For a lot of MD issues (including the “space invaders” error) people recommend unplugging and reseating the ribbon cables inside. It comes up a lot as kind of a general cure-all but if the physical component (pot) is failing this won’t do anything, however it’s a pretty non-committal act which you can try without doing additional damage or any soldering.
You’ll find it a lot if you look at old threads like this:
So certainly, it’s up to you if you want to even bother with this and don’t open the machine if you aren’t comfortable doing that, but it’s relatively simple and free to try.’
Edit: I was just thinking about this a little more and you might be able to run a couple of tests by routing tracks to the individual outs and determining if the volume drop is present at those outputs as well.
I can’t remember whether or not the individual outputs bypass the analog pot or if they just send the mono channel through it, but observing the behavior might give you a little more information about the nature of the problem to help you identify the cause until you do have a chance to go and get it checked out.