Haier LE39F32800 No Backlights PFC Not Boosting

[neilc6]

Tore down this TV to find that all the backlights were working so part of the 1% of cases where there is a bad power supply. I confirmed the TV was producing video by powering the backlight with my LED tester and turning it on. All voltages were good (V STB, 12V) other than zero readings going to the backlight.

Found a schematic and the PFC is only used by the backlight controller. Confirmed the BL-ON signal is turning on the optoisolator to enable VCC (20VDC) to the PFC IC FAN7930. PFC cap reads 180VDC in standby and the same when turned on so not boosting. Checked all power diodes and FETS in the PFC and they appear ok. If shorted, they would have likely blown the fuse.
LED controller is AP3616M28 but not the problem since it is not getting boosted PFC.

Power supply is TV4205-ZC02-01. There are different configurations depending on the TV size and model. Mine has one connector at CON203 driving a single LED string around 130V. TV is only worth about $40 so not worth buying a new board for it.

[R_J]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

Check R115 to see if it is ok and not open, You could still have a problem with the backlights shutting down the pfc ic via Q114.
One bad led in the backlight string will shut down the backlight circuit even though the led tester lights them but at a much lower current.
The pfc boosts the voltage for the main switcher as well, not only the led circuit.

[neilc6]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

I checked all the power resistors (some hidden under the heatsinks) and none are open circuit. I have a 300mA LED tester and the strips all lit up with no fluctuating voltages. TV menu showed the TV set to Energy Savings picture mode which runs at reduced current.

[R_J]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

Check the voltage on Q114 collector, or Q102 base, Did you see that each individual led lit up with the same intensity?

[neilc6]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

It is difficult to probe the back of the board with it connected for testing.

FB net is tied to the Restart and I can probe pin 3 (collector out) and pin 4(emitter out) of the optoisolator (U206). Emitter is 1.2V in standby and 1.7V on and 0V on pin3 in both states so not turning on.

Comparing to U201 Opto for PFC_VCC, collector pin 3 is 0.4V standby and 20V on so turning on and providing power to PFC IC.

Probing the output to the LED string, one side is 0.4V in standby and the other 0.01V. When turned on, there is no spike in voltage and both sides drop to 0V.

When I checked the LEDs, voltage was stable with tester and all LEDs showed equal intensity.

Vaux 16.2V Stby 19.4 On
Vcc 16.9 Stby 20.4 On
Correction PFC Voltage is 149V

[R_J]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

U206 is the feedback for the main switcher U101, it is working correctly
PFC_Vcc should be on Q110 emitter, I will guess it is there. So pfc ic has most everything to work except RDY pin may be wrong.
For the led's to work they need LED_Vcc provided by Q102
FB pin 4 of U206 should connect to U101 pin2
FB pin 4 of U203 should connect to U203 pin2
The FB points are not all tied together.
I see that Q114,D115 etc have a dashed box around them, is that circuit present on this board?

[CapLeaker]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

I am with RJ… problem is that FAN XXXX or around it. That RDY is like a reset pin, so it surely can prevent the PFC auto work.

[R_J]

Re: Haier LE39F32800 No Backlights PFC Not Boosting

The rdy pin is an output from the fan7930, but I suspect something is preventing Q102 from turning on. I don't think it is the fan7930 causing it.