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Here are the pictures of the power jack assembly I have taken (click on a picture for a larger version).

When you remove the two allen-head (hex-head) screws on either side of the switch plate, this is what you see: 

1

The switch plate can be pulled out fairly far by gently working the wires out of the scope base.

Here is the switch plate viewed from above, the power jack is on the left and the switch is on the right:

2

Here is the power jack viewed from the side:

3

There are 3 connectors: #1 is on left (relative to viewing the power jack from the outside of the scope), #2 is on top, and #3 is on the right.   #3 is connected to the positive terminal (center pin) of the jack, #2 is connected to the negative terminal (outer barrel of the plug), #1 is connected to a switch such that it is connected to #2 when the power plug is out and disconnected from everything when the power plug is in.

As others have remarked before, there is a diode (a one-way valve for electricity) between the power jack and the battery pack in the scope base.   The diode prevents external power from flowing into the internal battery pack.   This diode is inside the plastic sleeve shown below:

4

Apparently some scopes were made without adequate insulation over this diode such that short circuits could occur in this area.  Mine is a fairly early scope (Serial 60317 making it the 317th off the assembly line) and it is well insulated.

Here is another shot of the sleeve with the diode:

5

Note also that there are two red "quick disconnects" here (the big red things), one is connected to the diode and therefore is in the positive wire going to the battery, the other is in the negative wire going to the battery (I verified these connections with a continuity tester).  The wires that go off to the scope electronics are shown on the right (labeled "scope positive" and "scope negative") and the "mystery wire" from terminal #1 is shown going off to the left with the battery wires.

This is what I have figured out for the schematic of the jack: 

Note that the barrel of the plug does not make contact with the outer metal of the jack (e.g. the threaded mount) when inserted.  If you push hard on the plug you can make it touch but it doesn't by default.  Also, the threaded mount isn't connected to any of the 3 connectors.  Finally, I can't find what the mystery wire connects to.  It is connected to battery minus when the plug is out but not when the plug is in.  It is not grounded to the scope frame (in fact, nothing appears to be grounded to the scope frame though I have seen a ground wire in one of the pictures in the "inside_n5_pics.zip" file in the eGroup file area but this wire isn't grounded.  I can't tell who supplied these pictures but they are great - thanks!  I have posted them here: N5 Guts).  When the power plug is in the jack, negative on the plug connects to the scope electronics negative wire but not to the mystery wire, so the mystery wire appears to be truly isolated when the scope is running on external power.

Any insights would be appreciated (russbag@mail.com).

February 16, 2002