48K issue 4B upper memory issue

[mikeh_nz]

I’ve just figured out the problem

It was the frigging diagnostic card.
I’m guessing some bad joint on edge connector of the card.

Swapping it with a diagnostics cart purchased from retroleum and the machine is passing reliably now.

And I have now fixed the original diag cart

I ended up

• removing the edge connector solder and resoldering it (with a ton of flux).
• putting a ton of flux and reflowing the connections on the SMD 74LS chip.

Something in those two actions did the trick.

To be fair to the maker of the cart, I had already reflowed various bits without the flux. I quite likely could have been “root cause problem” all along!