This is the circuit that lets the drum sequencer skip over selected steps. Without this every tune would have to be sixteen steps long. It is controlled with the zapper and another pair of buttons. When the delete button is pressed the step being pointed at by the zapper is set to be skipped over. When the insert button is pressed the step pointed at by the zapper is no longer set to be skipped.
The circuit uses a pair of 74HC164 shift registers to form a 16bit
circular buffer. Channel draw select 2 comes from the same binary count
as the four bits that select which step is being drawn, but one bit
lower. This means using it to clock the pair of 164s makes them cycle through all 16 bits as the 16 steps are being drawn to the screen.
A point is pulled to either 5v or ground by pressing the insert or delete button. There's a 270Ω
resistor between the insert button and ground so there won't be a
short if both buttons are pressed at once. When the zapper picks up the
raster beam being drawn it charges up a capacitor through a diode.
This capacitor then discharges slowly producing a signal that remains
high for several scanlines after the zapper detection. This signal
closes a 4066 that lets the new value set by the buttons into the
circular buffer. The new value needs to be present as the shift
registers are making a transition which is why it was necessary to
stretch out the zapper pulse.
Skip draw signal is an output to the TV which brightens lines that have been set to be skipped over. Skipper is a signal that goes to the drum step count 4040. It makes the counter increment again right away if the present step is set to be skipped over. This works by ANDing the skip draw signal with the 74LS85 output. If both of these signals are high at the same time the drum sequence count must be on a line that's set to be skipped over.
The 4066 responsible for this circuit is the same one used for video porching and the zapper sampled sawtooth.