HISENSE F42K20E LED Tv - No backlight

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

I have no idea. You may want to look up the spec sheet of the LED driver UC701 to see if they have any explanation on how it works.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

Yes I wish I knew the specs of it.

I was playing with color options/brightness etc and with the random inductor I have in currently, there was no change to brightness or anything. Most likely bc the LEDs are being overpowered.

Would that be a result of a lower inductance or higher? As the voltage to the LEDs is 15v more than its supposed to be

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

You need to get the right inductance, current rating, dc resistance.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

Could I use anything else that'd be similar?

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

FERRITE pot, i have no idea where to get the replacement, the print on top looks like LEI-302B.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

Here's a side view shot

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

I can't see the windings they're incased. Did you see the picture of it I provided in an earlier post?

I do not see the side view.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

I can't see the windings they're incased. Did you see the picture of it I provided in an earlier post?

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

132v on c848 and 260v to led pin 1+ which is over what the board has labeled at 245v

Is this due to the random inductor I soldered in for L702?

Backlights work still but L702 gets pretty hot to the touch

OK, I thought you are still getting only 132V with the sub inductor you have installed.
Ot sounds like the sub is not the right one to use.
Can you look at the old inductor to see if the open circuit can be fixed.

[caphair]

Re: HISENSE F42K20E LED Tv - No backlight

132v on c848 and 260v to led pin 1+ which is over what the board has labeled at 245v

Is this due to the random inductor I soldered in for L702?

Backlights work still but L702 gets pretty hot to the touch

[budm]

Re: HISENSE F42K20E LED Tv - No backlight

What DCV do you have on C848 (Input Voltage to boost converter)?
The Cathode of the LED string should be connected to the Drain pin of the PWM Controlled MOSFET U703, the Source will be connected to the current sensing resistor, and another end of the resistor is connected to GND return. The Vdrops on the current sensing resistor is used for feedback to maintain constant current.
If you look up the spec sheet of IC U701 LED driver IC it may have example circuits on how it is used.
BTW, I think the MOSFET designators are 'V'xxx, and ICs are 'U'xxx.

[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?

[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

[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 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?

[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

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?

[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

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

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?