Fail in LED Output of Power Supply in LG 42LB5600

[jfhojat]

Hi, I have LG 42LB5600 TV. When I power on without LED Strip, LED+ and LED- Voltage changing around 0 to 48 V just a few second but after about 4 sec it fixed to zero. I chassis LED- route and see 114 v diff in Drain of Q803 and LED+.
I test Q803 Transistor and it seems OK. The Gate Voltage is Zero and the transistor is off. I Check D803 and every part to BD9486F (IC801) but the Dimout Pin has 0V. I cant find problem. Can anyone help me?

[budm]

Re: Fail in LED Output of Power Supply in LG 42LB5600

Please report LED connector (connector in place) pin Voltage with ref to chassis GND.
When making the measurement you need to leave the meter's probes in place then turn the TV on to see how high it jumps up to and then wait 10 seconds to see how low it goes down to.
BTW, DRV-ON (BL-ON) control signal (>2.5VDC) is present when the TV is turned on, correct?
You can also see the LED Voltage chart that shows the loaded Voltage.
We also need to see the picture that shows how may wires there are on that LED connector.

[jfhojat]

Re: Fail in LED Output of Power Supply in LG 42LB5600

V(LED-)-V(GND)(Maximum in t=0~10s)= 0.2V

V(LED+)-V(GND)(Maximum in t=0~10s)= 288V

V(LED-)-V(GND)(t>10s)= 0.1V

V(LED+)-V(GND)(t>10s)= 120V

V(DRV-ON)-V(GND)=3.47V

Voltage of output must be 208V,235mA.

[R_J]

Re: Fail in LED Output of Power Supply in LG 42LB5600

You have an open led string. The voltage goes high because there is no load (open led string) the circuit detects the open circuit and turns the boosted voltage off so it drops to 120v

[budm]

Re: Fail in LED Output of Power Supply in LG 42LB5600

you have an open led string. The voltage goes high because there is no load (open led string) the circuit detects the open circuit and turns the boosted voltage off so it drops to 120v

+1

[jfhojat]

Re: Fail in LED Output of Power Supply in LG 42LB5600

Thanks.
How can modify power board to reduce output voltage results no fail in new bought LED String in future؟

[budm]

Re: Fail in LED Output of Power Supply in LG 42LB5600

LEDs' are driven by constant current source. You can modify the circuit by changing the value of the currents sensing resistor or look up spec. sheet of the LED driver IC to see if it has pin for current level setting to reduce the current or you can turn down the backlights level to about 50%

[nomoresonys]

Re: Fail in LED Output of Power Supply in LG 42LB5600

Not a difficult procedure, just need to locate the proper resistor/s to change out. https://www.youtube.com/watch?v=ogtfojsIZK8