Today I put a Z0840004PSC Z80 CPU into my CPC. It's NMOS, 4MHZs, so I thought it should be fine. It properly turned on, I could run some programs BUT: as soon as I execute something that accesses Playcity sound chips, it will also switch the relay of the cassette port.
E.g. in Alcon while in menu, when the game shows the extras flying in from the side, the relay switches rapidly and it sounds like a machine gun. With the Reset10 demo disk, I also had these clicks, but not as many - so it seems to be related to specific commands only.
Any idea what could go wrong here? With another CPU I don't have that behaviour.
possibly a bad z80?
im currently running that same model z80 in my 464... but i dont have a playcity.... havent noticed any difference between the original Z8400APS and the Z0840004PSC (the PSC is used in speccy+2A)