You can turn it off with the little switch inside
the main batter
compartment. The machine should run from the main battery
(or a 5V PSU connected in place of the main battey) if there
are no otehr problems.
Both of my PX-8s (an earlier and a later model) will run with the 6V
wall wart connected into the proper socket when the main and wedge
batteries are removed. I wouldn't exactly recommend bypassing the
little bit of protection the charger input provides.
I would strongly recomend against doing that. There is no real regulation
in the input circuit of the PX8, it depends on the battery to clamp the
supply voltage. Yes, there is a protection zener (D19 iIRC, I had to
replace it on my machine, it was litereally blown off the board), but
it's something lik 7.5V so it doesn't offer much protection.
Running the machine from a _regulated_ 4.8V or 5V supply in place of the
battery is safe. The logic gets that voltage (less a little bit dropped
across the switching transistor), much the same as it would get if there
was a NiCd installed.
-tony