Logically speaking, yes. Electrically speaking, TTL chips can't always be substituted with HCT chips.

https://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/

Thanks for clarifying that. I was curious about the answer, and the article you linked seems to offer a clear and detailed explanation of the difference between the "LS" (TTL) and "HCT" (CMOS) parts.

If I understand correctly, a person could get away with substituting an LS part for an HCT part, if they are sure that the circuit's voltages are limited to 5v, and that the LS part is not going to be providing any inputs to any CMOS parts. But otherwise, the fact that CMOS parts may be used in circuits where voltages are much higher than 5v, and the fact that LS parts may output signal levels that are outside the range for reliable operation of a CMOS part, means that one would be unwise to use an LS part where an HCT part was specified.

Given the difference between the basic needs of a CPU board (strictly logic) and a sound board (deals with higher voltages and currents for sound amplification), I would guess that the right answer here is that the LS chip should not be used on the sound board where HCT chips are expected (nor vice a versa...the HCT part is mostly more accommodating, but would still be problematic if it were fed signals coming from any TTL part in the circuit).

Do not confuse HC vs HCT. HC is strictly CMOS. HCT is CMOS with TTL compatibility.

Compatibility, in general, depends on the circuit and how the IC is used. I am definitely no expert on this subject. I would ask Ed (@ G-P-E) to comment as he is learned in the way of these things.

If I understand it correctly, true compatibility depends on current sourcing but more importantly current sinking capabilities listed in the datasheet. If the circuit is a truly enclosed digital logic circuit then TTL and HCT are likely to be compatible due to the small amounts of current being used.

The one big potential difference I have seen is timing. Rottendog boards used HCT because they are often cheaper than their TTL counterparts. I have seen incompatibilities with these Rottendog boards vs OEM (or other reproduction boards) that use TTL.

YMMV.

I just wanted to say thanks to the original poster and the other people who have commented here.

About four days ago I started repairing a Rottendog board, substituting a 74HCT part for the bad part that was 74LS, and now I'm pretty sure that's why my repair hasn't been successful.

I'm sure the board wizard guys have lots of stories about this, but it really made me pull my hair out.

For this Rottendog board I'm now going to get exactly the part number I pulled out, and I'm going to source it from reputable suppliers. It's quite possible that the 74HCT parts I had in stock were from when I had no fear buying from Chinese suppliers of potentially counterfeit parts, so it's possible that the parts I put in are marginal.

Good parts. Correct parts. No substitutions unless you REALLY know what you are doing. That's my new watchword.

You know, until I get in a hurry... Hehe.