I have an RCA RT4038 tv with a non working backlight issue, 2 LED's were blown. I replaced the strips & tested them, all tested good with 15.5 v required to illuminate all. I also replaced the led driver IC SN51DP but still no backlights. When I checked the voltage output (unloaded) I get 42v. I know that driver is suppose to adjust to the proper voltage when connected to the load but for some reason I am not getting illumination when connected. What am I doing wrong? What might be at fault? I checked most of the related components but everything seemed ok. Maybe I missed something? I did not remove any resistors for testing.

Testing with 3 pin connector attached, I have -35v in standby, not sure there should be any voltage there until powered on? . Powering on, voltage climbs to -90v before settling down to -41v steady, no illumination. Voltage appears to be reversed at connector. Connection from LED's has black as positive and red as negative, connector on the board is the opposite. That doesn't seem right because polarity matters unless there's some type of reverse bias that's suppose to happen. Getting myself confused & tempted to remove the connector on the board and spin it. :thinking:

Update: I spun the board connector around so that the positive side pin went to the positive side of the LED's. When I applied power. I got 3.3v, rose to a max of 14.5v at the connector, and the LED's were full bright & strobing. Not what I was hoping for so I just put back the way it was and here I am, head spinning. It appears that there must be some type of reverse biasing that suppose to place to turn on the LED's that isn't happening for some odd reason. I'm stumped. Maybe the replacement driver IC I got from China is defective?

 

Images of original connections showing positive from board going to negative on LED strip. I don't know how these ever worked being connected this way - adding to the confusion.

 

From what I can tell, It looks like the issue might be related to polarity or possibly a faulty driver IC. The reverse voltage and strobing I observed suggest there’s a problem with voltage regulation or feedback.

 

From what I can tell, It looks like the issue might be related to polarity or possibly a faulty driver IC. The reverse voltage and strobing I observed suggest there’s a problem with voltage regulation or feedback.

Thanks for your reply. I am really puzzled on this one. When it's connected the way it is nothing happens and the switch goes from red to blue when I power it up. My guess is the polarity connection is wrong. When I configure it the way it should be, with positive to positive, it starts strobing as soon as I plug it in and there is no red standby light. It doesn't make any sense even though the customer said it was working. Rather than continue to waste more time I think I'm going to order a replacement board and hope that will do the trick.

 

UPDATE: Issue has been resolved. I came to the conclusion that since the unloaded output was 42v and I was getting 15.5v with my tester, it just didn't add up. The backlights were in parallel. I modified the connections so that 3 strips were wired in series which voltage requirement was more inline with the voltage output, powered it up and all good!