As far as I remember it could be an defective U901/CP2102
This chip must be recognized when the service port is connected without powering on the box.
The chip itself uses the 5v from the usb to activate.
More about this, see here at jb8a8f8.com.

But...
It may also be a issue with the crystal Y800, or an inductor interruption near it.
However, when that happens, then you should have the issue that all the connections on the back not working.

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Johnny B.

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