Insignia NS-48D51NA15 48" TV - No Backlight but image is present

[axlmastr]

The symptom is no backlight. If looking at the screen from the right angle or using a flashlight I see the image and of course sound works. I remember repairing earlier monitor backlight issues with a CCFL tube or inverter, but haven't done and LED. I was reading through another Insignia thread and checked my voltages as requested there by BudM a long time well respected electronics guru here. My power board is slightly different than the other thread.

Referring to the other thread I performed the following tests: Black wire at power on rises up to 238-245VDC and bounces between +238 & 245 consistently. White wire at power on rises up to -68-73VDC and again bounces between those two numbers consistently. When powering off the numbers slowly count down to below +5 or -5VDC respectively. I see there some caps from Chang and Capxon used on this board the doesn't make me happy since I've replaced hundreds of those brands over the years.

Where to look? Junk caps not visibly bad but causing voltage issues? New LED strips? 10 total needed.

[neilc6]

Re: Insignia NS-48D51NA15 48" TV - No Backlight but image is present

Those Insignia LED models are very prone to LED failures. I have yet to seen a capacitor failure on an LED TV but have replaced LEDs on dozens of LED TVs. I would invest in a LED tester if you plan on repairing more TVs. It only takes one bad LED to take down the backlight.

[axlmastr]

Re: Insignia NS-48D51NA15 48" TV - No Backlight but image is present

Thanks for the response. At least someone did.

I don't plan on repairing other tv's so an LED tester is not an option. There is a thread posted by Robert Phelps in Dec 2017 titled "insignia ns-48 no backlights" that I read and based my initial post info on. In it Badcaps veteran budm responds with his usual descriptive posts of test procedures that yield a solution. I realize with any entry level consumer product there is a propensity to contain sub-standard components. I have repaired several hundred boards since I joined here for a variety of symptoms. Some were answered here while others were not. Bad capacitors were often the culprit, but micro-fuse, SMD failure, or EEPROM re-flashing were also issues. I'm looking at the other thread mentioned above and noting the test procedures used in it for my post and seeking a response relative to the possibility of over-voltage to the LEDs causing a protection circuit to act as it should. Capacitors control voltage and my experience with repairing motherboards and power supplies has provided plenty of knowledge of how their involvement yields erratic or atypical behavior. I'm sure I can mock up a way to least fire the LEDs with some voltage input without a tester and can't believe that a voltage >200 VDC is needed to fire 60 LEDS to brightness. Are the readings I provided within spec for this or similar LED units? I didn't see an answer to confirming that. I was trying to abstain from splitting the screen to get to the strips for individual testing by having some definitive answer or test results for the power board first in order to rule it out. I assume your reference to the "it only takes one bad LED" is similar to the the Christmas lights scenario of one bad bulb....

[axlmastr]

Re: Insignia NS-48D51NA15 48" TV - No Backlight but image is present

One additional thought I just noticed upon review of the board. The CapXon caps in the corner nearest the black heatsink are rated 68uF 200v. My voltage output at the 2P connector (blk/wht wires) to the LED array was over 200V as posted. The 2P connector having its own two caps there are rated 25V. The 2P connector is where budm stated to check the voltages I posted at power on and off. He stated a 100 ohm 2W resistor on + lead to GND would light the array if voltage protection circuit was enabled by bleeding off the excess. I have replaced a ton of CapXon caps in LG & Samsung CCFL monitors over the years. Many with not visual signs just no power on symptom of the display.

[budm]

Re: Insignia NS-48D51NA15 48" TV - No Backlight but image is present

The filter cap for the LED driver power supply are rated at 33uf 250V (there are two of them), if the load is present and the load has returning path for the current, then the Voltage measured between the tow legs of the cap should be around 200VDVC (180Vf for the LED string and about 20V for the PWM controlled MOSFET for maitaining constant cursent) or so, but if the load is not present (or has very high resistance the the Voltage on the caps will rise up very high like what you are seeing. The LED's are connected in series to form one large LED string, the LED will light up with Voltage aroun 2.8V 2 30mA, so you need about 60 x 2.8V = 168VDC supply that can put out about 30mA to test the LED string. You can try using about eighteen 9V batteries connected in series as the Voltage source for testing the LED string.