Bad transistor

if the transistor is near the amp power supply terminal it might be a

MOSFET

if the label on that component cannot be read, try reading the one next to it.

if all fails you can try these part numbers

IRFZ44NPBF-ND

NTP45N06

but you need to replace them all to be sure.( All means all the MOSFET on the power supply circuits only).

does it looks like these?