                      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. 
 
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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. 
 
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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
       !         !     !
       <resistor !     !
       >1k       !     ! +
       <ohm      !     :--!]---to RCA
       !         !     !  10 uf
       !         !     !electrol cap
       !         !     !
ground---        !     !
       -         !     <resistor
                 !     >1k
                 !     <ohm
                 !     !
                 !     !
                 !    ---ground
                 !     -
                 !
                === .001 uf
                 !  capacitor
                 !
                ---ground 
                 - 
 
 
I ran the 12 VDC in shielded cable to reduce noise.  You can get 12 VDC for
the transistor from pin 28 of the SID chip.  It is actually 9 volts in the
C64c.  Use the shield to connect ground from the computer to the perf board.
Put some spaghetti (empty insulation) over the bare shield wire so that it
will not short out anything it touches.
 
Solder the ground wire from the RCA plugs to the main grounding strip
between the serial and video ports. 
 
Use shielded audio cable to connect the right channel center wire from the
center pin of the RCA connector to the negative side of the 10uf
electrolytic capacitor on the perf board that you constructed. 
 
Connect the shield to the ground tab of the RCA connector.  At the perf board
end leave the shield disconnected.  Cut the shield off close to the wire,
and wrap tape around any bit of shield wire sticking out.  This floating 
shield method keeps current from flowing through the shield and causing 
noise in the audio.  (The shield of the wire carrying 12 volts must be 
connected on both ends). 
 
Connect the left channel center wire to pin 3 of the audio/video connector.
If you mounted the RCA connector close to the audio/video connector, you do
not have to use shielded cable for this connection. 
 
Here is how to identify pin 3 of the audio/video connector.  Use an ohm 
meter to find the pin that connects to the negative side of the 10 uf 
capacitor that is close  to the original sound amp (a transistor close 
to the SID chip).
 
If you need any help constructing this circuit, check out any of the many
books that have schematics on the C-64.  This is an exact duplicate of the
one already inside the C-64 with the exception that a slightly different
transistor is used.  The additional transistor has the traditional EBC
configuration, whereas the transistor in the computer does not.
 
Page 4                                                     Add 2nd SID

Bend the tab on the top shield so that it lays flat on top of the heat sink
strip that you added.  Reassemble the computer. 
 
Connect a stereo cable from the RCA connectors to unused inputs on your
stereo, or connect to amplified speakers (amps in the speakers). 

TESTING BOTH CHIPS
 
Find a basic program that uses the SID starting address as a variable.
 
ADDRESS 
 
The address of the second chip is $DF00 hex 57088 decimal.  The address of
the original chip is $D500 hex 54528 decimal.  Any basic program that uses
the SID starting address as a variable, can be easily changed to access the
second chip.  Example: 
 
10 s=54528
  Change to:
10 s=57088 
 
Try running the program with the variable set to the original value.  You
should hear sound from the left speaker and TV or monitor.  Normally you will
want to turn your TV volume down and listen just from the stereo since you
get better sound that way. 
 
Then run the program with the variable set to the new value.  You should
hear the identical sound from the right speaker only. 
 
USING STEREO SID PLAYER 10.3 
 
Stereo SID Player does not check to see if you have a second SID chip.  You
have to go to the second menu and change NORMAL to STEREO.  Also make sure 
that the address for the second chip is set to $DF00 (if you used pin 
10 on the cartridge connector for chip select).  Save the setup on disk.  
The next  time you load Stereo SID, it should remember the change you 
made -- you do not have to change the menu every time you load Stereo 
SID Player.
 
Enjoy the world of 6 voice stereo sound. :)
 
 
C64c NOTE: 
 
Does this sound familiar?  You had some neat games and SIDs that ran fine on
your old C64, but when you upgraded to a C64c, you were missing some sounds
or voices?  You probably thought that there was something wrong with the SID
chip in your C64c. Nope.  Commodore built the C64c computers with 470pf
capacitors in the SID filter circuits.  This causes any voice with the high
filter turned on to be inaudible.  That is why certain games and SIDs sound
fine on an old C64 or C128, but don't play on a C64c.
 
While you have the computer open you could replace the original capacitors
with the correct value.  Alternatively use a SID edit program to turn off
the filters on all voices that will play on the original SID chip.
  
 
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