Good evening. I'm Bruce, nickname "DrKlahn."
Grinding noises in a turntable indicate that it is a gear-driven turntable. Looking at a photo, I see it is one of the plastic-gear-driven tables that were common from the mid-60s through the early 80s in all-in-one stereos. The plastic tends to harden over time, become brittle, and fracture. This is what I think happened in your unit and unfortunately there's no way to fix those broken gears; they must be replaced.
The way to repair
the unit is to have a trained technician open the unit, disassemble the gear trains, find the problem gear(s), replace them and reassemble the unit. This can be done but it will be an expensive proposition, figuring two hours of work at $100/hour plus parts. The parts may be difficult to locate so add in $50 for parts. Total, around $250.
In this case replacement
of the turntable looks like a better option. If the unit was here in front of me, I'd suggest buying an identical unit and removing the turntable for replacement into your unit. This can be done relatively quickly by any electronics repair shop or electronic hobbyist. You'll then also have every spare part you could need for any future problems.
At present there is one MC3050 on ebay.
MC3050 on ebay, $75 (delivered): http://www.ebay.com/itm/161703148283
Don't hesitate to ask if any questions remain unanswered.