I have an old sony str-6036a my father bought new in 1973,now left to me...the left side speakers have no sound.i have cleaned it, sprayed the pots and checked the balance control pot with ohm meter is the same reading when i turn knob left and right...that seems ok...i'm not an expert and not sure how to check amps,there seems to be two#stk-054...any ideas would be greatly appreciated. thank you