Since N770 doesn't have internal USB charging - the +5V USB line comes directly do TAVHO USB chip without any additional power processing.
5V 850mA charger - what I think is: the cheap plug-in chargers are far from specs since if the device supports charging - it has to have built-in protection and current limiting devices as well as a kind of voltage limiter.
Even oryginal 770 charger marked as +5V has circa 6V at the output without load. On 770's schematic charger line is noted with "6V with charger connected". What I'm pointing is that all plugin chargers have poor voltage regulation capabilities since they are made to be cheap and the battery equipped with carger circuitry can withstand this.

My diagnosis is that You fried Your tablet. Sorry but I don't believe in voltage driven software failure.