Quote:
Originally Posted by rca2000
LED BL's's DO go bad...guys. I have seen a good amount of them bad.
What usually happens is only one of the LED's opens...BUT usually there is only one or two strings..(except for the sets that use a "modulated BL", for "dynamic dimmng", these can have as many as twenty seperate LED strings all over the panel).
So--when an LED opens--it interrupts power to ALL of them. You can test this. Upon power on, you need to see if you are getting LED drive power to the BL. It should be at least 75 volts and as high as 250 volts--depending on the umber of LED's in the string and so on. If you see this voltage...and it then DROPS of, but you get NO BL..you probably have an OPEN string. There IS a way to prove this...but you MUST be fully aware of the BL driver ckt and ALSO be aware that is you make a mistake...you could BLOW the WHOLE PS !!.
Looking at you PS board--I was not able to tell if you have only one or two strings or not. I need to see the FOIL side--as that is where the LED driver transistors are. Normally...these are driven by the B+ I talked about...and it is on ONE side of the string. The OTHER side...goes to the dimming transistor. It then determines how close to ground it will brig the string.
If you are getting the drive to the string...THEN...you CAN "bypass" the driver transitor. But do NOT simply short the transistor to ground...it WILL blow the LED's !!
Take a 220 ohm 1 watt or so resistor and CAREFULLY..touch THIS to the side of the string that goes from the BL to the transistor, and the other end of the resistor to cold ground. Remember...the other side of the BL string goes to the B+ supply for the BL. WARNING !! Do NOT do this for more than a second or so--long enough to see if you get a BL flash on. The resistor WILL get hot--BTW. If you DO--the BL is probably ok--and you have a drive transistor or control ckt problem. If you do this and get NO BL and the voltage does NOT drop on the LED side of the resistor..or if there is NO voltage there...the bL is open somewhere.
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Nice trick. I'll keep that in mind if I ever run into a tough LED problem.