Both operate at TTL levels ....Sorry, Michael, but you are wrong. The input true and false thresholds are different for CMOS and TTL. Only a 74HCT (notice "HCT" for TTL interface compatibility) part may be used with TTL parts only because its input thresholds are offset to meet TTL output levels.
Sent: Saturday, January 04, 2020 6:33 PM
Subject: Re: [TekScopes] Trying to bring back a AA501
It appears to me that the MM74C175N has a VCC (max. operating) of 15V and the 74HC175N VCC operating range is -.5V - 7.0V. Otherwise they appear to be similar. Both operate at TTL levels. Quad Flip Flops with common clock and clear to all 4. As long as your operating VCC was compatible, it looks like the 74HC175N could sub in.