How can I fix my marantz Amp model 250M?

When I increase the volume the left channel disappears.
If I turn off and on, then the left channel works again but only at low level. I tested the output of the left channel: I connected the power output not to the loudspeaker but to a digital multemeter set to AC V which has a large input impedence. In this case THE LEFT CHANNEL WORKS WELL as I can see at Amp left meter and at the multimeter display. Which components of the left channel amp board are guilt for that?