Scantronic 9827 alarm keypad?

Hi. I have replaced a 9800 keypad (on a 9800 control panel) with a 9827 keypad for someone as some of the old keypad numbers has stopped working and sourcing the 9827 from e bay was the simplest option negating redecorating etc as it's the same size as the old one. The NVM of the old panel was removed etc and the installation was straight forward. However the day after installation the alarm activated for a 'tr' fault ( tamper return to sounder). Inputting the user code did not work and everything had to be powdered down/reprogrammed. I disabled the tr by using pins across the tr connections and the alarm worked fine for another 10 days before failing for 'tr' fault again. Once again user codes would not disable the alarm and all power had to be removed. I suspect there's a fault with the keypad but would welcome any helpful comments