Have you tried the reset procedure I am outlining below at the end of this post?
If you have not done it exactly this way, try it. The "protect" that is displayed is the protection circuit being activated. Most times it is activated is to prevent further damage to the unit. Sometimes it can be activated from the unit being over-driven (played at very loud volume for prolonged periods or having the improper speakers or a bad speaker).
If this reset procedure does not work, you need to take it in for
service preferably at a Marantz authorized service center because the problem you are having is not something that can be fixed by any sequence of keys being pressed. It normally happens when the unit has a blown channel. That is not the only reason, but it is the most common reason.
If you have the proper tools, a schematic, and knowledge of how to check electronic components to know when they are faulty, you can then fix it yourself. If you do not have these tools and the proper skills, you can cause much more damage to the unit than you started with, even to the point of starting a fire or making the unit un-fixable. This type of unit is not an easy one to repair, even for a well trained electronic
technician. I work at an authorized Marantz service center and have repaired many of these and other receivers over the past 17 plus years.
Be aware, by using this procedure and then turing the unit on can sometimes cause more damage to the unit if it has a shorted channel or other problem that makes it draw a lot of current. This method is meant for qualified
service technicians to perform after they have checked the output stages for any problems and the unit is still in the protect mode. You have been warned, so don't say I didn't tell you it should be taken to a service center first. If you see or smell any smoke or buring smell, unplug the unit immediately. You may want to have one hand on the plug when you do this, just in case.
Here is the proper reset procedure:
SR4500 Turn the unit on (standby without pressing the power button), press and hold the 7.1CH INPUT and ATT buttons simultaneously for3 seconds or more.
If you have found this free information helpful, a "FixYa!" rating would be appreciated, and if you need any more help I am happy to offer it, just ask!
Dave
p.s. I have told you all the warnings and suggested a service center because this is a very good receiver and I would hate to see it damaged more than it may possibly have already. It is possible that this reset procedure will return the unit to normal operation, but more often than not, the "protect" circuit activating means that you have a serious problem that can not be fixed by anybody without the proper training.