If you run the test tones from your receiver, do you hear the tone from the left tower speaker? If not, do a battery test on the speaker, and see if you get sound connecting and disconnecting a 1.5 volt battery. If you hear no thump, thump then the speaker is dead. Alternatively switch the left and right speakers, and see if the same speaker has no sound, or if the right is now dead. If it switches, then the receiver is defective. If the receiver is at fault, which is most likely, then do a hard reset. If this does not cure the problem, then your receiver needs repair or replacement. With the age of that receiver, it is probably the receiver that is defective. It is not worth the cost of repair.