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Panasonic LCD TX-L37D28E - power on failure - no blinking codes available

I have a LED TV Panasonic code TX-L37D28E, the P-board is a PS-309WW, the A-Board is a TNPH0851-1A (TXN/A10QDB), the problem is:
when I turn on the power switch, the front led remain red (no diagnostic blinking) and the P-board relays (which enable 12 and 17Vdc to the A-board) continuously switching between on and off, TV remain OFF 
I have measured all the voltages indicated in the service manual (during 12V and 17 V availability) and all of them are in the expected range.
I have checked the "power on" sequence, it seems the CPU in the A-Board command correctly the closure of the P-Board relays (relay command) but, for a problem, the P-Board generates the SOS signal which request a turn off to the CPU, then CPU removes the relay command and the sequence, after about 2 seconds, restart.
What I don't undertsand is why the P-Board send an SOS signal to the A-Board; is it possible is due to overcurrent? 
I don't know what other points have to be checked to try to understand if the problem is in the P-board or A-board and, eventually, how to solve the issue.
Thank you very much to anybody is able to help me, let me know if more data are needed!