Sony music system MHC-GZR77D eject message disply & set is stand by mode

Could be an open internal fuse or power supply failure

that was in a MHC-GZR77D that the drive belt was the answer to

In this problem change the cassette player belt it will be OK 100% .I just solve the problem in my set

Try unpluging the power then wait thirty seconds then plug it back in. I have a feeling there is a disk stuck in it.
Terry

There is most likely a short in the amplifier that will need to be repaired before the unit will power on normally.

If only demo mode is playing,it's mean set is in standby mode.Normally
in standby mode it can't be operated.You have to power on the set by pressing power botton during demo mode.Once it gets power on,it will
start to work according to user required function.If the set not get power on after pressing power botton,then it's an electrical problem
and need to visit your nearby repairing centre.

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This should be because you have not used the player for many years. Does the system eject the CD tray? Use the CD head cleaner CD that removes dust on the laser head if any. Try using the audio CD which are recorded in .cda format and not ion MP3 format.