INSTALLING A RESET SWITCH ON YOUR C-64 (and C64C)   02-25-2010

     You don't really need a schematic... I can talk you through it.
You need a momentary contact pushbutton switch (available from Radio
Shack), a few lengths of wire and some hand tools including a drill and
soldering iron. The switch momentarily grounds the input to a chip (pin
8 of U20, a 556 timer IC in the C64) which is used to generate a single 
reset pulse each time the computer is turned on. It's a bit different 
in a C64C as explained later.
     Connect one side of the switch to the junction of capacitor C105
(a 0.1uF disk ceramic), resistor R50 (a 1 meg 1/2 watt), and pin 8 of
the IC. NOTE: C105 is C34 in some versions of the 64. It's easiest to
tack-solder onto the resistor leg nearest the capacitor on the top of
the PC board. You don't have to pull the board out of the case. The
other side of the switch goes to the nearest ground foil, found at the
edge of the PC board near the resistor. By the way, don't miss-wire and
connect to the far side of the resistor... that goes directly to the +5
volt supply! Grounding that line will kill chips! Want to make the
modification fail-safe? Add a 100 ohm 1/4 or 1/2 watt resistor in line
(in series) with the switch. Then if you make a mistake, it will do no
harm... it just will not work.
     With the proper lengths of wire, you can mount the push button
anywhere it will fit on the case. I like to install it on the lower
left side near where the wires connect to the PC board. If mounted to
the lower cabinet half-shell, the wires will not be disturbed when the
keyboard (top half shell) is removed/installed. Drill the appropriate
sized hole for the push button and mount it so that it doesn't touch
the PC board and cause a short. Solder the wires and you're done.
Solder quickly, by the way... those little switches are plastic and
will melt from too much heat.
                    
                      A RESET SWITCH FOR THE C64C
     
     You must remove the metal shield (heatsink) from the top of the
board to get to the junction of capacitor C43, the anode of diode CR5,
and pin 5 of IC U23. Tack solder one wire of your added pushbutton to
pin 5 of the chip or to the anode side of diode CR5. The other side 
of the added switch is connected to ground. Again, for safety's sake, 
it's advisable to install a 100 ohm 1/4 or 1/2 watt resistor in series 
(in-line with one wire) with the push button switch.
     Mounting the switch in a C is a bit tricky. You must prevent the
switch terminals touching the shield when it is reinstalled. Bend the
shield or cut a small portion of it away if necessary to avoid a short
circuit. When reinstalling the shield, make sure the little fingers
touch all the chips. It is a heat sink and the metal against the IC's 
case draws away their waste heat and helps to lengthen the life of 
those precious ICs. Don't wipe off the white paste. It's heat sink 
compound. If necessary, bend down all the tabs slightly before  
re-installing the shield so all tabs press firmly on the chips. 

Ray Carlsen
CARLSEN ELECTRONICS... a leader in trailing-edge technology.
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