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Philips Logo Flashes, Then Turns Off

Philips 75PFL5603/F7 flashes logo on screen then completely shuts off. Voltages on power board are correct when plugged in. The only sign of life on the main board is the red light in the digital audio optical port. The red light stays on for about 5 seconds when pressing the power button, but eventually turns off. There are no other standby lights that I can see. Research on the matter has brought up a possible main board fault or maybe bad backlight cuasing the lights to go to 100% very quickly and overheating and the TV going into protection mode and turning off. Has anyone seen this problem before? I'm trying to determine what parts I need to get to fix it.

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thats not a Philips tv

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Backlight array needed changed. Only one of the 16 backlight strips was faulty, but changed all 16 with new ones. Also needed a new main board as the tv wouldn't turn on with the old main board even after changing the backlights.

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@rohansolo "Voltages on power board are correct when plugged in." we need to know where you measured for those and what voltages you got. Then we need to see your boards. Remove the back cover and post a couple of good, well focused and large size images of those boards, with your Question. Adding images to an existing question

Let us know what voltage you measure on the backlight connector and what happens when you dc the backlights? Does your TV stay on/turn on?

This can have a lot of causes. From a bad main board, to a bad panel etc. and yes, the backlight strips as well.

Update (07/20/23)

@rohansolo test the voltages on your T-con board. Let us know what you get.

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The boxes on the right hand side are for the gamma chip. Start with the highest point and work your way down (GM18-GM1)

Also check the 6R8 coils and see if you get power on those. Black to ground and REd to one of the legs.

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I attached a bunch of pictures of the TV to the OP. When checking voltages more carefully this time I realized a lot of them fluctuated. On the power board the 5v, 20v and GND rails were all showing the correct readings. The RES, ADJ, and BL rails would be around 280mv until I pressed the on button, then they would shoot up to 3.3v. Note that the ADJ and BL voltages would be zero when the main board was unplugged from the power board.

The LED Driver was also fluctuating and very confusing to me. The LED 1, LED 2, LED 3, and LED 4 rails would go from 66mv to 0.6v and then shoot up to as high as 50v for a split second when I pressed the on button, but go back down quickly. I'm assuming that quick spike correlates the the Philips logo flashing on the screen for a split second. The NC rails went from 7.7mv to 11mv. The FB rails all read zero. Note that these reading would all go away if I unplugged the main board from the power board even though the LED Driver is on the power board.

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The main board doesn't have any indicators by the connector that comes from the power board, but from top to bottom the road like this: zero, 289mv that would shoot up to 3.3v when pressing the power button, zero, zero, zero, 20v, 20v, 20v, 20v, zero, zero, zero, 5v, 3.3v

The connector to the left speaker read 3.3v, zero.

The connector to the right speaker read zero, 41mv, 41mv, zero

The connector to the standby light that never turn on is, from top to bottom, 3.3v, zero, 3.3v.

Note that the picture of the main board shows the digital audio output port lit up red. That turns off after a few second. Also note that every once in a while when I press the power button I will hear a very noticeable buz and high pitch noise coming from both speakers that won't go away until I unplug the tv, but it's random and doesn't happen very often. Let me know if you need any more info. Thanks!

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@rohansolo does this "up to as high as 50v for a split second when I pressed the on button, but go back down quickly" change when you unplug the backlights from the power board?

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Yes, when unplugged from the power board LED 1 and LED 3 would only go up to 0.66v after pressing the on button. LED 2 and LED 4 would go up to about 380mv when pressing the on button.

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@rohansolo with the backlight connected again etc. Turn your TV on and measure the voltage on the fuse of the T-con board. So far, all measurements were made in DC mode, correct?

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Wasn't my power board, looked on the back of the main board and see this. Anyone think my mainboard is fried?

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@blainetaylor looks like superficial corrosion. Whats on the other side of that board (same location)?

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Didn't matter the new board i bought didn't help either.

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@blainetaylor if you are thinking of changing the backlights I would suggest getting two bug suction cups with handles the help lift the panel. It was actually quite easy, but nerve racking. I placed a couple towels down and put the panel on the towels.

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RohanSolo will be eternally grateful.
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