Z80 CPU (ZX Spectrum 16 48k)
After the ULA is checked and you are seeing something on screen (even it if is messed up), the next thing to confirm is operational is the Z80 CPU.
The Z80 CPU is driven by the ULA IC.
Firstly the ULA checks to see if there are any expansion port ROM available (such as a Diagnostics ROM) and if so, loads that.
Otherwise, it reads the BIOS ROM and runs that on the Z80 CPU.
The best way to test the Z80 CPU is to use a diagnostic ROM such as DiagROM. The diagnostics ROMs only need a working ULA and Z80 CPU (along with all traces between them working).
The BIOS chip and all RAM are not needed to test the Z80 is working if using a Diagnostics ROM.
Presuming you have checked your power supply. the ULA and basic output of something to screen, and using a diagnostics ROM if it doesn't load properly (outputting test sounds or the screens you would expect) then it is only one of a few things:
- Bad Z80
- Bad ULA (unlikely if previous tests worked)
- Bad traces between Z80 and ULA
Often a bad trace such as D7 being broken between the Z80 and ULA would result in a screen with vertical stripes, or horizontal lines, or total garbage, with no sound. Often there are flashing blocks too like random cursors.
If using the diagnostics ROM now shows something on screen or the diagnostics beep sounds, then it is pretty certain the Z80 is ok, and you can use the diagnostics ROM to now diagnose further issues with the system.