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I have 2 of these. Sometime back, one of them went into protect mode. I couldn't make it work, so I put it aside and purchased a different model. A couple days ago, the other one began to fail. Initially there was some loss of volume and a fairly loud hiss. When I would rap on the case, it would cause noise through the speakers. I took it out of service to troubleshoot. Initially the hiss problem was still there, but fairly soon that quit, and I discovered that I was missing the right channel for both front and rear outputs. Subsequently, I pulled the other one out of storage and it seemed to be working ok, so I put it back in service. About all I've determined so far is that it is not the output transistors. I have the tech manual. I could use some pointers on how to troubleshoot. I've taken the main board out and inspected the bottom and saw no obvious problems. The only test equipment I have is a multimeter.
It turns out the problem is some broken solder joints on an IC.
Further edit: It appears that the two power amp IC's run hot enough that over time, the solder joints will break. They were probably not of good quality to begin with. After resoldering all of the pins on both IC's in both of the failed receivers, they are now all working. If you have one of these, you can do what I did, but you have to take it completely apart to get at the bottom of the main board (It's not that hard). In the center of it you should see some discoloration around the connections to IC701 and IC702. Inspect the solder joints and if they appear cracked, then just resolder.
It turns out the problem is some broken solder joints on an IC.
Further edit: It appears that the two power amp IC's run hot enough that over time, the solder joints will break. They were probably not of good quality to begin with. After resoldering all of the pins on both IC's in both of the failed receivers, they are now all working. If you have one of these, you can do what I did, but you have to take it completely apart to get at the bottom of the main board (It's not that hard). In the center of it you should see some discoloration around the connections to IC701 and IC702. Inspect the solder joints and if they appear cracked, then just resolder.