Sometimes the tone board can cause protection errors. If the signal coming from the CD Player is too high you can at high volumes overdrive the tone board causing clipping of the signal once it gets bad enough the protection circuit will see the clipping as DC voltage and put the unit into protection. Some CD players can output up to a volt of signal. I have had this problem and had to reduce the output level of my CD player. The input sensitivity of the AUX and Tape inputs is 150mv. There is no maximum stated but I have found that levels of 300 to 500mv can cause overload of the tone board. As the tone board is after the volume control it is volume level sensitive. You will need a scope to see this but I will bet that is the problem.