Which power adapter do I need for Roku 3930X? And where can I find it at?
You want the first listed after the remotes the PW10 here:
Not the second one that is also a PW10, it has usb extender instead of the micro usb cable which you need.
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If it's anything like my 3900 Express, it's just a stock micro-USB adapter and cable, same as used for many phones. It will need to be rated for a minimum of 1A power.