Nemoide wrote:There is still audio output when the video cuts out, which makes me think it's MOSTLY working fine.
Yeah, so I would have said that potentially increases the probability that it's a sync issue, but it occurred to me that some of the component signals are going to be superpositions of the RGB signals, and the circuitry might actually lose video output with a problem on one of the RGB signals, if it's a mixed signal process. It's probably not going to be an issue with 5v or ground, but you could test it anyway, just to be sure. The RGB and sync outputs would really require an oscilloscope to verify any sort of serious signal distortion, or external coupling from other noisy sources.
Nemoide wrote:The unit is currently at a friend's house who has more soldering/electronics skills than I have; they did the work and while I'm not sure what solder they used I trust it was something appropriate.
Yeah, the only potential issue here is that mixing alloys can potentially alter the heat profile, and might reduce the integrity of any rework. It's probably not a concern in general, but lead solder mixed with unleaded solder would be a pretty dramatic mixture, and I know some people who just refuse to use lead solder.
Nemoide wrote:I'll also try checking the AC adapter; I feel like this is a long shot, but maybe my American model 1 AC adapter will provide better results for some reason.
US power supply would technically be better, but Japanese power is nominally ~17% out of US specifications, and most of the critical components in the Mega Drive are given a regulated voltage, internally. That said, the unregulated DC off the power supply is used for the rails on a lot of the analog audio and video circuitry. The only real concern is that the rails might be reduced enough by the difference in transformer winding ratio that the signals would start to clip, but I doubt it because those signals probably aren't greater than 3.25 V in peak magnitude. You may as well use the US adapter, anyway, though. There's at least a possibility that it would solve the issue.