LG 55UB850V doesn’t turn on

[tehmatix]

Hi,
The red led flashing continuously.
I checked the power supply board and found 3.5V fluctuating down and up
I replaced the capacitors and the voltage , 3.5V still varying
On bridge rectifier 320V

[Diah]

Re: LG 55UB850V doesn't turn on

disconect the main boards from PWB and check yours STB 3.5V stable or not !

[tehmatix]

Re: LG 55UB850V doesn't turn on

I disconnected from main board and it does the same thing

[Diah]

Re: LG 55UB850V doesn't turn on

check your replacment capacitors if the legs - + correct or one faulty / stock one.

[nomoresonys]

Re: LG 55UB850V doesn't turn on

hit the ones you didn't change out with a hairdryer for a couple minutes, see if the voltage gets stable for a bit.

[nomoresonys]

Re: LG 55UB850V doesn't turn on

I test simple things first, including all diodes and resistors on top of board and all solder joints, look very closely at the solder joints, reflow any that look suspect, careful not to bridge any, examples of bad joints there if needed: https://www.google.com/search?q=bad+...ih=607&dpr=1.5

[nomoresonys]

Re: LG 55UB850V doesn't turn on

If those things check out, check the diodes and transistors on the heatsinks, youtube is your friend for transistor testing.

[tehmatix]

Re: LG 55UB850V doesn't turn on

Hi,
I re-soldered pads and checked the diodes and capacitors. 3,5V it still variable.
On pin 7 at ICE3BR4765J , Vcc voltage is variable. I think the optocupler on FB pin is damage. I will change it and I will come back latter

[nomoresonys]

Re: LG 55UB850V doesn't turn on

Did you try post #5.

[tehmatix]

Re: LG 55UB850V doesn't turn on

No, I didn't tried to heat the board/components

[nomoresonys]

Re: LG 55UB850V doesn't turn on

And...no hairdryer?

[tehmatix]

Re: LG 55UB850V doesn't turn on

Yes, hairdryer no effects
Optpcoupler changed no effects
Output diode for 3,5V changed no effects
Moving forward I replaced the entire 3.5V psi with an external one:
- red led stay on solid
- when press button on remote control on main board those 2 leds flashing one , tv doesn’t turn on, front red led turn on
- no other aux voltage 12v or 24v or backlight
I suspect a software glitch caused by 3.5V pulses