The first thing I would do is to trace where the
from the serial port go.
Opening it up I learned via the prominent
rectifier that the power is AC at least.
There's a big Toshiba TMP80C49P that looks like a 'keyboard'
MCU to me. Some minor things like Motorola SN74LS(05N,138N).
Nearest the SIO port are just a few resistors and two
transistors. At least one leg of the SIO goes directly to
the toshiba. Looks non-RS232 then... Whatsit 5V instead of 15?
If one of them is at a -ve voltage, ...