So I've looked at the schematic and made some measurements:
power supply 1: transformer 31.5VAC --> 1B4B1 rectifier 19.6VDC measured at v-in on 7812CT regulator, 17.08VDC out. I measured R8 to be 56 Ohms.
power supply 2: same transformer ------> 1B4B1 rectifier 18.2VDC measured at v-in on AN7812 regulator, 13.38VDC out. R8 is 54.5 Ohms. This one working OK. (or should I say, I'm not afraid to plug this in to the motherboard
)
Now, looking at the schematic I noticed capacitors C4 and C6 are on +12V and took a look on both boards... C6, according to the schematic, is supposed to be 10uF but is 1uF on both boards....
Does it matter? I don't know.. Equally, C9 should be 10uF on -12V but is 1uF on power supply 1.
Well I swapped them out for 10uF and no change, infact v-out on 7812 PSU1 is now 17.78V, 7912 out is -13.3V.
Next to try is putting the 7812 from PSU2 into PSU1...
...and now I have 13.6V out from the old 7812.