I bought an amplifier Yamaha A-520 on auction. The CD-Player plays (it's certainly working), but I have only mono signal on speakers (the same on phones out). The signal is the sum of left and right signals. When I turn the mono-stereo switch nothing changes. When I turn the balance left or right nothing changes, too. But when I turn it completely to left or right - there is no sound on speakers at all. Do you have any propositions for me to repair it to have stereo?

edit: my mistake - when I turn the balance to left I can hear louder signal which is recorded to left (accordingly to right of course)