Denon AVR-687 cannot power on, no power light lit around power

https://www.manualslib.com/manual/38357/Denon-Avr-1905.html
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Greetings
the red flashing LED is that the Device is in protection mode.
the internal power supply . +15 volts and -15 volts are defected you need to be replace it ... Regulators 7915 and 7815 .... these you should change , might also need to check capacitors .
these are common defects for Denon . AVR units.
Good Luck
Hope this helps

It has to have one---follow the power cord inside the unit--if fuse is good consider taking it to a shop for an estimate.

Need to check on the power supply to the output drivers, if there is no response on the speakers it is possible that the output replay is under protection. You need to remove power plug and leave aside for an hour before the amplifier is plugged back. If all functions show on display but there is no sound check the PREAMP output- audio out. If there is audio then the fault is with a non switching of the power amp or a power amp protection. Since these are mounted boards with little access to testing you need to be skilled to go further to probe and proceed. Also Denon might refuse to service if the amp is tampered which also can be expensive.

Was it put into standy from the remote control and then just turned off at the wall? This can be a common cause, as it will be stuck in standby

Check power cord connection.

may be break part insied microphone jack...

There should be a switch that you could set to the various voltages. Normally a round switch in one of the corners on the rear panel that you can turn with a flat screwdriver

Try this, turn off the unit and remove the AC power cord from the electrical outlet, Hold the AUTO button and PCM button and plug the unit back into the electrical outlet, if the display starts flashing with 1 second intervals then release the 2 buttons, this initialized the microprocessor

Replace the internal fuse