The transistor and diode shown in red below, must be added to the main PCB, the pad positions should be present but unpopulated.
Note: The pinout of the 2N3904 is not identical to the original transistor used by Amstrad and should be mounted as shown belowwith the Base and Collector pins swapped! Should you be able to source the original transistor (now sold as 2SC1815) these pins should not be swapped.