Hooked up brand new Dual XR 4115 am/fm receiver. Works fine for about 15-20 minutes then no sound. All the lights are still on and all the buttons will work. Went back and returned it and got another one same thing worked only about 5 minutes this time and just as soon as I turned "Loud" feature on it --the sound went out. Lights still on, but no sound again. Went and reset to factory default, but still no sound. CAn you help???