INSTALLING A RESET SWITCH ON YOUR C-64, C64C and SX   
             latest updates and/or corrections: 1-3-2014

There are two versions, warm and cold. I'll explain both here beginning 
with the installation of a simple button switch.

     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 from disk and you push reset, the screen will freeze, the 
computer will lock up, and the program will not be not eliminated from 
memory. A soft reset will work if you're running a cartridge program. 
     To enable a "cold" reset with a ML program running from a disk, 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 screen. Another program can then be entered 
as needed. That cold reset is roughly equivalent to turning the computer 
power off and on again. 

     To add a simple warm 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 at power up to generate a single reset pulse 
each time the computer is turned on. The original circuitry is a bit 
different in a C64C and for the very early version C64 board as I'll 
explain later.

THE RESET SWITCH INSTALLATION IN A STANDARD C64

     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 opened. Drill the appropriate sized hole 
for the push button and mount it so that its terminals don't touch the 
PC board compnents 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.

THE EARLY C64 (BOARD # 326298) SWITCH INSTALLATION

     The design of the original on-board reset circuit is different in 
this C64 board and the electrical connections to the switch are 
therefore different. Rather than grounding the IC trigger point as in 
other board versions, one end of the reset switch is connected to the 
+5VDC line! The other end of the switch goes to pin 12 of IC U20. These 
connections effectively put the switch across capacitor C24. 
     The cold reset modification is a bit different for this board as 
well. The input side of it needs to be inverted, and an additional 
transistor is used for that. See the photos of the warm and cold reset 
circuits for early board 326298. All of these modifications can be made 
on the top of the board, making removal from the case unnecessary.

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. I would put 
the switch a bit closer to the front of the case to avoid touching the 
shield. The switch wires will need to be routed through an open space in 
the shield so they are not pinched when the shield is put back. 
     When reinstalling the shield, make sure its little "fingers" touch 
all the chips. Those fingers are heat sinks, and that metal against the 
top of the IC's case draws away the waste heat and helps to lengthen the 
life of those precious chips. 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. If the 
heat sink paste is very dirty, It's OK to remove it. Grit in the paste 
will keep it from making intimate contact with the chips anyway. If the 
paste is dried out or missing, don't worry about replacing it. It's more 
important that the metal fingers sit flat and firmly touch all the chips!

HOW TO INSTALL THE COLD RESET CIRCUIT MODIFICATION

     I discovered that if a delayed pulse is applied to the /EXROM line 
at reset time, the ML program screen will completely clear. To create  
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 to logic low. The 
collector is tied to the /EXROM line in the computer and 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 JiffyDOS. 
     If you use a DPDT center-off switch, both warm and cold reset 
circuits can be implemented. By moving the switch one way, you get a 
warm reset and by moving it the other way, a cold reset. That switch 
can be either a center-off (three position) or a center-off momentary 
contact two-way switch. One photo shows the installation in the most 
common 250407 board but the same circuit can be installed in any C64 
motherboard. 
     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 into C64 mode as 
if a cartrige were plugged in! If a cold reset is preferred in this 
computer model, one could install a switch to open and close the cold 
reset input connection as necessary or install the dual position 
switch as described above. 

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