When the screen goes black, is it really black (like when the TV is switched off) or a shade lighter i.e. backlights on, but just with no picture?
If it is black i.e. no backlights on, then try shining a torch at an angle close to the screen to see if you can detect any images at all. They will be very faint so trying this in a darkened room may help to see them.
If there are backlights it may be the tcon, given that there are vertical lines before going black? but it could also be the mainboard if there is also no audio at the same time. The tcon board is associated only with the video signal to the screen i.e. interface between the mainboard and the screen and not with the audio at all
If there are no backlights but there are images then it may be the mainboard or possibly the power board. If there is also no audio at the time then again it could be the mainboard
Sorry to be so vague but without schematics of the boards (I couldn’t find them anyway, you may have better luck) it is difficult to know what voltages to look for and where.
Perhaps try sourcing a replacement board from a supplier that accepts returns for only a small restocking fee so as to minimize the cost to you in case it isn’t the problem