use of 7493 instead of 7490

This circuit generates a `six pips` signal indicating the exact hour. At the instant the first `pip` is to start, an eight-input NAND gate, which receives BCD-coded minute and second marks from a TTL clock, transmits a negative-going edge to MFl (at A). Each igit requires not more than two inputs, the logic `l` level only being used for decoding. The code shown in the dia- gram corresponds to the moment 59` SS".