That components below R5 are not easy to solder , but they look aligned . The input jack looks nice , good solder joints. Great Photos

Since my last post, I've known that the trace from R10 to the asic was in bad shape. When measured, the line wasn't open but there was a significant amount of resistance. I'm assuming I messed it up accidentally when soldering on the resistors. This really only left me with 1 option, which is to pass a jumper from R10 to the 5th pin from the top on the left of the ASIC. Easier said than done
Of course, the pins on the ASIC are extremely small and close to one another and I don't have any fancy micro-soldering equipment...
So anyways, that's what I did today and I somehow managed not to short a bunch of pins in the process

Its a bit hard to see, but I used 36 AWG insulated jumper wire.



Here is a mechanical pencil lead next to the joint for scale



I measured to make sure everything was good and closed it back up... Unfortunately, this didn't fix my issue. Just like before, the input lines still work and at least one of the output lines doesn't. On the bright side, I can rule that out as a potential issue.
There's almost certainly a couple cold welds though since the soldering job leaves a lot to be desired (done with basic soldering tools and no flux). I'll probaly try to reflow the ones that look the worst and if that doesn't work, I'd probably just try again from scratch with proper tools and some flux (I have a bunch of half-dead calculators anyway)

Its been a few years, I have gotten much better at micro soldering.
I decided to revisit this project by attempting it again from scratch with two new calculators (I do not trust what I did back then and trying to fix it would be harder than just trying again)

This time, it worked!

Here is the board before and after, and a close-up view of the comm circuit that was added:


I did exactly what I described in this post except moving J14, stealing the circuit from a rev Q TI-82 instead of rev U (because its what I had on hand, it is the same circuit)

Of course, I had to cut a hole out of the bottom of the calc to accommodate the link port:


As some of you may know, the TI-80 only supports one operation through the link port, taking screenshots, so that's exactly what I did.
I managed to obtain this screenshot:


Finally, here is a video of the screenshot being taken:

He he, nice work

That's really cool! Glad to see it work this time!