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39LN5300 with black screen, no image with flashlight

I have a LG 39LN5300, manufactured 2/2014, with a black screen, but with sound (static). I looked at trouble shooting guides and confirmed there is no image even with a flashlight. I replaced the motherboard but this did not solve the problem.  Now I am suspcious of the power supply board, but would like to verify if mine is bad first.  I am able to use  a volt meter.

I took meter readings from the P201 9-pin plug for ribbon cable to motherboard.  Next to the plug there is a chart with 2 columns of 8 numbers each:

Left column says "PWR-ON", lists bottom to top:

3.5, 3.5,     GND, 24v, GND,    12V, 12V, GND

Right column says "DRV-ON", lists bottom to top:

PDIM, N.C.,     GND, 24V, GND,     12V, N.C., GND


Readings from bottom to top as follows:

a.  Plugged in but not turned on:          0,       3.3, 3.3,       0, 0, 0,         0, 0, 0

b.  Plugged in, turned on with remote:  3.0,    3.2, 3.2     0. 23.8, 0,   12, 12, 0


Can one tell from these readings if my power supply is bad?

Thanks.

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If no answer forthcoming here you might want to check out:

https://old.reddit.com/r/TVRepair/

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I see P201 above is an 18-pin plug.

Note the following per the trouble shooting flow chart link:

1) “Check stand-by Voltage. P401 3, 5pin : +3.5V_ST. “ P401 is on the motherboard and receives the cable from P201 on the power board. I have 3.38 volts on pin 3, i have zero on pin 5. SO I am not getting standby voltage.

2)The next step if I don't have the 3.5 v is "Check 18pin Power connector". i unplugged and re-plugged both ends, and it doesn't exhibit any damage. I could try to check continuity across all 18 pins but it seems unlikely to me that the connector or plugs went bad. BUT….i did have 3.38 v out of two pins at P201. Thoughts?

3) If OK, the next step is "Main B/D 3.5V Line Short Check" . Not sure how to do that: I am not sure where the 3.5 v line is, nor how to check it for shorts. Suggestions?

4) If it is OK it says "Replace Power Board."

I am thinking that it is worth the gamble that the power board is bad, but would love to be able to do steps 2 and 3 if anyone can guide me.

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Hi @ribertgropius ,

Here’s the power-up boot check troubleshooting flowchart from the service manual for the TV.

This should help you to find the problem

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Hi @ribertgropius ,

As there is no circuit diagram with the manual I'm going on what you're saying.

If there is power at pins 3 & 5 on the power board but not at pin 5 at the mainboard and assuming that it is a direct cable, wire for wire between the two boards then the problem may be either in the cable or the connector at either end.

You have to prove continuity of the respective wire in the cable from end to end, just to eliminate it as a possible cause of the problem.

If the cable tests OK, you'll also have to visually inspect the connector on the boards (cable?) at both ends to see if it is OK on pin 5 and not damaged or has lost tension or if it is soldered OK to the board, again just to eliminate it as a cause of the problem.

To check for a "shorted line" on the mainboard would be difficult without a circuit diagram. What this means is that there is +3.5V going into the board (which you say is not there) but due to a fault there is not the required voltage appearing at various points in the board itself and the entire Standby voltage detect circuit on the board would have to be traced to find out where the problem is.

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