Can some one explain the yorkville xm200 bass schematic?

There's any number of shut down circuits from U7A onwards including the photodetector determining U7A's gain. Suspects for frying R84 would be Q15,16,17 and 2 or the associated transistors controlling them but I'd start by checking Q15,16,17 and 2 out of circuit and working backwards. Have a look at the voltage between TP1 and TP2 and adjust for 2mV with P7 as recommended on the diagram first before you break out the soldering iron. If you can't adjust it or it's a zillion miles away from 2mV that's another clue as to where things are going wrong.