LOW D+ voltage to leds Samsung PS BN44-00666A

[gavric]

Hello, i am working on Samsung 40led tv with power supply BN44-00666A . Tv had no backlight and upon replacing 1 complete led strip all of the leds inside tv power on now but not to full brightness.

Voltage to leds should be 135 VDC as per print found on ps but I am getting 96 VDC across D+ and chassis ground. If led connector is disconnected i get 215 VDC on D+ and chassis ground. With led strip tester all leds turn on nice and bright at 102 VDC

All leds test out good and power on together but not to full bright even when backlight is set to max. When I purchased replacement strip it listed same part number but when it arrived its same looking strip but different revision number of it. could that somehow account for 40 VDC missing.

Thank You

[budm]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

https://www.badcaps.net/forum/attach...2&d=1510788361
What Voltage do you have on cap CM808 in the LED power supply section?
I would also remove that cap and inspect the bottom of the cap to check capacitance and see if it has any sign of corrosion on its legs.

[dick_barton]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

What were the strip numbers of each?

[gavric]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

Thank You for reply, At cap CM808 I get 218-220VDC when everything is hooked up. After checking around ps board i have found that 4 diodes that are close to that cap CM808 are not acting as diodes, multimeter on diode mode dont even register them in either way when power is off. all 4 seem to be interconnected somehow and i am not sure if this messes up regular diode testing way. voltage on one side of diode is different that other side. example of most drastic on is - one side of diode is 247vdc and chassis ground , other side is 87vdc. others test 33Vdc more than other side of diode.

[gavric]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

strip part number is BN96-28766A

on led strip tester original 2 leds strips when tested on actual leads to screen are using 35.1 VDC and replacement is pulling 32.4, not sure what is happening because replacement strip difference is only version number on it, replacement strip is later version. same size, same shape, same number of leds, 13 per strip. Output of power supply i get 96VDC where i would need 102Vdc for full brightness.

[dick_barton]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

What's the TV model number?

[gavric]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

What's the TV model number?

Model of tv is UN40EH5300 and i have already ordered replacement power supply at cost of $20 its not worth messing around to try to fix it. Will update results here. Thanks

[dick_barton]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

The open circuit voltage to the leds appears to be high enough to indicate that the boost circuit is working and a quick google on your set indicates the top and bottom angled strips have 13 leds each and the centre having just one straight strip with 12 leds.
So it would seem to indicate that the leds are all in series.
(13+13+12) leds @ say 3.2V = 121Volts

I wonder if the led current sensing resistor has changed in value reducing the current flowing through the leds, resulting in a lower voltage across the leds.

The led tester does not push out a very high current so although it's useful for testing for open/shorted leds or poor connections it does not drive the leds hard so does not catch all faults.

Can you post a photo of the power supply from your set (not the Shopjimmy photo) we can inspect.

Just my thoughts.

[gavric]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

Thanks for your reply, on led tester led is being powered on by 2.5 VDC and it glows bright. i have already ordered replacement power supply at cost of $20. its not worth messing around to try to fix it. Will update results here. Thanks

[gavric]

Re: LOW D+ voltage to leds Samsung PS BN44-00666A

UPDATE:

Replacement of power supply has resolved dim picture issues and voltage going to leds is at 130VDC at brightest setting. I have it set in backlight controls at 12 out of 20 and its is well bright and it is running at 112VDC. Seems as if initial bad led somehow damaged power supply and after replacement of led light strip there was still problem with power supply that caused low brightness of screen.

Thank You for your help.