran]

Re: Samsung 913V blinking LED

can you post a picture of the back side of the board ?

also, is the 13 volts doing the same thing ?

[budm]

Re: Samsung 913V blinking LED

Did you check and replace the start up cap to the left of the SMPS ICI IC101 heat sink as shown in your picture?

[Half-Saint]

Re: Samsung 913V blinking LED

Did you check and replace the start up cap to the left of the SMPS ICI IC101 heat sink as shown in your picture?

I have no equipment to actually check the caps. The cap you're referring to is 47uF 50V and it hasn't been replaced yet. I don't have any such caps lying around and would have to order one specifically for this purpose.

[budm]

Re: Samsung 913V blinking LED

Try heating it with hair dryer (no power applied to unit) for a few minutes, then apply the power.
You already verify the three rectifier diodes (one on the heat sink), and two axial-lead type? Also need to check the SMD Power MOSFETS which drive the inverter high voltage transformers to see if they are shorted out or not.

[Agent24]

Re: Samsung 913V blinking LED

The only resistor nearby is a 37kOhm right next to the cap and it's in series with the cap. Shouldn't a bleeder resistor be in parallel with the cap?

It may be going through something else as well or that might just be the startup resistor for the PWM controller. Where does the other end of the resistor go?

[budm]

Re: Samsung 913V blinking LED

I will not worry about the bleeder resistor at this point, the SMPS IC circuit is connected to the filter cap so when the AC is removed, the IC is still running, it is an always on power supply, it will keep on running until the voltage threshold is too low for the SMPS IC to run.

[Agent24]

Re: Samsung 913V blinking LED

I was just thinking in terms of if Half-Saint wants to know what that resistor is for

[budm]

Re: Samsung 913V blinking LED

If he can show good clear pictures of the bottom side of the board and the designator of that resistor, then we probably can trace out the function of that resistor.

[Half-Saint]

Re: Samsung 913V blinking LED

Here's the picture of the bottom side. Please ignore the half ruined fuse solder pad. I used wick to remove excess solder and it almost cost me the solder pad...

The resistor in question is designated R106 and is soldered perpendicular to the PCB.

[bammbammfran]

Re: Samsung 913V blinking LED

I ask again: do you have the 13 volts ?

also: when you checked the 5 volts that was going on and off along with the led - did you check it at pins 6 and 7 or at pin 9

[budm]

Re: Samsung 913V blinking LED

Lift up one end of the jumper J308 (this will isolate the inverter power circuit from the power supply) and see if the 5V/13V become stable.