Platter wobble

So let's see it sounds like the platter when removed lays flat. The bearing when you check it turns true. So the problem is probably what teo_audio outlined, the platter got bent, probably somewhere near the center. 

What you need then is some way of bending it back straight. Easiest would be if you can lay it down upside down on something solid and flat and then tap it out flat from the underside. Hold something like a dowel or block of wood to the high spot and hit it with a hammer. Don't hammer directly, you'll make dents or maybe even crack the platter. 

This might just maybe be a good time to scroll up and read my post one more time. I am not kidding. Totally serious. Now maybe you got another platter and can see how fubar it is you begin to understand. You could totally take a piece of MDF and make your own better platter.

Nice find. Gonna be hard to beat that.

Or if MDF isn't good enough you could upgrade to IKEA bamboo
https://www.youtube.com/watch?v=1sp37DxnKDY
Note this whole turntable cost less than the Technics. 

Always wondered how anyone sells a complete turntable so cheap. It has to come in something that will at least survive transportation, and so at a certain point the box it comes in becomes one of the more costly "components". 

When it gets to the point where the parts are so cheap and flimsy one of the key components whose whole job is to just lay flat, and it doesn't even do that, well then Houston, we have a problem.

But you're determined to fix it for free, or at least cheap. If you can't bend it flat then here's what you do: Make yourself a new one out of MDF. Even if you have to do this entirely by hand my bet is it will be a big improvement over what you have now.

Is the cartridge worth saving?