> I think they are actually rechargable nicads

Three AAA Nicads in series, yes.  If they've gone bad, it's easy
to construct a replacement pack out of a 3-cell cordless phone
battery pack.  In my neck of the woods, replacement packs are
readily available at Wal-Mart or Radio Shack.

To check the condition of the batteries, run the machine for a couple
of hours with the battery pack plugged in to establish
a minimal charge in the batteries.  Then turn the machine off
and check the voltage from the pack while it's still plugged in.  If the
voltage drops below 3.6V in less than a week or two, you need a new pack.

If you don't want to wait a week or two, a 100 ohm resistor will
simulate a 36 mA load.  A fully charged battery ought to be able to put out
36 mA for a couple of hours before it goes low.  (The capacity of
a new AAA nicad is 100-150 mAH, for reference.)

Often only one cell in the pack fails, but only a true cheapskate
would bother replacing the single failed cell.

Tim.