Sounds like there is at least one open LED in each array. Are the LEDs individually accessible for probing? If so, you can try putting a jumper across each one in turn (shorting that particular LED) until that string lights up. Remove the short and replace that LED. Of course this method of troubleshooting will not work if more than one LED in the string is open, but it is "quick and dirty" requiring no "test equipment" other than the jumper wire. Be careful not to short out, or make physical contact with, the 75VDC while doing this.
If there is more than one open-circuit LED in a string, you will need to test each one individually using an external low-voltage power source (a three volt battery... two dry cells in series is fine) and a current-limiting resistor, say 120Ω, 1/2 watt, to start. If this appears to "light up" the LED too much, increase the resistor value. All you are trying to see is whether or not a particular LED lights up. If none of them light up, increase the supply voltage by adding another dry cell in series.
Do not power up the TV when doing this test! If possible, disconnect the backlight wiring. IMO, an isolated battery is better than a line-operated low-voltage power supply, even if you have one of those handy on your workbench.
Please let us know if either of these troubleshooting methods helps you solve your problem.
