> By the way, since I am a Commodore SID music enthusiast and I
> require a Commodore 64 first series just for the 6581 SID, a C64C
> with a 6581 would be just good enough for me. Is there a way to
> install a 6581 on a Commodore 64C (I mean, those with the cost
> reduced boards sporting an 8580)? I would be happy if you could give
> me some directions

     I would be happy to do that. You probably already know, but the
white case C64C can have any one of three different motherboards
installed, and to my knowledge, only the latest short board will have a
8580 from the factory. The two SID versions are pin compatible but use a
different voltage on pin 28, and two different filter capacitors for
voicing. The capacitors go from pin 1 to 2 and from 3 to 4 of both SID
versions. The 6581 has +12VDC on pin 28 and the two capacitors C10 and
C11 are both 470pF. The 8580 has +9VDC on pin 28 and uses 22000pF
(.022uF or 22nF) for capacitors C70 and C71.
     Changing a C64C short board (250469) to fit a 6581 will require
changing the zener diode CR7 from 9V to 12V to up the supply voltage
from +9 to +12. The schematic for that board also indicates a resistor
R8 (1K) should be removed (probably already done by the factory) for an
8580 and installed for 6581. Changing the two voicing filter caps will
complete the job.
     To run an 8580 in an earlier board, a hack of sorts is necessary
for the supply voltage changeover. 3 volt zener diodes are kind of rare
so I would install a 9V regulator IC to feed the SID. Another difference
between the two SID versions involves the 6581 and its "voltage leak"
that enables a user to produce digital samples. To simulate that leak in
an 8580, tie a 330K resistor from SID audio input pin 26 to ground. I've
never tried to change an early board to an 8580 since those chips are
even harder to find than the 6581.