IIRC the reason why it can accept both AC and DC isn't because of the step down transformer but of the quad diodes. This is readily apparent and easy to spot in the NES-001 schematic. When you drive the NES with AC, the diodes rectify (is that the right term?) the current to DC at a cost of a couple of volts (7v- 2v=5v just right for a computer). When you feed the mess DC, you just use 2 out of four diodes.
Quite right, SL.

The four-diode arrangement is called a Bridge Rectifier.
Tutorial
here for those who want to know how they work.
You can see the Bridge Rectifier in the upper left corner of pg 2 of the NES schematic
here.
The symbol to the right of the Bridge Rectifier is a polarized capacitor. (It's not a commonly used symbol version, found it
here)
After some smoothing/filtering capacitors and the power switch, the 9v feeds into the
7805 voltage regulator.
Based on the schematic, a 9VDC input
should work as well as a 9VAC input, but it will probably cause the 7805 to dissipate a bit more heat since the voltage and current flow is constant instead of the pulsing "waveform without capacitor" (dotted line) shown below -- not sure if that will shorten the component life on the 7805.


Scott