In this case it was because of a defective 74LS174 chip. To be more accurate, he was experimenting with a doner board since it's 74LS174 was used to repair anot

In this case it was because of a defective 74LS174 chip. To be more accurate, he was experimenting with a doner board since it's 74LS174 was used to repair another machine. I think the actual application in this case was education, likely for himself but interesting enough for his audience. Adrian Black's channel has gone through interesting transitions through the years. He went from an everyday hobbiest who made simple, but common, repairs to someone who tries to understand how the electronics works to diagnose problems and make targeted repairs. While the host of some channels will say, "old caps, let's replace them," Adrian will see if the caps are actually source of the problem before replacing them (unless they are of a type guaranteed to leak or explode). Rather than swapping chips, he will look at the signals generated by the chips to see whether they (or surrounding circuitry) are the source of the problem. Why is experimentation like this valuable? I would suggest there are two reasons. First of all, people should be encouraged to experiment with what they have. While Adrian has a lot more than most people, his resources appear to be limited (and a lot of his more exotic stuff appears to be the product of goodwill, since people appreciate what he does). Second, it lays the foundations for projects that may have more utility. For example: much of this particular journey would be applicable to expanding the memory on machines that don't have an integrated way to expand memory. (Say you wanted to upgrade the RAM on a Macintosh 128k.) EDIT: elaboration on capacitor's comment.