Audio Systems: speakers, rectifier diode, filter capacitor

Question
I have a pair of Acoustic Research Powered Partner speakers that are over 20 years old. These speakers each plug into an electric outlet. One of the speakers has a loud hum, the other is fine. I have tried using different plugs, and I have tried using different outlets, but I keep getting the humming noise so I'm guessing that it's an internal problem. Can you give me any advice? Thanks

Answer
ONE: Reverse the speaker inputs to determine if it is the speaker system or the source/tuner/amp feeding it.  If the sound switches channels when reversed the hum is coming from the source unit.

TWO:  Assuming you are right and the amp for the humming speaker is the cause you can be very certain it is the power supply section and either a bad rectifier diode or an open electrolytic filter capacitor. In either case the repair is DIY level if you have basic electronic skills, tools and soldering know how.

THREE:  The cost of repair should not be $$$; probably under $75.  More like $50 would be my guess.

Hope this helps.