Hi folks! I am a new to this site and was hoping to find a "expert(s)" that would be generous enough to provide some troubleshooting suggestions. I have two separate amps with two different issues. I understand basic electronics and have schematics for these units, I am in need of a "shopping list" of things to start checking to point me in the right direction so I am not going down a blind path. AU-517 Amp powers up and plays fine up to a point. I have found that with no low level inputs (cables disconnected) and just speakers connected that as the amp begins to warm up a low level hum appears on the left channel only regardless of volume level. Flipping switches/turning pots etc. on front or back of unit does not affect this. As unit continues to warm up the hum or the level does not change. After about 1 hour the hum gets louder and there are crackles and high frequency "screetches". When the amp is powered down and cools off the entire process is repeatable. AU-919 Unit works fine until it warms up for about an hour and the power "protector" circuit kicks in with the flashing LED. It stays this way for 10-15 seconds and then turns off and operates normally (this may happen 2 or 3 times). Note, than when I power up the unit cold or hot it takes ~11 seconds for the protector circuit to "close" and the LED to go solid red. This time seems long and is much longer than the 517 protector that switches in after 2-3 seconds, don't know if this suggests anything. thanks in advance for any guidance.