If I remember correctly the 2500 had 2 battery packs with 4 AA batteries in them. I had problems with a couple of these as sometimes if left batteries in it will in storage  corrode the contacts of the packs   , or they will sometimes the plastic will crack  so the batteries wont make contact. With the alkaline AA battery in each pack  check that there is the 6 volts in each back as there should be 2 pack in it. Each pack should measure 6 volts on a VTM meter, so it is supplying 12 volts total.   This way you know it has the power to run it, then is the contact in the detector are cleaned to make good contact. If no go then there has to be something in the control box has a problem.
      Check it out and see if you have the voltage at the pack first and see if any contact has corrosion  on any of the contacts , if there is the eraser off a pencil normally clean off the slight corrosion, but if a lot of real corrosion it will need more help .
  I hope it is that simple.
                                                   Rick