Left channel is dead,but with max volume both channels are ok. Is this a known problem?

If you're saying the channel awaakens when you crank it, there is the slighest chance that a common control may have developed a high-resistance or 'dead' spot and is causing your symptom. Turn the POWER OFF and operate every control throughout its range a number of times, especially rarely-used ones like Tape Monitors and the Mute control.

If it still doesn't work you might want to prove if its a low-level signal loss or a high level amp failure. One way would be to run an RCA cable pair from the Tape Out to any other device (even a TV with speakers) you might have that accepts an audio input. If IT can't detect receive the DEAD Channel you