To help point you in the right direction for your specific repair, could you tell me:
A: The "Matrix" button upmixes stereo sources to 5.1 surround. If you are testing 5.1 output and only getting front speakers, turning off the Matrix mode solves the issue. logitech z906 service manual patched
Usually caused by a failure in the Switching Mode Power Supply (SMPS) circuit. To help point you in the right direction