Out Zone repair log #3

Received this Out Zone PCB for a repair:

Actually board was a factory conversion from a Zero Wing board (hardware revision ‘TP-015’).When I powered it up, I got nothing of screen, neither the  black and white wavy lines sign of the system being initialized like it happens in this Toaplan hardware :

Probing the 68000 main CPU releaved it properly received /RESET signal as well as clock was present.As usual I did my check on CPU/RAM/ROM data/address bus and all was fine.But I went to check the clock signal of the Z80 it was stuck high:

The clock is generated by a 28MHz oscillator then divided by a 74F163 and inverted by a 74LS04 before reaching pin 6 of Z80 with a frequency of 3.5 MHz:

On this kind of Toaplan hardware the Z80 commands not only the sound system but also the I/O, this would explain why the whole system didn’t get initalized since the 68000 could not find the Z80 running.Confident I went to remove the 28MHz oscillator :

Fitted a good one and :

Board risen from its grave!Mission accomplished.