Optronica amp one channel out.

Question

Channel A and B left side output has no output and nothing but a buzz on the earphones for one channel - assume left channel Meter for left side shows no level. Checked the 2 side by side fuses and both are testing Good. Right side works good.

Inspection shows no burned parts. Circuit board looks good. Rotary switches Look good. Volume controls measure same Resistance
Two variable resistors on the board measure about the same Resistance between terminals.

Do you have schematic or suggestions on what to test next?