TOD signal generator.
The C64 uses the 9 volt AC to generate a 50Hz (60Hz for NTSC) clock pulse for the Time Of Day (TOD) clocks of both CIA’s
We should need a replacement clock pulse for this TOD signal when we want to run the C64 on DC (power 5 and 12 Volt).
The UC3843 PWM chip can help us.
This chip is used to generate a pulse up to 500kHz
This frequency is set by the R/C combination at one of it’s inputs.
The formula for this is
fr = 1.72/(C*R)
So we will get the next component values:
C = 100 nF
R = 330KΩ
fr = 52,1212 Hz
(ideal R = 344KΩ for 50 Hz)
For NTSC this will be:
C = 100 nF
R = 270KΩ
fr = 53,7037 Hz
(ideal R = 286,667KΩ for 60 Hz)
you could actually add a variable resistor and cheat in certain games by speeding up or slowing down the clock.
The output signal of the chip should be connected to the TOD pin of both CIA’s in the C64
and both pins should be disconnected from circuit of the C64 mainboard by cutting a pin of a chip and removing a resistor (part depending on board version) or cutting a trace on the mainboard.