Tested on the new motherboard and didn't make any difference which way round the leads of the multimeter were, same findings. Did same test with my LED and battery home-made continuity tester and the light is brighter on the connections expected and dimmer on the ground and other voltage wires. Going to have to take a gamble and test my old system tomorrow, to see if works as continuity testing doesn't help this time round. UPDATE - got the LED illuminated on the console when turned it on but the system didn't boot. Also smelt something subtle like smoke, slightly; which therefore must mean the board is toast. Removed the ports from the new GC motherboard in preparation to installing that instead. Fair bit of work to do over the next days when get time.