Scantonic 9800 system?

Can any body help. I have scantronic 9800 alarm sytem fitted 20 years ago. Recently it started to play up. I have changed the battery only 1 month ago.
I set the alarm normaly it sets I have been told by neighbours that it goes off after 1/2 to 1 hour. When I come home the outside light is flashing inside the home there is no indication on the panel display. I set it off normaly with the code the outside light also goes off. Can someone tell me what the problem is and how can I fix it.