60inch TCL LCD TV no backlight power supply repair (help sought)

[Pauldg123]

Hi,

I have a 60" LCD TV from TCL which has sound but no screen (back light not working). TCL model 60D6 Power supply 40-L201W8-PWA1CG
Symptoms: when I turn on the TV, I do hear sound but the power led on the front does flash once and turns off.

The closest schematic that I could find was posted here https://elektrotricks.blogspot.com/2018/04/tcl-les9b21-lcd-tv-power-supply.html it comes in three parts but all is one power supply. There are only two optocouplers signaling from cold (led) to hot (transistor). No optocoupler signaling from hot to cold (but schematic does have that).

I measure 12 V DC on the cold side but the back light power rail does not has any voltage. I took the power supply pcb out of the TV and inspected it with no signs of burnt parts or burnt pcb areas. I wired up the BL-On and P-On pins to 12V via a 4k resistor to trick it in powering up. The backlight rails turn on for a fraction of a second and it goes zero volt.
I measured the PCF chip FAN7930 which has 15V Vpcf (or Vcc) and even disabled it with no avail.

The Backlight chip OZ9976 should have on the enable pin 13, 2-5.6V DC for 'on' state and less than 1.0V for 'off' state. I measured 0V. Even when powering it up, it stays 0 volt. Pin 14, Vref should have 5V but measures 0.88V DC. Pin 8 VSEN should have 3.0V but measures 0V at all times. I have added a load to the backlight rail (small 20W led light) to ensure the Vsen get some power if a current would flow. I also measured the hot side stage for the backlight (MOSFETS). All good and even replaced them. Measured all the diodes and transistors for shorts (all good).


The PCF chip and Backlight chip have been replaced with new ones but same symptoms. However the power led on the from does not flash anymore but just turns off as usual when the tv was functioning well. So how should I proceed since the datasheet for the OZ9976 is worthless (one pager diagram  ). I assume ENA, Vsen and Isen must be present to turn the back light on after a soft start. Anyone who has a good strategy to follow?

[Mario87]

Pin 14, Vref should have 5V but measures 0.88V DC.

This is what I would focus on first. The schematic you attached does not show where this 5V Vref is generated. I would look at that and first ensure there is a 5V rail on the board at all, if not (which it sounds like it), probably replace the voltage regulator responsible for stepping down from 12V to 5V.

Would be the next step for me at least.

[vintagepc]

The other thing to be wary of is some driver chips will have overcurrent sensing on their outputs.
If you consider this together with a typical failure mode of overdriven backlight LEDs (they short and no longer drop voltage) it can be a sign that there's too much current being drawn due to shorted or dying LEDs.

Don't overlook the simple - if you can get access to the LEDs themselves easily, it's worth giving them a visual inspection to look for skid marks or black spots.

[perieanuo]

yep, like Mario87 pointed, start checking voltages that powers the led driver: Vref (i doubt it's a ref voltage, it's a supply voltage from main power supply block), 12V
regarding the IC U401, check the old one if DRV1 DRV2 are shorted to his Vin pin (powered by 12V) or GND
Isen verification comes after the holy days, bottom of the list i mean

[gabiz_ro]

Vref may be generated by OZ9976 ( some internal pull-up and external zener D603)
But since enable pin is low, maybe entire IC is in shutdown mode and also Vref part.
Check PC3 voltage on 1-2 pin
If opto in ON that will pull down enable signal wich in fact is BL_ON

[RayRay]

You should have at least one (if not more) voltage regulators on the other (green side) of the board, better test em. Also, did you measure the resistors? Do you have a capacitance/esr meter? Electrolytic capacitors are generally the weakest link in any circuit (in terms of reliability) and just because they look physically okay, it doesn't necessiarily mean they truly are! The two largest (main capacitors) are probably fine, but unless you have a way to test the other ones, you'd be best replacing em, do note though that you'd need to replace em with proper, low ESR caps and not general purpose ones!

