MY Teac AG-370 reciever only plays one speaker , I know the speakers are good ,I checked them , what other things can I check?

If you've already swapped them left to right to prove the speaker works bt the channel doesn't then you know the problem is in the electronics. Is it one or more sources? Turn the receiver off and manually operate all the switches and knobs throughtout their ranges (particularly the Tape controls) and see if that helps. No? Take it to someone to look at it.