[Radio1950]

Solution.

When all else fails, get the manual out.
I looked for clues in 74LS90N datasheets.

In only one (Motorola) of many, I found this tiny footnote “tr<100nS” (“tsubscript r<100nS).
Pennies falling.

My clock risetime was many times this.

Severe and ongoing embarrassment. Basic. Old age.

Solution, use a 74LS14 Schmitt to give edge and buffer action.

I also forgot the old TTL rule, “buffer in, buffer out”, so the other LS14 sections now buffer all outputs.

Several forum members were straight onto this risetime and falltime issue, notably “Neutrino, lesmw0sec, and cmjones01”.

Good comments from all. Thank you.

I have cut up the Kewpie Doll into six sections, wrapped all, awaiting personal collection.