The Casio AC adaptor for the Z-7000 is the AD-B60080. It is rated at 6V, 800ma with a negative tip. The service manual calls for 6V 500ma. The unit will draw the current it needs, my guess is that 500ma is close to the minimum, below that, performance may be affected.
On the adaptor and on the back of the Z-7000, the polarity of the tip is clearly shown +---(o---(-) with the tip being negative. (Sorry for the bad drawing, look on back of the Z-7000 for the real one.) The tip must be negative. The adaptor from Radio Shack for their Z-PDA also should be a negative tip. They included a sheet in the manual explaining that.
Looking at the schematic, probably no damage was done. The postive voltage will simply not turn on the transistor and not damage the voltage regulator if the voltage is 6V. The Z definitely was running off batteries, not the adaptor. There is no way it will power the unit with a positive tip. It is difficult to test because good batteries need to be in the unit for it to turn on even using the AC adaptor. I would definitely change the tip to negative.
We have gotten several calls from Tandy Z-PDA owners who were told to use a positive tip and we explained that the tip is negative and they have not called back--so let's hope there is no permanent damage.
Bill and Scott Casio Tech Support