It looks like the power and signal grounds are open. A common problem with this amp is an open resistor/jumper R70-0 ohm 1/2watt, you may have a blown trace on the underside of board as well. But I would check from top side before you remove amp from chassis, because the resistor can be replaced from the top.
With the amp oriented so that power terminals are on your right, the resistor will be just above input filter inductor and behind the 820uf/160v filter caps. You can check and replace this resistor from the top. Make sure that you have a dead short from the Power Ground and Neg Spkr Ground terminals.