Alarm not arming

[jason taberner]

Wondering if anyone can help. My Grandad recently passed and his home has an Optima alarm. It was installed in 1990 and back then it was the latest model if that helps. The batery in it is over 8 years old and has caused the alarm to falsely go of (or this is what I was advised). I put the alarm into engineer mode so as not to go of and swopped the batery for a new one.
The problem is that now the alarm won't arm itself. When the cover went back on I pressed the clear button as it stays but the alarm went of as if it was being tampered with. I presed the code but it wouldn't stop. I switched of at the mains and took the cover of and disconnected the batery. This stopped the alarm but now when the mains is on the alarm is sounding and I'm having to leave the mains of and the batery connected to stop it from going of. Can anyone help. All I did was change the batery which should not of caused any problems. I swopped the batery on my dad's 6 months ago and nothing like this happened but that was a different alarm. Is there something else I should of done or is there a fault somewhere that can be rectified.

 

[sparkymarka]

Try 0123 to switch it off it may have gone back to factory set

 

[Doggit]

What lights come up when the alarm goes off. It's possible that the tamper spring has been twisted, so not rearming itself - push the front plastic in were the tamper spring is, and see if that corrects it. Daft suggestion, but you've put the battery back in the right way round.