Boot up with garbage, quickly clears screen and then nothing. Used Fluke with Z80 pod – read rom OK, and write ram OK, but timeout reading any Ram location. Check fluke status and ‘wait’ always set. Check pin 24 of Z80– stuck low, trace back through 7H,7C,6C1 to 6B1 pin 2 – always low, although pin 1 changing. Replace 7404 with new one, now working normally.
Buzzing noise, particularly when lots of bright colours on screen, loudest on white test pattern and main game screen. All boards I have had with this problem use the LA4422 Amplifier Chip. To solve this problem add a mylar capacitor of 100pf in the corner of the PCB at A9. There should be an existing electrolytic in the corner (C24) and near that a ceramic disc capacitor (C23). Remove the track coating from the track going to C23, and parallel to that on the ground track around the edge of the board. Mount the new capacitor on these two tracks where marked on the diagram below.
n.b. To disable the watchdog on this PCB add a link from pin 1 of 9C to ground. For safety it may be a good idea to also cut the track, otherwise you may take out the 74138 that connects to it!