After getting it back together, I see a slight reduction of the horizontal breathing during the flash test. I also saw that the Pincushion settings had changed a small but noticeable amount.
I believe that I must have stumbled on a poor solder joint or something that was causing a little bit of issues with the Horizontal, and reflowing a lot of that section helped.
It's still not "perfect" though. I'm seeing two remaining flaws.
1) the top of the raster is still a little pinched inward under optimal settings, on the upper left-hand corner. not sure how to fix it, but the "Pin Up" control is all the way down to get it to the best possible setting. I would assume that the default setting for the Pin Up potentiometer is center, but I have to turn it all the way down to achieve the best outcome. This seems suspect to me, and possibly indicative of another issue. I traced the signal from the Pin Up control, and it runs to Q201 on the D board. This is labeled V RAMP. The collector of Q201 measures 11V steady instead of 11.4V, so I just kind of let it pass, but maybe it's worth checking the other voltages? I have it closed up, so somewhat reluctant to open it again for a long shot.
2) the brightest whites don't get as bright as would expect them to before I start to get color smearing. This only seems to happen with the red, and that seems to be par for the course with 90s VGA monitors. I understand that red takes the most juice to drive or something, so it tends to crap out before the other colors.
However, it's happening right at the edge of what the SM states this monitor can handle, and I've generally seen it be okay with some headroom.
SM states it can handle 96-106 Nits, and I think the last time I did a white balance, I got it about 100ish Nits before it started to smear red towards the right of the whitest whites. It's not exactly dim, but it seems a little skimpy.
So my question to all here is: can I do anything to help with the red smearing, or is that just a function of the phosphors and nothing I can do? I'm new enough to this, and I've heard enough conflicting information about it to not conclusively know if it's fixable or not.
For example, could upping the value (or lowering the value) of a cap somewhere tighten up the color resolution at higher voltages?