LG 50PN650T sound but no picture

[PowerAmpFreak]

This plasma LG 50PN650T start up with a white screen but goes black after one second.
VS and VA stays on, sound is OK.
I need either a service manual or info how to activate logic board test pattern.

[dick_barton]

Re: LG 50PN650T sound but no picture

If my memory serves me correctly, shorting the 2 pads by R51/R52 and with the ribbon cable removed from the mainboard to run the self test.

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

Thanks for suggestion. I wasn't sure, so good to have one more!

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

I could not get it to enter logic test mode by shorting those pins and removing ribbon from mainboard.
The green led on logic board flashes in a special pattern possibly indicating a failure.
So for now I cannot tell if the failure is logic and/or Y-SUS.

The symptom looks like the classic 50PK650 failure with the open 22R power resistors. In this case, it's not the same chassis nor the same kind of powersupply. And in this set, VA and VS stays on.

A bad Y-SUS tend to not produce any picture at all, this turns white directly after start-up, but is shut down the moment after.

[Andrew F. Ali]

Re: LG 50PN650T sound but no picture

Since the Va and Vs are constant we could say the PSU is OK. Since the Vs is good then I would think the Y-sus and Z-sus are OK. The Logic Bd in my mind is your fault. Replace the Bd and your set will work again.

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

Yes, it points to the logic board. Thanks!

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

Logic board replaced, issue remains.
Could not activate testmode, probably another procedure.

[dick_barton]

Re: LG 50PN650T sound but no picture

Is it the correct Logic board. I cannot quite read the EBR number from your photo but the last few digits look like 60602

Spares2Repair list the last digits as 60502

https://spares2repair.co.uk/lg-50pn6...50r5-60r5-ctrl

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

Yes, both P/N match. They both are EBR75760502

[dick_barton]

Re: LG 50PN650T sound but no picture

Do you have the other supplies Vsc, -Vy, Z_Bias and is +5V on the Logic board

[Andrew F. Ali]

Re: LG 50PN650T sound but no picture

do you have the other supplies vsc, -vy, z_bias and is +5v on the logic board

+10

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

+5V is present on logic. I guess this fault is somewhere on Y-SUS and I will dig into the supplies to see all are up.
It seems like Y-SUS itself are turning off due to some protection.

[PowerAmpFreak]

Re: LG 50PN650T sound but no picture

Success!
I found a shorted UF4006 diode on the YSUS board in the floating supply section.
Now works nice again!

[dick_barton]

Re: LG 50PN650T sound but no picture

Excellent