||I have an old Scantronic ,this is what i did when had the same issue ,its worth a try ....
Take the power off the unit INCLUDING THE BATTERY (your external alarm will probably go off)
Open the panel and REMOVE the NVM chip.
Hold in the tamper switch and power the unit up with the front panel open. Keep holding the tamper in for the next few operations.
Key in 1,2,3,4
Key in 0, ENTER, 7890 and then release the tamper.
Key in 9,8, ENTER
Hold in the tamper again and keep holding
Key in 0, ENTER,7,8,9,0 and then release tamper
You should now be able to close up the panel and reprogram.
Felix Naven March 2019