Convert your Commodore C64C revision B to use a SID 8580 instead of a SID 6581
I had a C64C that was otherwise fine, but its soundchip 6581 was broken, missing its filter capabilities.
Because I only had 8580 type soundchips as spares, I decided to try to modify the motherboard components to accept the 8580 chip.
I have quite some knowledge of the two chip types from building a synthesizer, MidiBox SID, using both the chip types.
(More of MidiBox SID synthesizers at www.ucapps.de)
The main differences of the two SID chip types:
SID 6581 needs 12 V DC voltage and two 470 pF filter capacitors
SID 8580 needs 9 V DC voltage and two 2,2 nF filter capacitors
So changing from 6581 to 8580 needs the motherboard components to be changed to supply a different voltage to the SID and different filter capacitors.
Here is a short guide how to do it:
1. Open your C64C (models having the B revision motherboard have three screws under the front side)
2. Remove keyboard (a couple of screws and a connector with a bunch of wires) and power led connector
3. Remove tin coated shielding cardboard (a screw and a clip)
Now you should be looking at this:
(this C64 has Scandinavian keyboard, so the two chips with labels on are not original, neither are the heatsinks on two chips)
Let's zoom into the lower right portion of the motherboard:
In the middle of the picture there is a three footed component, and a marking on the motherboard saying VR1. The original component has numbers 7812 on it, indicating it's a voltage regulator that outputs 12 V DC. In this picture I have already desoldered it and soldered a regulator 7809 in its place. Now the motherboard supplies 9 V DC to the SID chip socket. Here is an image of the removed 7812 regulator:
Now we have the voltage changed to suit the 8580, let's change the filter capacitors. Here's a picture of the soundchip and the capacitors:
In the picture is the new 8580 chip, on the original chip it reads 6581. The motherboard has markings C10 and C11 next to two capacitors (probably bearing the numbers 471 to indicate 470 pF). These capacitors must be desoldered and replaced with 2,2 nF ones.
Once you have done this, you can install the 8580 chip into the socket, put the machine back together, turn it on, load up your favourite piece on C64 music and put a nostalgic smile on your face!
You can probably do this on different motherboard revisions, too, but the components might be placed differently. It should also be possible to convert a 8580 equipped C64 to use a 6581, replacing a 7809 regulator with a 7812 and 2,2 nF capacitors with 470 pF ones.
Guide written by Juha Partanen aka Jurbo. Use at your own risk, writer takes no responsibilities whatsoever.