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Hello Edward,
Thanks for the update, and for the kind words... I think I was just in a good day.
In your case, I confess, when I jumped in I was believing the problem could be related to the ROM, only because I had a clear remembrance of reading about many incidents of ROM rots on some oscilloscope model(s) of that vintage.
But then, I decided to double check and learnt that that ROM in particular was not among the ones known to suffer from problems... AND ULTIMATELY, when I got to look a little bit more in detail to how the circuit worked, it wasn't that hard to see that it wasn't likely to be around the ROM chip, because the signal that tells ALT from CHOP doesn't even go into the ROM and doesn't matter to the operation of that circuit... and that was important to notice, because we knew CHOP was working.
Finally, to this particular control logic, if we rule-out the ROM and the LS175, and we also rule out a the Chop clock generator that we knew was working too... there was really very little left to get busy with.

Still, Dieter could have been right all along, because those Texas IC sockets are indeed awful.
They're particularly bad if the mating IC is one of the kind in which the pins are only tinned before the pins are cut-out to form the IC bonding spider (or whatever it's called in English).
On those ICs, the pin faces that were formerly the cross-section of the plate from which the pins are cut, are left exposed to the environment and ugly copper/brass oxides develop.
The evil in those Texas sockets is that they grab the pins exactly in that direction, that is prone to become badly oxidized (on the IC pin)... the IC socket contacts themselves are good quality gold-plated, but it's of little help if they contact the pins on the oxidized surface.

I`m glad that it worked,
Rgrds,
Fabio