Second SID Chip Installation ---------------------------- (c) 1987 Mark A. Dickenson Adding an extra SID 6581 chip This is not a project to be tackled by the sqeamish or people who are deathly afraid of opening their computer just to take a peek inside. Now let's get rid of the nasty stuff first. No liability is assumed with respect to the use of the following information. In other words if you screw-up trying to install this modification, then it's your responsability. YOU DO THIS AT YOUR OWN RISK!!!! If you do not feel up to it PLEASE take it to a Commodore repair center or a repair service that can work on computers and let them do the installation. You can make your own piggy back board or you can do what I am going to describe (since it is a little hard to put a schematic in a text file, well not hard just cumbersome). You should ground yourself with a static guard wristband (such as what Radio Shack sells). Even though the chip is quite durable, just the right static discharge can ruin all or part of the SID chip. For those of you that are not familier with the way pins are numbered on an IC chip here is a short explanation. 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 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 ---------- You need the following parts. 1 - 6581 SID chip From Jamco or K. Boufaul 1 - 2N2222 transistor Radio Shack 276-1617 2 - 220pf capacitors Radio Shack 272-124 2 - 1k ohm 1/4 watt resistors Radio Shack 271-1321 1 - 1000 pf capacitor Radio Shack 272-126 listed as .001 mf this is the same as 1000pf 1 - 10k ohm 1/4 watt resistor Radio Shack 271-1335 1 - 10 uf electrolitic capacitor Radio Shack 272-1025 1 - 5 inch length of wire 1 - 5 inch length of shielded cable 1 - surface mount female RCA plug (this is what you normally find on the back of your stereo. On the C-64 the SID is IC U18. It is usually located in the middle of the circuit board, next to the metal video chip case or up between and just below the serial and monitor jacks. On the C-128 the SID is IC U5. It is located at the back of the circuit board just to the right of the metal housing for the 40 and 80 column video chips. First bend out pins 23, 24 and 26 and cut them off of the 6581 SID chip. These are for the two analog 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, and 27. Solder one of the 220pf capacitors to pins 1 and 2 then solder the other 220pf capacitor to pins 3 and 4. The capacitors control the upper and lower frequency range of the SID chip. The reason I am using 220pf capacitors is because of problems with the filters in the SID chip. The C-64 first came out with 2200pf capacitors, but they were changed to 470pf. The reason for this was because the filters of the SID vary from chip to chip and using 2200pf caused a lot of them to sound muffeled when the filters were on. I have found that by lowering the capacitor value to 220 pf helps even more. If you wish, you can use 470s if you feel it would be better, but DO NOT use 2200pf. Solder one end of your wire to pin 8 of the SID chip. This is for the chip select line. We will connect this to the main mother board, so as to provide the addressing for the new chip (described later). 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 sid chip currently in your computer. You may have to bend those pins inward just a little for them to get a good grip on the SID chip. Be very careful not leave the soldering iron on the chip TOO long as you could ruin BOTH SID chips. Make sure there is good sized gap between both chips to provide air circulation. Since they do get very hot at times you may have to heat sink AT LEAST the bottom one (both if you can). Now that you have the chips soldered together (place the SID chips back in the socket if you removed them), solder the wire from pin 8 (on the SID chip) to pin 10 of IC U15 (chip 74LS239 or 74LS139). This selects address $DE00 for the new SID chip. (U15 can be either a 74LS239 or 74LS139). I am no longer describing how to connect for address $DF00. This address causes problems with the RAM Expansion Units and numerous other cartridges. From now on address $DE00 is the ONLY address for the SID chip. (FOR 128 USERS ONLY) Here comes the hard part, I have access to TWO C-128s and guess what, the silk-screened chip number for the IC you need to connect pin 8 of the added SID chip has been partially GOUGED out, ON BOTH! They cut a trace and took part of the number with it. All I have is U3(gouge). If you know the number LEAVE ME EMAIL! I will try to explain where it is (it's chip 74LS138). As you are looking from the front of the 128 (like you are getting ready to start typing), it is the chip just to the left of IC U6 (a big 40 pin IC - 8502). This chip is UPSIDE- DOWN in comparison to IC U6 and it is a 74LS138 (since these are very early versions of the 128 that have been fixed by the company they may have changed the chip number as they have on the C-64). Connect to pin #9 on this IC (74LS138) for address $DE00. Now partially reassemble your computer (be careful that nothing shorts out the pins still sticking out). Turn the computer on and load a program that will produce sound on all three voices. If you get sound then so far so good. Turn off the computer and disassemble the case. Drill a hole in the back end of the computer just large enough to anchor the RCA plug. Then solder the center wire of the shielded cable to the center post of the RCA plug. Insert the wire through the hole you have just drilled and anchor the plug to the case. Now solder the ground wire to the ground tab on the RCA plug. Here comes the difficult part to explain. This is the coupling circuit for the audio output. Here is a rough schematic. Pin 27 on 12volts dc SID chip resistor ! --. 10k ohm !collector 27!----.--/!/!/--.-----O 2n2222 --' ! ! !emitter ! ! ! 1k ! ! + 1k !