THE FACTS:

• AH-3 is currently out of production. It was first introduced in mid '80s.
  


• The manufacturer does not provide the Service Manual for AH-3.
  


• AH-3 is a derivative of AT-120 (a Marine Tuner).  It uses the same PCB as AT-120 but the hardware and software are different.  A Service Manual for the AT-120 model can be purchased from the manufacturer.
  


• AH-3 software has no test mode and all diagnostics have to be performed in the operating mode.

If you have materials related to AH-3 that could be helpful to other Amateurs, please contact me.

73, N4PL