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When A0 is stuck LOW, the processor can't communicate (proper) with any of the other chips (RAM/ROM/ASIC).
The problem is that all chips that are handling address lines can be responsible for this.
Replace processor first. It is on a socket
Quoted from ravve:following the fantastic Sascha V. layout
great stuff 
Quoted from ravve:So I am right asuming that A0 should be pulsing and that I should focus on that first (not D0-D7 that are not pulsing either)?
They all should be pulsing, but I don't know how the processor responds when communication is not possible.
Quoted from ravve:Could there be some other reason than failed ram/rom/asic for A0 not pulsing
The ICs on top of the CPU-board do fail sometimes (U1, U2, U3)
You have to start somewhere. It is handy when there is a spare CPU-board where components can be swapped en tested good or bad.
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