If you have access to instrumentation then try having a look at it yourselves, not in an attempt to repair but simply as a diagnostic. You're correct in that Stereo Exchange is asking WAY too much just to have a peek at it; a complete repair job might not even cost that much at a reasonably competent shop whose prices are FAIR.
As JC advises, first ensure that it isn't an interface issue by disconnecting all cabling in & out, but of course AC power must be connected. If protect mode is still triggering, then get access to an oscilloscope or at least an analog voltmeter. Observe electrical activity at the output terminals, one channel at a time, while powering up. Specifically look for transient voltages or DC offset (even less than 1 volt) at the output terminals. I suspect that you may observe some activity in that regard, confirming that indeed the unit is ailing. However the only benefit of this test is for you to help your repair shop save some time by steering them in possible directions of failure. JC very aptly observes that repair cost of this unit may not even be justifiable considering its original price.
As JC advises, first ensure that it isn't an interface issue by disconnecting all cabling in & out, but of course AC power must be connected. If protect mode is still triggering, then get access to an oscilloscope or at least an analog voltmeter. Observe electrical activity at the output terminals, one channel at a time, while powering up. Specifically look for transient voltages or DC offset (even less than 1 volt) at the output terminals. I suspect that you may observe some activity in that regard, confirming that indeed the unit is ailing. However the only benefit of this test is for you to help your repair shop save some time by steering them in possible directions of failure. JC very aptly observes that repair cost of this unit may not even be justifiable considering its original price.