How do I get my Ranger American PC1550 from "armed" to "ready"? ?

We have a Ranger American PC 1550 system installed in 1993. We moved in Dec 2006. No manual or security code was available for the alarm system. The system was in the "ready" mode and beeped whenever a door or window was opened. This is fine with us. The other day, kids messing with the buttons somehow armed the system and set off the siren with a door opening. I unplugged the system & battery to get it to stop. Now I can't get the system back to the "ready" mode using the 1234 code. Please help.