My avr 300 goes into standby mode and cannot be turn on. How can i turn it on and what causes this things to happen?

Please see my other answers for Harman Kardon issues. If you can't find them, please respond back here.

It probably needs a new servo switch. I would recommend taking it to a qualified technician for repair.

Open the unit and measure the voltage across the reset microswitch on the transformer board (with the unit in standby). It should be a hi signal, over 3 volts. If it is low, below 1 volt (when you can't turn it on) it is sending a constant reset signal to the cpu. Remove the switch and try again. If the problem goes away, the switch was bad. Its a service switch so you don't have to replace it.

Is the red light flashing at first?
The unit may be going into protect mode. Lets hope it has done the job and the receiver has protected itself from failure.
Protect mode can result from speaker wires touching each other or the Amps metal case. Check that first.
It can also be caused by unstable ohm loads if too many speakers are connected to any of the outputs especially zone 2.
If you can remove all speaker wires and turn the unit back on. If it stays on, turn the volume down on the main zone and zone 2 if connected. Then reconnect speaker wires.

Make sure it is not in download mode!! a little switch on the back of the device

It has a blown channel, unless you are good at repairing them I suggest an authorized Denon service center. This Denon not only needs the shorted output transistors replaced, but a few resistors, a couple of driver transistors and a capacitor on the preamp board as well. alot of parts to be replaced. I have repaired about a hundred of these over the last few years and my shop charges about $200 for this repair, other shops should be about the same.

Unit is in protect mode there are a few problems that can trigger protect mode the most common is blown channel (shorted outputs). Sometimes one of the 15v regulators may go bad causing imbalance of supplys triggering protect mode. Also sometimes a bad solder connection could cause unit to go into protect also. I work for Denon authorzied repair center in Michigan if your nearby and Shipping to Michigan is economical let me know [email protected]

Going into protect due to either blown channel or probably a 15v regulator might be down which I have seen many times also. causing unit to go into protect.

press the off/on buttun and keep it pressed in while pressing the standby button... it should fix it