Sony KDL-55EX720

[AFajitas]

Good evening, friends. Here I have a Sony KDL-55EX720 that when plugged in the standby light is solid red. When I press the power button once, the red light disappears. When I price the power button again, the TV turns on and says HDMI in the upper right hand corner but turns off again after 1-2 seconds and then the standby light blinks red 4 times, pauses, blinks 4 times again indefinitely.

My first guess is a possible bad LED backlight or driver board, but how can I troubleshoot this? I have a DMM, but I just need some guidance on what to check for.

Here are pics of the TV's innards:









*edit - Meant to make the title more descriptive, oops.

[redwire]

Re: Sony KDL-55EX720

Some info here: http://www.avsforum.com/forum
Service Manual at Electrotanya

It looks like 4 blinks could be the main board (BALANCER) and checking the BL_ERROR signal from the BAT-V board is a first step.

[AFajitas]

Re: Sony KDL-55EX720

Thanks redwire. So I grounded out the BL_Error wire before the bottom power board and now when I plug it in it the standby light still turns red and I still have to hit power twice to get it to power on. The backlight will still shut off after 2 seconds, but the TV doesnt put itself into standby mode anymore at this point (no blinking, remains on but w no backlight) and I was able to verify I could see see an image with a flashlight and hear the sound when pressing volume keys.

I then grounded the BL_error before the top power board and the symptoms changed. Now I see a Sony logo when I apply power to the TV, and pressing the power button once turns it on. The backlight is still shutting off after 2 seconds tho, no blinking, can see image w flashlight. So it looks like it's either the driver board or a backlight strip. How can I test the voltages to narrow it down to the backlights or driver?

[AFajitas]

Re: Sony KDL-55EX720

Can anyone tell me how to determine if my problem is an LED backlight or the driver board? I might strip it down tonight and have a look at the backlights.

[redwire]

Re: Sony KDL-55EX720

I don't know this set specifically, sometimes I take an educated guess and start there. Like an LED might be open, or the PSU problem.

Check the VLED_VCC is good on the BLK board CN1002. I think it's 12V.
I'd take measurements and compare each of the 4 strip (OUT+, OUT-) voltages
Each strip gets constant-current feed with PWM control for dimming by IC1000 on BLK/LD (LED DRIVER) board.
LD_ERR trips if there is trouble with the LED's current or voltage or IC1000 says so.

You could also use a bench power supply (with resistor) to test/compare each LED strip.

[freakaftr8]

Re: Sony KDL-55EX720

These models are known for the array backlight controller failure. Have any dark spots in the screen?

[budm]

Re: Sony KDL-55EX720

I don't know this set specifically, sometimes I take an educated guess and start there. Like an LED might be open, or the PSU problem.

Check the VLED_VCC is good on the BLK board CN1002. I think it's 12V.
I'd take measurements and compare each of the 4 strip (OUT+, OUT-) voltages
Each strip gets constant-current feed with PWM control for dimming by IC1000 on BLK/LD (LED DRIVER) board.
LD_ERR trips if there is trouble with the LED's current or voltage or IC1000 says so.

You could also use a bench power supply (with resistor) to test/compare each LED strip.

And this the SCH for the LED driver board.

[AFajitas]

Re: Sony KDL-55EX720

These models are known for the array backlight controller failure. Have any dark spots in the screen?

I looked, but it's hard to tell as it's a dark background and only on for a second or two.

[AFajitas]

Re: Sony KDL-55EX720

I am getting a reading of 181 volts on the two VLED_VCC pins on CN1002. Is that possible or am I doing something wrong?

[budm]

Re: Sony KDL-55EX720

You need to look at CN1000 -OUT1, 2, 3, and 4.
You will need to leave the probe on -OUT1 then TV on and watch the DCV reading, take notes, turn TV off and then repeat it 3 more times for the other -OUT.

[AFajitas]

Re: Sony KDL-55EX720

Thanks budm! CN1000 gave similar readings on all 4 pins. Immediately after pressing power button reading is 39v-41v.. Then when the backlight comes on it goes down to 4-6v, backlight shuts off 2 seconds later and reading goes back to around 40v.

I also tested CN1001 header and the reading on all the pins, 180v before backlight comes on, 215v after, back to 180v when backlight shuts off.

[budm]

Re: Sony KDL-55EX720

If all four -OUTs (they are the LED Cathode return lines) show the SAME readings, that means you do not have open LED strings.
You also have the Boosted of 215V feeding the Anode of the LED strings and then it drops down to 180V due to error detection some where.
I will have to study that LED driver board circuit to see what is going on, it can be error condition or the dimming control signals, or the T-CON is telling it to shut down.
What DCV do you have on LD-ERRR PIN? See if the Voltage changes state when the backlights go on and when backlights go off.

[AFajitas]

Re: Sony KDL-55EX720

LD_ERR - When I turn the TV on I see 3.4v, then when the tv powers on it drops to .007 volts... When the backlight turns on it says 0.0 volts but makes my volt meter display blink 3-4 times then when backlight shuts down it goes back to 3.5 - 3.6 volts.

[budm]

Re: Sony KDL-55EX720

Looking at the diagram of the LED driver board. there is 1A fuse PS1000 on pin 9 of the connector CN1003, see if you do have 12VDC between GND and both ends of the fuse.
The T-CON sends 4 PWM control signals (PWM 1, 3, 5, and 7 of the IC) to the LED Controller IC1000 (BD9276). T-CON also sends the Vref (pin 4 of CN1003) to IC1003 which provides VLED_DAC to pin 33 of the LED Controller IC, this signal is also fed to the detection circuit that generates the 'FAIL' (fed to pin 2 of the LED Controller IC) and the LED_ERR (goes to pin 9 of CN1002). The 'AOUT' is also fed to the detection circuit.
It can also be one of the PWM control signal is not working.
We may be able to defeat the protection circuit but we should find out what trips the detection circuit first.
Dam, I keep getting server error trying to upload picture with only 971KB.

[budm]

Re: Sony KDL-55EX720

LD_ERR - When I turn the TV on I see 3.4v, then when the tv powers on it drops to .007 volts... When the backlight turns on it says 0.0 volts but makes my volt meter display blink 3-4 times then when backlight shuts down it goes back to 3.5 - 3.6 volts.

It looks like it (LED_ERR) needs to stay low, I will have to look up the spec sheet of the LED Controller IC to see what kind of signal it requires on the FAIL pin.
So look at the 'FAIL' pin2 of the IC1000 to see what it is when the backlights are ON.

[budm]

Re: Sony KDL-55EX720

BTW, did you check the DCV at the fuse?

[AFajitas]

Re: Sony KDL-55EX720

Fail pin on IC1000, 2.8v when I turn the tv on and backlights come on. After backlight shuts off it goes to 1.8v and then drops to .4v after aprox a second.

[AFajitas]

Re: Sony KDL-55EX720

Budm, DCV on both sides of fuse reads 12.6v and remains constant.

[AFajitas]

Re: Sony KDL-55EX720

Any ideas? Im not against just buying a new led driver board if its too difficult to troubleshoot.

[budm]

Re: Sony KDL-55EX720

You can put the jumper wire on the resistor R1116, this will cause the 'FAIL' to stay stay HIGH and the 'LED_ERR' to stay LOW which should make the BACKLIGHTS stay ON.
Or you can try new LED driver board.