Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

[jason123]

Hi Folks,

got this M70-c3 TV off craigslist for few bucks. Owner claims to have sound only. He had purchased a main board and that didn't fix the problem. Front LED responsive, Optical port lights up on the main board, and boards seem clean in general.

Flashlight test shows image. Don't know if it's the driver or that actual LED strips. Not familar with this configuration and seeking guidance. Anyone knows the pinouts of these ribbon cable? Looks like each strip has it's own driver circuit.

Not looking forward to tearing down the panel to replace LED Strips if I don't have to
Thanks

[DXseekerMO]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

On the backlight inverter board, test the voltage going to the LED strips with a voltmeter. Does the voltage come up then go right back down? I'd say if so then the backlight strips/LED's are suspect

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Quick update... TV is definitely passing the flashlight test. Checking the power going to the LED driver board (CM301) none of the 24 V are present even with the LED driver disconnected. Does power supply not turn on until it sees the LED board connected? (I see a SENSE pin)

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

On the backlight inverter board, test the voltage going to the LED strips with a voltmeter. Does the voltage come up then go right back down? I'd say if so then the backlight strips/LED's are suspect

I am not seeing any activity on the LED DRIVER BOARD.. 24 v coming into the driver board doesn't seem to turn on. There are 4 lines going to each LED strip.. and there are caps by each cable.. there is no voltage on these caps.. whatsoever.. no transient peaks either

[budm]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

I am confused, in post #3 you indicated no 24VDC feeding the LED driver board, now post #4 you have 24VDC?

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Hi budm.. There is no 24VDC at all coming out of the power supply going to the LED driver board. Maybe #4 is confusing.. but no change in the story.. still no 24VDc

[budm]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

The connector on the power supply board that goes to the main board has 15 pins, what are the Voltage on each pin when TV is OFF and when Power switch is turned on? Make sure to put down pin name for each pin.

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Here you are.. Only thing worth noting.. Pin 13.. labeled on the board the same as Pin 14, except it doesn't have any readings (confirmed Pin 13 is not connected to anything on the board).. I take the board off and see if there is solder joint issue..

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

In Doing general probing on the board, I found one FET (Q301) shorted on the board. I took the heatsink off and see what I found.. Looks like the FET is damaged. Will a replacement be fine? Tested the pads on the board and there is no short.

[budm]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Those 2 FET's are for the 24V SMPS, the IC that drives the Gate of the FET's may be damaged also, so you need to check all the component that are connected to these 2 FET's for damages.
https://www.shopjimmy.com/vizio-09-7...upply-unit.htm

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Quick question for you guys.. The FET (Q301) that I have blown on this board is marked with 50R350CE - it turns out there are several flavors that are marked with this same marking. They have the IPA50R380CE and the IPP50R380CE and so on. the one I have looks like it's all plastic without a tab. The improved performance one, IPP50... has the metal tab and superior performance. Can I use the one with the metal Tab? I don't know from the datasheet what the Tab is connected to, so I don't want to short it to ground. I haven't received the parts yet, so I can't do continuity on it. Thoughts?

IPA Datasheet: https://cdn.badcaps-static.com/pdfs/...7150c11ef5.pdf

IPP Datasheet: https://cdn.badcaps-static.com/pdfs/...5c2d791dea.pdf

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

So I installed the one with the tab - looks like the center pin is connected to tab, so I used an isolation kit. I'm getting 33 V instead of the 24 V on the rails. Does that mean the IC / Regulator is bad? or these FETS are not suitable for this application??

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Good morning people of the forum..
Before you get too busy with the Superbowl.. I wanted to run something by you.

I finally got the plastic FETs and they too generate 33.1 Volts instead of the marked on the board, with is 24V. The set seems to work fine. I checked the resistors around the controller to see if there is an issue with feedback, but all check out. Could it be that the voltage is mislabeled on the board??

[budm]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Are you 100% sure that the meter's battery is good and meter not set to AC? Just to be sure.

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Yes.. Positive.. Just checked with another meter and the reading is solid 33.x VDC

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Just got a the new board, and sure enough the 24V node is 33.24 V on that board as well. To test the battery theory, all the other voltages seem right on.

Thanks guys!

[budm]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

Well, keep an eye on it.

[jason123]

Re: Vizio M70-C3 70" 4K Smart TV - Possible dead backlight

I don't know what's with Vizio and their boards.. I just got a 60" (M60-C3) and the power supply board has 20.000 labeled on the voltage going to the LED Driver, and it too, is reading 26.4 VDC!!!