Thanks for the replies James Wilson and al-yeti.
I will fit a 680 ohm resistor at each end of the AB Data Bus and remove the link in the panel.
The replacement PCB is coming from an ebay transaction. I know that is a risky source but the guy seemed to know his stuff and the board was £25 so worth trying. He was using the board for training purposes and has reset to factory default. Just not sure about running on an earlier firmware version. I suspect that flashing new firmware, should I obtain the file, is quite tricky.
Main objective is to clear the power fault and RS485 issue where the keypad is flashing with row of stars in display. Notwithstanding the power rail fault the flashing stars display is down to poor Data Bus comms or the peripherals not being registered in the system. The spare PCB is as much a diagnostic tool as it is a replacement. If all fails then I will need to stump up for a new board such as the Flex 50 as al-yeti kindly suggests.
Can I keep the two existing RF portals and single keypad with a Flex 50 ?