Most common reason for channel to drop out in head unit?

[ringnalda]

When I received my (new to me but used) F900BT the USB port was not working, even with the correct cable.  After some troubleshooting and dissassembly, I found the plug to the circuit board had been pulled to the extent one of the terminals had become disconnected. Easy fix and a knot in the cable will stop it from being pulled out too far again. Everything worked fine for a while, but now one of the four output channels has gone dead. I have heard of the small fuses, but is there one of these on each channel?  Where can I find a wiring diagram?

 

Thanks for any and all help,  Jan