The PSU switching FET (IRF740) is shorted so I expect the drive IC ( UC3842) is also dead. There appears to be NO source resistor with the FET for current sensing.
The PSU produces a *single* DC rail for the bridge mode amplifier of about
140 volts - there is a 1000uF, 160V electro. I guess the speaker outputs rest at 0 volts and the DC rail electronically splits into +/- 70 volts.
Question is, how in hell are only four *puny* TO220 mosfets supposed to form a bridged output stage and drive a 4 ohm woofer from such a supply voltage ?
Looks to me like a real piece of Chinese s*it.
.... Phil