MegaST board with strange CPU mod

[exxos]

[dml]

So this MegaST board is definitely a basket case. I thought it was fixed when I saw the desktop and put it aside - but checking it again today, I notice it only boots half the time - and the keyboard doesn't work.

So I go poking around again and scope out the pins of the IKBD ACIA. It looks mostly normal but pin 8 looks weird. Tapping the keys also doesn't seem to produce any activity but its difficult to tell without some annoying setup at the scope to capture one-shot in digital mode (its not a new scope, so that part isn't amazing).

The fact it only boots half the time - and only from a warm reset, never from a cold boot - seems like a floating line somewhere.

Pin 8 on the ACIA is wobbly/fuzzy and that's the only lead I have.

Schematic says pin 8 is the 6850 CS chip select. So that would explain why no keyboard-worky, if no chip-selected. (!)

Schematic says 6850 CS comes from GLUE. Uh oh. That's where the PCB track apocalypse from previous posting was discovered. If that went under the PLCC socket it means I need to take the socket off -__-

I scope out pin 42 on GLUE and it looks ok. Not wobbly/fuzzy. Hmm.

Pin 8 on ACIA still wobbly/fuzzy.

No continuity from IKBD pin 8 -> GLUE pin 42.

Microscope time....

....can't find anything wrong. Also can't find where the tracks go to the GLUE. Much poking around with probes.

Grab the working MegaST, probe around and find the relevant track/vias. Now look for the same on dodgy MegaST.... there's a break somewhere around the GLUE socket -__-

Microscope again.... zoomy zoomy..... scraping at the track at different points to narrow down the break.... then, what's this?

Screenshot 2025-06-13 at 22.44.22.png (2.68 MiB) Viewed 1018 times

...right next to the bit I just scraped.

Screenshot 2025-06-13 at 22.45.13.png (1.44 MiB) Viewed 1018 times

That's so annoyingly tiny it's not funny.

Screenshot 2025-06-13 at 22.45.55.png (3.26 MiB) Viewed 1018 times

...fixed.

Screenshot 2025-06-13 at 22.46.37.png (1.42 MiB) Viewed 1018 times

If anything else goes wrong with this it might become a sharp green frisbee.

[dml]

forgot to add - it boots 100% of the time now and the keyboard works. hence: 'fixed' - until the next thing