Insignia NS-50L440NA14, Backlights Shut Off

[toddw65]

Picked up this tv and it powers on, standby LED flashes red 7 times, backlights come on and display evenly, and then backlights shut off and LED stays green. Nothing on the display with a flashlight. When I first got it, the Insignia logo would come on when powered up, but after a dozen on/off cycles that stopped happening.

Appreciate your input on this.

Thanks,
Todd

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Gotta love this stuff. I took off the main board to get a pic of the back. While it was out, I hooked it up to check voltages on the back. Lo & behold...the backlight now stays on...even with the board put back in place. And it goes on and off as it should with the power button.

Now on the the next part of the puzzle.

Interestingly, voltage to the TCON is 8V and I assumed this would be either 5V or 12V.

[budm]

Re: Insignia NS-50L440NA14, Backlights Shut Off

What DCV do you have on that white connector with red and black wire on the FRC (Frame Rate Converter) board (the board sitting between the Tcon and the manin board)?

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

What DCV do you have on that white connector with red and black wire on the FRC (Frame Rate Converter) board (the board sitting between the Tcon and the manin board)?

5.1V at each red

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

I've been trying to research why there is 8V going from the main board ->FRC -> TCON - > Panel. Although I'm a newbie TV repair, 8V just seems unusual to me and I'm thinking it should be 12V. With that in mind I started tracing from the vcc on the connector that goes to the FRC and I'm looking at Q1 on the main board. This looks like where I'm losing my 12v.

[budm]

Re: Insignia NS-50L440NA14, Backlights Shut Off

I've been trying to research why there is 8V going from the main board ->FRC -> TCON - > Panel. Although I'm a newbie TV repair, 8V just seems unusual to me and I'm thinking it should be 12V. With that in mind I started tracing from the vcc on the connector that goes to the FRC and I'm looking at Q1 on the main board. This looks like where I'm losing my 12v.

I do expect 12v FOR 50" panel, the switched 12V should be fed through a switched MOSFET which should be on the main board some where close to where the RED/BLK wire connector is.
You do have 12VDC feeding the main board, right?

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Yes, but it's actually 13v coming from the power board to the main. If I looked it up correctly, I think Q1 is a P channel mosfet.

[budm]

Re: Insignia NS-50L440NA14, Backlights Shut Off

It is P-CH because the +13VDC is fed into the Source pin, so for this MOSFET to be on, the Gate DCV must be at least 3V less than the Source pin when TV is powered ON, you can force on this MOSFET but connecting 1K resistor between Gate and circuit ground to see if you will get 13V at the Drain pin.
Right now the MOSFET is OFF but it may be leaky that is why you are getting 8V even though there is no Bias to turn on the MOSFET.
You should check the resistance between S and D, G and S, G and D.

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Adding the 1K resistor made no change, but here's what I noticed next. When I disconnected the cable to the FRC, the voltage at the drain pin of Q1 jumped up to 13V and stayed there even when I removed the resistor.

I then reconnected the main to FRC board and disconnected the FRC to TCON and the voltage stayed at 13V.

Disconnected the panel from the TCON and reconnected the FRC to TCON harness and voltage dropped back to 8v.

So the problem appears to be something pulling down the voltage on the TCON.

[budm]

Re: Insignia NS-50L440NA14, Backlights Shut Off

So the Gate Voltage stays at 13VDC when you get 13V at the Drain?
Is there a fuse on the T-CON board? You can check the resistance between the fuse and the GND to see if it shows low resistance, or you can try new T-CON board.

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Yes. 13V at gate & drain. Fuse to ground is infinite so something on the board must be shorted.

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Got another TCON and put it in and no change. Took out the 3407 MOSFET and since I don't yet have another MOSFET to replace it, I sent 12V to the drain location on the board. Bingo. Fired right up and the menus, picture, and sound all work. New 3407s should be here on Friday. It'll be interesting to see if a new one fixes the issue long term.

Thanks for your help Bud!

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Just wanted to say thanks again. Got the FET in today and she fired right up.

[budm]

Re: Insignia NS-50L440NA14, Backlights Shut Off

Thanks for the follow-up.

[toddw65]

Re: Insignia NS-50L440NA14, Backlights Shut Off

No problem. Now on to the next conquest.