So i have a samsung un65nu6900f in for service. Its doing the power cycle 2 blinks thing. Many other people have worked around this by taping the LVDS cable at the expense of losing some color quality on the affected side of the panel.
My customer is not super happy about that compromise so i spent all day tracing this thing out today. I can confirm that the issue is not related to the clock signals, but rather the VGH signal right next to them. It is supposed to be 25V, it is 0v while the whole cycling thing is happening. If i remove R13246 from the main board the tv will power on with the slight discoloration. This is breaking the signal path from VGH on the main board to VHG on the panel. Same as taping the cable but easier to play with back and forth.
No detectable shorts on that line and in fact it measures identically to the VGH on the other side of the panel in terms of resistance, diode mode, etc.
Here is where it gets even stranger, if i feed the 25VDC into the VGH test pad on the panel PCB from a bench supply the color clears up and it looks literally perfect.
I can confirm that with that resistor removed the mainboard is putting out the expected 25V, but with the resistor in place that same measurement point on the main board drops to 0V. That screams short circuit to me, but no measurable short exists. One would assume if a short exists on the panel, then my bench supply should reflect that, and it certainly shouldn't just work perfectly.
Already replaced mainboard just to be safe and issue is exactly the same.
Anyone have any ideas?
If it comes down to it im just going to put a step up regulator somewhere inside it for the 25V, but i would love to figure this one out for real since it is a rather common problem.
My customer is not super happy about that compromise so i spent all day tracing this thing out today. I can confirm that the issue is not related to the clock signals, but rather the VGH signal right next to them. It is supposed to be 25V, it is 0v while the whole cycling thing is happening. If i remove R13246 from the main board the tv will power on with the slight discoloration. This is breaking the signal path from VGH on the main board to VHG on the panel. Same as taping the cable but easier to play with back and forth.
No detectable shorts on that line and in fact it measures identically to the VGH on the other side of the panel in terms of resistance, diode mode, etc.
Here is where it gets even stranger, if i feed the 25VDC into the VGH test pad on the panel PCB from a bench supply the color clears up and it looks literally perfect.
I can confirm that with that resistor removed the mainboard is putting out the expected 25V, but with the resistor in place that same measurement point on the main board drops to 0V. That screams short circuit to me, but no measurable short exists. One would assume if a short exists on the panel, then my bench supply should reflect that, and it certainly shouldn't just work perfectly.
Already replaced mainboard just to be safe and issue is exactly the same.
Anyone have any ideas?
If it comes down to it im just going to put a step up regulator somewhere inside it for the 25V, but i would love to figure this one out for real since it is a rather common problem.
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