Well,It appears to be the Z-80 CPU.With all the chips associated with the D3 line are pulled,D3 still swings between 0-2V.No shorts to ground,or other D-bits,that D3 line just won't swing full TTL levels.
Question though,all the Z-80 CPUs are either higher clock speed,NMOS/CMOS.Does it matter what Z-80 is used as long as it meets spec Original CPU is a Mostek.Another question.Have replaced a few of the 6.8uf caps.How critical are the values.Most of the caps. I find are at least 10uf.Acceptable to replace the 6.8 with 10uf.And still another question.Why Tantalums as output coupling caps. I have seen them in some "British" Eq circuits.Unique distortion
Jim