Tamper won't go off?

My Scantronic 9448 had 12v at the pcb but the pcb and lights did not light up, so I replaced it with my spare, a completely new panel. All good, can program etc, except the tamper light comes on so can't set it. Loop the tamper terminals and no change. Just the single beep when I put the code in. Any idea folks? Thanks, Martyn