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On plus4/plus4-sidcart.c it says on line 62:
return sid_sound_machine_init(psid, speed, cycles_per_sec);
Changing that to speed*2 seems to fix the issue. On a real cart the main phi2 frequency is halved before it goes to the SID.
Marco van den Heuvel
Fixed in rev 27831, thank you for catching that bug.