How to work out unknown PSU replacement

Hi folks, This week I managed to get my paws on a machine that I only ever saw in ?coming up!? type magazine articles in the mid-80s. It?s made by a UK manufacturer of Viewdata set top boxes and home micro modems called Tandata who were a split from Tangerine, the company that gave us the Microtan 65 and eventually the Oric 1 and Oric Atmos. Documentation on the Tandata PA is zero, if you search for it you get my Binary Dinosaurs page and nothing else so tonight I set about trying to work out the power inputs from its 4 pin socket. Going clockwise pin 1 is definitely GND/0V and pin 2 is not connected. Pin 3 goes to the input of a 79L05 -5V regulator which via a capacitor seems to be used as the GND pins for 3 CMOS 74 series chips. Pin 4 goes to a 7805 5V regulator. I?ve never seen a -5V reg be used in a GND circuit so before I continue searching am I barking up the wrong tree? The trace literally goes from socket to 79L05 pin 2, output goes to a capacitor then to the GND pins on a CD74HC74E, CD74HC86E and CD74HC4066E. There?s a VARTA battery nearby too. Board pic is here: http://binarydinosaurs.co.uk/tandatapa-13.jpg <http://binarydinosaurs.co.uk/tandatapa-13.jpg> Any insight much appreciated! -- adrian/witchy Owner of Binary Dinosaurs, the UK's biggest home computer collection? t: @binarydinosaurs f: facebook.com/binarydinosaurs w: www.binarydinosaurs.co.uk