A hum like this is most often associated with a failure in the power supply. Unfortunately, troubleshooting that is something a novice would find difficult at best. From the photo it looks like a bipolar analog supply, which are the easiest to work with, but I can't give you much more than that. You might see if there's a service manual online, or I would suggest having the thing serviced. I use a particular guy for amp servicing, but he's in California, perhaps that's not good for you. I ship stuff to him, he fixes and sends back. Doesn't seem to matter what it is. PM me if you need his contact.