HISENSE F42K20E LED Tv - No backlight

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  • caphair
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    Ok thanks. Can I just short the bad ones to complete the circuit?

    Also I'm curious why my meter only detected that momentary 300v spike while on auto range but didn't pick it up with that feature turned off?

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  • budm
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    The site screwed up again.

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  • budm
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    I would check the wiring to the LED string first, then look at each LED to see if any one of them has burn mark or dis-color.
    You can test each LED if your meter has enough Voltage to make the LED conduct or you can use small power supply with current limiter. Some people use 3V coin battery as the source.

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  • caphair
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    Ah very nice explanation thank you!

    As I've already done testing of the other pins I don't get any voltages only the quick spike to little over 300v on 1+

    If LEDs are suspect. If I tear down the panel, how would I go about finding the defective LEDs? Test each individual one with a small voltage?
    Last edited by caphair; 11-30-2015, 10:54 PM.

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  • budm
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    The Two LED strings are connected in series to form on big long string, if you look at the bottom side of the board the 1+ (Anode of the string #1) should be connected to the B+ of the LED power supply, 1- (Cathode of the string #1) should be connected to the 2+ (Anode of the string #2), then the 2- (Cathode of the string #2 should be connected to the PWM Controlled Power MOSFET, the Source of the power MOSFET should be connected to the Current sensing resistor (one end of the resistor is connected to GND) V drops on this resistor is the feedback to the LED Driver controller IC to maintain constant current through the LED string.
    So you can leave the meter red probe (Black probe on the chassis) on 1+ turn on the TV and take notes on the Voltage reading, turn the TV OFF, then move red probe to 1- and leave it on that pin and then take the reading again when you turn the TV on, turn the TV off after you get the reading, do the same thing again on 2+, and 2-.
    It will help to have good clear pictures of the bottom side of the board to verify the connections.
    Last edited by budm; 11-30-2015, 10:24 PM.

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  • caphair
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    Originally posted by budm
    The LED Voltage output went up to 300V because it does not have any load due to open circuit in the LED string somewhere then it goes into OVS (Over Voltage Shutdown).
    As you can see the Voltage should be 245V (with load) as printed on the board.
    So is it safe to say the driver circuit is fine and the LEDs are the fault?
    What are the 1+ 1- / 2+ 2- points? Are the - points the return/feedback? I measure nothing on those pins, which I guess would make sense if there's an open in the string somewhere?

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  • budm
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    The LED Voltage output went up to 300V because it does not have any load due to open circuit in the LED string somewhere then it goes into OVS (Over Voltage Shutdown).
    As you can see the Voltage should be 245V (with load) as printed on the board. The filter cap C715 for LED power supply is rated at 350V.
    https://www.badcaps.net/forum/attach...6&d=1448933014
    Last edited by budm; 11-30-2015, 10:09 PM.

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  • stj
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    everything.

    capacitance, esr, resistance (incase it has an internal partial short)

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  • caphair
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    How do you check caps like 716? Esr or?

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  • stj
    replied
    Re: HISENSE F42K20E LED Tv - No backlight

    c715, and c716 should be checked.
    716 looks bulged.

    Leave a comment:


  • caphair
    started a topic HISENSE F42K20E LED Tv - No backlight

    HISENSE F42K20E LED Tv - No backlight

    Powers on and there's sound but no backlight. This is my first time troubleshooting an LED backlit TV so I'm a little lost as to what to look for.

    The backlight connector with the +/- do you probe those points or keep negative lead on cold ground?

    I probed the capacitor c715 right next to the connector positive side and measured over 300v that quickly decreased while the Tv was on, this was both with led connector plugged and unplugged. When powered off and back on, the same behavior happens. Not sure if that helps determine something?

    This psu looks intimidating for some reason, can someone explain each section to me? I'm used to looking at a CCFL psu
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