INSTALLING A RESET SWITCH ON YOUR C-64 (and C64C)   03-2-2013

There are two versions, warm and cold. I'll explain both here...

     A reset switch is handy if you want to return the computer to its
power-up state without repeatedly turning the power switch off and on 
again. A "warm" reset, implemented by adding a momentary contact push 
button switch, will reset the computer back to the opening screen, but 
that works only with BASIC programs. If a ML (machine language) program
is running and you push reset, the screen will freeze, the program will 
not be not eliminated from memory and the computer will be locked up. 
     To enable a better reset, a small circuit can be added to your 
computer consisting of a diode, resistor, capacitor and transistor. 
With that circuit wired in, the ML program, although still in memory, 
will be stopped and the screen will clear back to the opening logo. 
Another program can then be entered as needed. That is a "cold" reset
roughly equivalent to turning on the computer cold. 

     To add a reset switch, you need a momentary contact pushbutton 
switch (available from Radio Shack), a few lengths of wire and some 
hand tools including a drill and a 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 I'll 
explain 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 accidentally can 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. 


     I discovered that if a delayed pulse is applied to the /EXROM 
line at reset time, the ML program screen will completely clear. To 
implement that pulse, I took the pulse output of the reset generator 
IC in the computer, fed it through a diode for isolation, and used it 
to charge a capacitor. That charge goes though a resistor and onto 
the base of a transistor, turning it on and bringing its collector 
lead low. The collector is tied to the /EXROM line in the computer. 
The charge on the cap stays a fraction of a second longer than system 
reset, so the /EXROM line is held low for that delayed time.
     Although this circuit works well to clear the screen, the old 
program is actually still in memory although it can be overwritten 
by loading another into the computer. One way to tell the old program 
is still there is to hit the RESTORE key. Another negative effect I  
noticed is that an EPYX FastLoad cart no longer works with the cold 
reset circuit in place (blank blue screen) and the computer is 
locked up. All other cartridges seem to work fine, however as does 
     Just for an experiment, I tried this circuit in a C128 which 
already has a factory reset button. Of course it works to clear the
screen as described above, but it locks the computer in C64 mode! If 
a cold reset is preferred in either computer model, one could install 
a switch to open and close the /EXROM connection when necessary. 

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