It was switching noise.
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
I hadn't added the bypass caps to the 74xx chips
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
Lesson learned.
Never underestimate the power of the bypass cap across Vcc & GND to clean up switching noise.
Ignore them at your own peril and frustration.
BTW:
The hint was that moving C13 to smooth the signal going into IC6 p1&19 worked.
For that to make a difference it had to be a very short blip. Like, say, noise on the line.
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
Positive:
I learn so much more when I make mistakes.
I now know how pretty much every piece of this works.
Cheers,
Matthew