I tested the TV, using the PC input, and the computer image appeared on the TV screen, so everything was perfect.

put a rca cable into the video output of a DVD player and the other end of this cable into the SPDIF input of the TV, the picture did not appear.

I am beginning to suspect that the main board is faulty, only replacing it.

And if this does not work, take the service manual and proceed to repair. Or try to reflash.

S / PDIF (Sony * / Philips * Digital Interface Format) is an audio transmission interface format that supports the transfer of digital audio signals from one device to another without the conversion procedure to an analog signal, thus avoiding degradation of sound quality.

In your case it is better to replace main.