[perieanuo]

i adjust my post, Vref is generated by the oz chip, see some other tv: https://elektrotanya.com/konka_kip_l100u04c1-01_oz9976_fan7530_fsq0465_sch.pdf/download.html
i haven't see some clear answer from op, oz9976 pin 15 what voltage does he have?
you said ENA(BL_ON) is 0, so the other block (the main processor doesn't activate backlight, this may be irrelevant for now, i'd focus on this block)
d603/c606 pin11 are ok? no Vref can mean short on Vin rail
c603 (pin 15, Vin) is ok? just replace him
oz9976 take current from Vin 15 pin? how much? damaged chips take very small or very big current from Vin pin
final stage, just diode-test DRV1/2 pins regarding Vin and gnd, final stage is some weak point for those ic's

[gabiz_ro]

 ENA can be grounded (pulled low) by PC3 even when BL_ON is high.

I measured the PCF chip FAN7930 which has 15V Vpcf (or Vcc) and even disabled it with no avail.

First part is clear, PFC_VCC is ok,15V,second part ? disabled how?why?
PFC_VCC 15V seems OK,
PFC_OK must also be same or high enough not to switch optocoupler.
CS must be low enough to keep open C-E Q602

But even CS increase over normal limit,if BL_ON is high there will be some few failed start sequence of OZ9976 that can be identified on cold side.
(since BL_ON is high,ENA is also high,OZ9976 startup,CS switch on,Q602 and PC3,ENA then drop to 0,and OZ shut down,and CS is 0 and ENA signal can go high again if BL_ON is still high and another startup is tried again and again as long as BL_ON is high)

I wired up the BL-On and P-On pins to 12V via a 4k resistor to trick it in powering up. The backlight rails turn on for a fraction of a second and it goes zero volt.

That seems like ENA become low.

[perieanuo]

seems like protection is activating from led overcurrent or something alike
anyway, we need to have a clearer picture with all voltages involved measured, even for the case that bl_on is 'forced' to enable leds
if we don't have at least the important feedback values, like Isen Vsen, and oc return (ENA) and of course all the ps values, this discussion can last 3 months
led testers, they exists, or fabricate one if you want, shouldn't be that complicate for that led line/matrix

[ActiveRegion]

were you able to fix your TV backlight problem?
I think im having the same issue with my TV. My PSU model is 40-L141W4-PWC1CG tho.
What solution did you make?

[sicco]

My 55" TCL TV model is 49S405, symptoms were the same: no backlight.  The LEDs were fine. The power supply board L141WA2-PW220AB was faulty.

The OZ9976 was not the problem. In this TV on this board the GND and VIN pins are hardwired together... So no, it does not have a short on the chip between pins 2 and 15. The short is a PCB track under the IC... The IC here gets its power externally into VREF. So from elsewhere. Elsewhere is the 12VDC main power supply corner around the two opto couplers. I suspect something like transistor Q406 and or Q405 that was broken (though not shorted!), but I'm not sure. Because I eBay ordered a replacement board from China. $37.84. Advertised as new. But when it arrived, it was clearly used. And a different part number, minor differences. And... broken on arrival. It did not even have 12VDC on its main output, so no white LED on at all on the TV... It had two faults actually:
1) the TL431 U402 did not function as expected (I pushed an external 12VDC on the unplugged 12VDC power supply output - and noticed the cathode that connects to the optocoupler was below 2.5 V.   
2) the OB2273 (?) apparently was faulty. That's U401, the 6 pin PWM controller for the main 12VDC.
Re-using those two ICs from my original (no backlight) power supply unit onto the broken eBay unit got me a working TV again.

Schematics wise, what came closest for me was  https://elektrotanya.com/tcl_eo402c1_08-eo402-pw200aa_sch.pdf/download.html
But then my U402 is their U201 and my U401 is their U101.