I have a Master Control Panel 5500 that has a non funtioning volume ring ie. nothing happens when you turn it. All other keys work fine. I've installed new batteries; checked the micro switch with a multimeter and appears fine. Having searched the forums, this sounds like not an uncommon fault. However, I'm not sure where to go from here. Could this be a faulty opto sensor? If so, where could I purchase one from? Any other things to try?
Thanks
Martin