If its missing a pot screw, then is it like...broken off from the pot itself? If that's the case then I would say that needs to be replaced for sure.
Could you elaborate? I'm sorry, it's just that I really don't know what he's referring to, or how I could get a replacement for that; and he's about as good at explaining this stuff as he is at taking photos.
"Pot" is short for potentiometer. Its basically the same thing as the volume knob on a stereo only the ones inside a game system are really super small and need to be turned with a tiny screwdriver. If a person turns them too far they break. CDROMs often have little adjustments that can be made to fine tune the laser. It usually won't fix a problem, but someone must have thought it would and broke the thing.
If yours is broke you need to replace it because whoever broke it probably also messed with the adjustment a lot as well before they broke it, and its probably not in the original setting.
As for a replacement, I really don't know. Possibly Radio Shack, but they are stocking less and less of that sort of thing nowadays. You could get one from some broken thing laying around the house. A PS1 might work, and at least half of those are broken.
Just for you, I opened my Duo R and took a picture. I've never owned a standard CDROM2 unit, so I have no idea if the pots in your system are like mine, or if they are even broken in the first place.
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