I have a crack with speedball update that I purchased approximately 8 years ago. I recall I had some issues with it and had to send it back for repairs. Anyway, I hadn't used it for some time and I turned it on recently and there was excessive hum. I can't find the manual. I've looked at all the connections. They all look fine, particularly the ground connections. I have also heated up several of the connections to see if that would help. Anyone have any thoughts? The hum seems to be in both channels it is not affected by the volume. I have tried different power sources, also different input sources and moved it away from any other electric equipment