ADDING AN EXTRA SID 6581 CHIP by: Charlie Young Adding an extra SID chip adds 3 voices for a total of 6, and allows you to play stereo SIDs. Some of this information is from the author of Stereo SID Player, Mark A. Dickenson. I added a second chip to my C64c following Mark's instructions and a C64 schematic. One of the chips that Mark referred to does not exist in my computer. This updated set of instructions should work for any C64, C64c, or C128 computer. This is not a project to be tackled by the squeamish or people who are afraid of opening their computer just to take a peek inside. [If you were one of them, you would buy the stereo cartridge from CMD]. However, the instructions are fairly simple. If you mess something up, it will probably be the sound chip. Hopefully the rest of the computer would not be affected. No liability is assumed with respect to the use of the following information. YOU DO THIS AT YOUR OWN RISK!!!! OVERVIEW You will piggyback a second SID chip to the original. All data, address, and power pins will be soldered to the original chip. Several other pins will not be soldered to the original, so pay attention to the instructions. You will build a simple audio amplifier on a small piece of perf board. Please read through the instruction before starting construction. PARTS 1 - 6581 SID chip 1 - 2N2222 transistor 2 - 2,200pf capacitors 2 - 1k ohm 1/4 watt resistors 1 - .001 uf capacitor 1 - 10k ohm 1/4 watt resistor 1 - 10 uf electrolytic capacitor 1 - small DPST switch - insulated hookup wire - shielded audio cable - silicon heat sink grease 2 - chassis mount female RCA plug (like you find on the back of your stereo). HOW TO DO IT Print these instructions, and check off each step as you do it. Disconnect the power supply. Take the computer case apart. Remove the keyboard. Take off the top shield. You do not have to remove the main board. If you connect the power supply to check voltages, disconnect it before doing any soldering. You should ground yourself with a static guard wristband (available at Radio Shack sells). Even though the chip is quite durable, just the right static discharge can ruin all or part of the SID chip. Page 1 Add 2nd SID Note which way the original SID chip is installed. The notch probably faces the back of the computer. Remove the original chip. Pull it straight up using a chip puller or pry with a small screwdriver at both ends. Put the second chip in the socket and try it out. You will have to reinstall the keyboard to do this, but you do not have to put the case back together. If you bought the chip from Grapevine, it may be bad -- especially if it has pins missing. If the second chip is good, then you are ready to begin piggybacking. Here is a short explanation of the way pins are numbered on an IC chip. On one end of the IC you should find a little notch. Looking at the chip with the notch at the top the numbering goes this way. The upper left corner of the chip is pin 1 and they are numbered consecutively, counter-clockwise around the chip. Some chips do not have a notch in one end, but instead a dot is placed in one of the chip corners to designate that pin 1 starts in that location. notch ----,,---- 1-!. !-8 2-! dot !-7 3-! !-6 4-! !-5 ---------- First bend out pins 23, 24 and 26 and cut them off of the 6581 SID chip. These are for the two analog (paddles) and one audio input lines. They will cause problems if connected and since they will not be used it is best to remove them. Now bend out pins 1, 2, 3, 4, 8, 27. If you want things to run cool, you can make a heat sink. Find a thin narrow strip of metal that is longer and narrower than the SID chip. Cover the top of the original chip with a thin layer of heat sink grease. Lay the strip on top of the original chip. You might fasten it with a small spot of glue to make sure that it does not move and short the pins later. Now smear the bottom of the second chip with a thin layer of heat sink grease. Make sure the notches face the same way, and make the sandwich. Bend the heat sink strip over the end and top of the top chip. You will want this strip to touch the top shield when you replace it. Prepare to solder. You will be soldering some pins of the top chip to the bottom chip. Solder one of the 2,200pf capacitors to pins 1 and 2 then solder the other 2,200pf capacitor to pins 3 and 4. The capacitors control the cut off frequencies of the SID chip. See the C64c filter note at the end of this file. Now solder the remaining pins (excluding the ones we have bent out and also removed 1, 2, 3, 4, 8, 23, 24, 26 and 27) to the original SID chip. You may have to bend the top chip pins inward just a little for them to get a good grip on the bottom chip. Be very careful not leave the soldering iron on the chip TOO long as you could ruin BOTH SID chips. Now that you have the chips soldered together place the SID chips back in the socket. Page 2 Add 2nd SID DESELECT SWITCH Your second SID chip will work without this switch, but some of your cartridges may not. You can skip this section and install the switch later if needed. With the switch off, the second chip is invisible to the computer -- the chip select pin stays high all the time which keeps the chip from responding to any reads or writes. If you turn the switch off while chip 2 is generating sound, the sound will continue, but the chip will not respond to any further instructions until you turn the switch back on. Drill a hole and mount the DPST switch in the computer. I used the space between the RF modulator and the video/audio DIN connector. An original C64 has more room above the RF connector, so that may be a good place. 5V Solder a piece of wire to pin 2 of the cassette connector. Looking at the back of the computer that is the second pin from the left. You could make sure you have the right pin by powering the computer on and checking that pin with a meter. Solder the other end to the top terminal of the switch. Chip Select Solder a piece of wire to pin 10 of the cartridge connector. With the computer facing you, the connector is at the back right. Pin 10 is the 10th pin from the left. You can wrap the wire around the pin that angles from the top of the connector to the board. Make sure the wire does not short to pin 9 or 11. Solder the other end of the wire to the bottom terminal of the switch. Solder a wire to pin 8 of the second SID chip. Solder the other end to the middle terminal of the switch. Alternative SPST You could probably use a SPST switch and a pullup resistor. Alternative address You could use pin 7 on the cartridge connector for address $DE00. You could actually have a total of 3 SID chips. Chip 2 - $DF00 Chip 3 - $DE00 Don't know of any player that addresses 3 chips though. AUDIO AMP Mount the two RCA connectors on the back of the computer. Here is a schematic of the audio output circuit. Construct this on a piece of perf board. I put this audio circuit under the keyboard. Page 3 Add 2nd SID Pin 27 on 12volts dc SID chip resistor ! --. 10k ohm !collector 27!----.--/!/!/--.-----O 2n2222 --' ! ! !emitter ! ! ! 1k ! ! + 1k !