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
       !         !     !
       <resistor !     !
       >1k       !     ! +
       <ohm      !     :--!]---to RCA
       !         !     !  10 uf
       !         !     !electrol cap
       !         !     !
ground---        !     !
       -         !     <resistor
                 !     >1k
                 !     <ohm
                 !     !
                 !     !
                 !     !
                 !    ---ground
                 !     -
                 !
                === 1000 pf (.001mf)
                 !  capacitor
                 !
                ---ground
                 -


You can get the 12 volts you need for 
the transistor directly from pin #28 of
the SID chip.

If you need any help on constructing 
this circuit check out any of the 
many books that have schematics on 
the C-64.  This is similar to the one 
already inside the C-64.

The ground wire from the RCA plug can 
be soldered to the main grounding 
strip between the serial and video 
ports.  The center wire will be 
connected to the negative side of the 
10uf electrolitic capacitor.

I still think you should have someone 
familier with electronics install 
this circuit for you.

If you have a problem with some 
cartridges, you will have to install 
a switch between pin #25 of BOTH SID
chips.  This will CUT the power to the
extra SID chip, effectivly turning it
off.  I would suggest that you turn
OFF the computer before you turn the
extra SID chip ON or OFF with this
switch.

A good place to mount the switch and
RCA plug is on the back of the
computer and above the monitor jack on 
the 64.  I still haven't found a GOOD 
place on the 128.  A suggestion was 
made that if you are not going to use 
the RF output on the computer, you can 
cut the wire going to that RCA plug.  
Then connect your audio output wire to 
the center connector of the plug.  This 
does work but BE CAREFUL!

Good luck on the construction.


Mark A. Dickenson
