Just for future reference, the power supply in these machines is pretty rough-and-ready. Here is the circuit diagram (I have a copy of all the circuit diagrams for this machine, courtesy of samdal.com).
And here is where the regulators sit:
The 7805 and 7812 are +VE voltage regulators - 5 and 12 V respectively. They can handle 1A each which should be enough... although they have very high relative voltages going into them... I'm almost surprised they didn't suffer from failures over time. The pinouts when looking at them from the front are 1.input, 2.ground, 3.output
The 79-series are -VE regulators - the 7912 is a -12V reg, but the pinout is different on 79xx's. Pin 1 is ground, 2.input, 3.output. Once again, 1A current capacity.
The 79L05 is a low current version (100mA) and hard to see in my crappy photo - but its in a small black transistor casing. The pinout is the same as its big brother, taken when looking at the flat face with the pins facing down. Its 1.ground, 2.input, 3.output. (The output pin is closest to the back edge of the pc for the 79L05).
Having said that, all 4 rails are at the right voltage in my one so its not that... but I thought I would document it in case anyone else has a faulty one.
Old stuff I own: Casio PB-100, Spectravideo SV-328, AY-3-8500 IC (Just waiting till I have time to build a Pong game with my daughter)