PJMBigDog

My Promedia 5.1 speakers are pretty old (guessing about 6 years or so) and never had any problem other than the volume control knob being jumpy (numbers jumped around a lot when turning the knob), but this was an ongoing thing ever since initial purchase. The other day, I just got no sound at all. The control tower still turns on and adjusts but I get no sound. Verified with other speakers that sound card, computer setup, etc are all okay. How do I determine if my subwoofer board is fried or if this is a problem with the control tower unit (which is easily replaced for $33)?