A 4040 binary counter keeps track of where the drum sequence is. It's clocked by a 555 astable circuit with a variable frequency set by a potentiometer. This changes the tempo of the drum machine. The output of this binary counter is compared to the display count using a 74LS85 4-bit binary magnitude comparator. The 74LS85 produces a 1 only when the two sets of input bits match. When this runs back to the screen it produces a line that highlights where the drum sequence is because the 74LS85 will only output high when those scanlines are being drawn. The signal also triggers four sample-and-hold circuits that copy the values stored in capacitors indicated by the drum count.
The 74LS85 has inputs labelled "A=B IN", A>B IN", "A<B IN". These allow two or more 74LS85s to be cascaded to compare larger binary numbers. For this application A>B and A<B should be at ground and A=B at Vcc.
The 555 runs to the 4040 clock through a diode which forms an OR gate with another diode. The drum sequencer skips over selected steps by sending a second "skipper" pulse to the 4040 through this gate.