Yamaha CX5M - Power Connector Pinouts
OK, after a bit of investigation inside the machine, I think I've solved this one. I post the information here in case it is useful to others.
The diagram below shows the pin arrangement, as seen looking at the socket on the back of the computer:
Pin assignments as follows:
1 +5V
2 0V (Ground)
3 +12V
4 -12V
5 Switch
6 +5V
7 0V (Ground)
8 +12V
9 Switch
10 +5V
11 0V (Ground)
12 0V (Ground)
13 +5V
Pins 2, 7, 11 and 12 are all 0V / Ground, and go to internal Black wires.
Pins 1, 6, 10 and 13 are all +5V, and go to internal Red wires.
Pins 3 and 8 are both +12V, and go to internal Yellow wires.
Pin 4 in -12V, and goes to an internal Blue wire.
The power switch on the side of the computer and the cartridge slot safety switches are connected in series to pins 5 and 9 of the power connector. Both switches must be closed (i.e. pins 5 and 9 on the power connector shorted) to enable the power supply.
If you rig up a non-standard power supply, these switches will not function, and you must switch the power on/off manually, being careful not to remove or insert a cartridge with the power on.
Note that the safety switches are probably not designed to switch the full power supply current, so care should be taken if you wish to make use of them with your own power supply.
The original 13-pin DIN connector is difficult to find, but the socket is located on a small daughter board inside the computer and is easy to change for a more normal connector type.
Alternatively, I believe the Atari ST computer uses the same type of connector for its monitor output, so you may be able to steal the plug from an Atari ST monitor cable, if you can find one.
Confession: I have not actually tested any of this yet - that's a job for tomorrow, but I'm prety confident it's correct. That said, don't hold me responsible for any damage you may cause by blindly using this information - check for yourself before doing anything!
Hope this is useful.
Great info, thanks! 
