Ok, I read the earlier thread. The diagram is the same one I remember using.
The drawing showing the pin positions is of the face of the female connector on the MX100 when looking at the back of the unit from the outside, which I think may be causing the confusion. The label that says "inside the MX100" means the jack, not inside the actual cabinet.
What I did back in 1990 was I cut the male connector off the the A/C plug that came in the box with the unit. I used a current tester to label each of the stripped wires as they matched to each of the male pins.
I then used a small radio shack DYI box and a set of 9V snap connectors to wire 6 9V batteries to the identified wires on the plug, 3 each in parallel (not in series VERY IMPORTANT!!) so that the system was still 9V, but would run longer.
So, again looking at the back of the MX100, you would want the bridged positive and negative battery terminals running to pin #1, at 3 o'clock-the lower far right hand side of the female jack facing you, the open negative connected to pin #3, at the 11 o'clock postion, and the open positive connected to pin #4, at the 9 o'clock position. Again, these are based on your facing the rear of the MX100 looking at the female connector. If you are trying to pack batteries internally, or do this without the 4 pin male plug, you just need to make sure you are following the wiring of the female plug end to match the diagram.
If this does not work, then there has to be something wrong with the unit itself.
Hope this helps.