If you can read schematic circuits and have and know how to use a DMM (digital multimeter) then here’s the service manual for the receiver. It has the full wiring diagram for the receiver, some troubleshooting flowcharts, disassembly instructions and a parts list, so basically it has everything you need to locate and fix the problem.
It may be something simple like a faulty connection to the left speaker output connector or as difficult as tracking down the problem from the left speaker output terminal back through the left speaker channel audio amp stages etc. until you locate where the problem lies. You won’t know until you start testing.
If you don’t know how to read wiring diagrams or how to safely test “live” circuits with a DMM, then it will be extremely difficult.