scott@max.u.washington.edu (12/01/90)
Since it seems that there are several people who are interested in the
Mark Dickenson's instruction on how to add a second SID chip on to
the motherboard of the C64/C128, but have no means to getting it
directly from Milton.
The following text is the instructional text file found inside the archived
"stereoplayer10.3" file that guides you on how to add a second SID
chip to give three more voices to the C64/C128.
Sincerely,
Scott K. Stephen
----------------------- cut here -------------------------------------
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