Please Help my Sony ht sf 1300 amp is playing up.

[Big_daddy2k45]

I recently bought a Sony Ht Sf 1300 Home Cinema system & it worked great till just before christmas. One day i turned it on & it flashed up protect on the display then shutdown. So i read the instruction manual & it says I have irregeular current from the speakers. I checked & replaced all the speaker wire as i had extened some of the cables. No joy! Sometimes it does work but the next day there is the same problem. I then took it 20miles to the nearest Sony repair centre they say there is no fault. I get it home & plug it in without the speakers & it flashes Protect then shuts down. If anybody can help it would be greatfully recieved.