First of all i have a player like that and once i used a different charger. the charger had different polarity and when i plugged it in the mp5 nothing happened. then i noticed that near the arrow buttons it became hot. When i opened it i found that i fried a small chip.
From that time on it didn't charge any more. I tried replacing the small chip but could not recognize it. Then after some testing with multimeter I found that if you solder a diode instead of this it began to charge but the charging animation didn't turn on.
The second thing. The button you are searching is probably hidden. You'll have to reach it by inserting a small screw driver in the line-out jack and then connect to usb.
Hope this helps!!
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