HISENSE F42K20E LED Tv - No backlight

[caphair]

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

[stj]

Re: HISENSE F42K20E LED Tv - No backlight

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

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

How do you check caps like 716? Esr or?

[stj]

Re: HISENSE F42K20E LED Tv - No backlight

everything.

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

[budm]

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

[caphair]

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.

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?

[budm]

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.

[caphair]

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?

[budm]

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.

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

The site screwed up again.

[caphair]

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?

[budm]

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?

I do not know the exact function of the protection circuit if there are less LEDs connected (bad LEDs bypass), it is constant current circuit but it may not have enough LOW range (ability to reduce the output Voltage to maintain the set constant current).

What range did you put the meter in when you are not using the Auto range? Make and model?

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

Would it cause any problems if I shorted the bad one/s if it's a constant current circuit? Could I use a regular diode in place or another component? Assuming I couldn't find replacement LEDs for it.

This is the meter I have:
http://www.amazon.com/AideTek-VC97-M.../dp/B008GTEZPI

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

You can use 5 Diodes (1N4007) in series to simulate 1 White LED, or use 3.3V 1W Zener connected in reverse bias. Edge lit LED is harder to replace the LEDs.
using diodes
https://www.badcaps.net/forum/showpo...&postcount=181

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

Ok finally had a chance to rip the panel apart and I tested each led using 3v battery. Every one lit up, now I'm stumped.

Also checked wiring going to the strips using continuity checker and they checked out. So it's the power supply?

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

I must have misdiagnosed originally, when I probe + of the 10uf 350v capacitor connected to 1+ terminal of led strip it measures in millivolts and slowly climbs to 1v then drops.

Here are some pics of backside of the board, some connections looked questionable so the flux residue is from my resoldering some points.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

So the mosfets are getting a signal from the ic driver as my meter showed a frequency of 200-240hz on the gates.

There's 134v coming off the secondary side of the transformer that feeds the drain of u701 mosfet, however I noticed one side of L702 has 134v but the other side that connects directly to u701 only has 1v

The resistance of L702 is 5mega ohms. Is that the culprit?

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

So the mosfets are getting a signal from the ic driver as my meter showed a frequency of 200-240hz on the gates.

There's 134v coming off the secondary side of the transformer that feeds the drain of u701 mosfet, however I noticed one side of L702 has 134v but the other side that connects directly to u701 only has 1v

The resistance of L702 is 5mega ohms. Is that the culprit?

L702, U701 MOSFET, UD702, C715 are part of the Boost converter, if it is not working then the input Voltage (diode VD853, filter cap C848, xformer T831) will be equal to the output Voltage.

L702 should show <10 Ohms.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

So would L702 be bad preventing the 134v from being boosted closer to 300v to power the LEDs?

Is U701 the only mosfet being used to boost the voltage to the LEDs? I'm confused about u703's role in the circuit

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

Removed L702 and it tests open. Problem is I can't find anything online using the part numbers. Ended up soldering in a .02mH inductor from a scrap board and backlights came on. Reads 134v to 1+ of led connector.

Is it safe continuing to use this inductor? It's smaller, most likely the original was a higher value

What I'm wondering is if L702 is part of a boost converter, wouldn't the 134v fed into it be stepped up rather than maintaining that voltage to the LEDs?