Platter wobble

I have a Technics SL-BD20D and the platter has a noticeable wobble while spinning. ive tried a couple mats - cork and acrylic - which made the wobble much more apparent.  The rigid acrylic platter would not lay flat on the platter. 

Anything I can do? Is there a way to test what the issue is?  Is there an quality aftermarket platter that I can use?

Is the cartridge worth saving?
Cart is OEM. 

Not looking to spend money. Looking more for a DIY fix...  or a mat/platter that will improve sound without significant investment. 
As platter is not stiff you can carefully bend some areas and bring it to shape. It will take some time, even though you may get close enough it will never be perfect. Don't spend your money on buying a new one as wobble will be apparent also.
Does the platter lay flat on a table? If so the problem is probably elsewhere. One thing that might work, if the acrylic platter is warped, is placing the platter between two sheets of glass and weighting the top sheet of glass with 5-10 lb. weight. Place the “platter sandwich” in oven at 150-200 degrees for 10 minutes. See if that works. If not, replace the platter. Maybe the platter is still on warranty. 
Post removed 
Nearly all aluminum type platters at this price point (due to thickness) suffer mostly the same. A bearing would not produce wobble but noise. SLBD20 does not have an acrylic platter. If mat does not sit properly on platter then platter has an issue.
Thanks guys.

@petg60 you've confirmed my assumption that an OEM platter off ebay is likely to have the same issue.

Does anyone make an alternative platter that might work?

The OEM rubber mat lays nicely on the current platter and the wobble is very minimal - But i've tried a cork mat and an acrylic mat and the wobble became very pronounced as the mats did not lay flat.  I'm most interested in trying alternative mat materials but can't seem to get them to work with my platter without wobble....
So, what material is the platter? At one point you said acrylic but that was probably a typo. Maybe time to upgrade the TT.
Hi @geoffkait platter is aluminum.  My mention of acrylic platter in the first post was a typo. That was meant to say: "The rigid acrylic mat would not lay flat on the platter"
leemaze, do not bother with other mat than rubber, it is fine (rubber takes shape easier and suits your tt fine). You will benefit a worthwhile improvement by going to an upmarket tt.
Your only hope, if you want to fix this problem without spending money, is to try the method suggested by GK. Otherwise, you have a platter that is not flat. A flat platter is a core requirement in vinyl reproduction. If one of your rigid mats lies flat such that it is plane parallel to the arc inscribed by a tonearm, then you might get away with using a rigid mat. Otherwise, I would dump the thing.
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.
The way I figure it since many LPs are not flat it will all average out. 
Or if MDF isn't good enough you could upgrade to IKEA bamboo
Note this whole turntable cost less than the Technics. 
Nice find. Gonna be hard to beat that.
Dear @leemaze: What are you waiting for ? , just from the very first post in your thread @noromance  gave you the rigth answer, now it's up to you.

Regards and enjoy the MUSIC NOT DISTORTIONS,
The OP has a belt drive turntable with a cast aluminum platter. The turntable’s prior owner(s) probably bent the platter by pushing it down one one side. These platters aren’t designed to withstand that kind of abuse. Given what it is, the OP could try to “correct” the bend in the platter by rotating it to the high point and gently pressing down until it turns without significant runout. Bottom like is these things are a dime a dozen, easily damaged by carelessness, and the p-mount cartridges tend to be lacklustre. With that said, a well cared for turntable similar to the OP’s turntable can produce much better sound quality than its value would suggest. 
Welp, I got it for $25.  I hope this one is flat!
Fingers crossed. If it is, put it on a stable table on a sheet of granite and treat yourself to an Audio Technica AT95E cartridge.
Thanks @noromance though it seems the AT95E will not work with my p-mount tonearm. Is there an equivalent p-mount?
Update: Sadly the new one wobbles worst than the original. 

Oh oh well was worth the shot. 
This is not good. Send it back. I was really hoping you could get up and running on the cheap.
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.

But @millercarbon I don’t have a jigsaw!

It would be a little tricky with this table as the belt circles a smaller inner ring on the the underside of the table. 

And No worries @noromance i got a refund. Thanks!
is the bearing shaft warped from the platter being slammed hard on one side/edge?

take the platter off, and re orient it so it sits on the bearing spindle in a different spot.

If the wobble changes, at all, you have at least part of your answer.

Most wobble in a platter on a older cheaper technics deck like that, is going to originate from abuse. Technics never sold any turntables with pronounced platter wobble, so it has to originate in abuse or damage to the spindle’s original acceptable limits of production ’eccentricity’.

If you can wobble it by hand, by gently rocking it, then it may be the spindle and the platter are both damaged from a hard side impact on the platter edge when it was mounted. This is actually quite common in cheap turntables that have had some nasty near endings to their lives. It takes almost nothing, re force applied.

They are quite fragile in this area of abuse or impact, as all the force of such a blow goes right into the fulcrum which is also the mount point and also the area where mechanical platter edge stability originates from.

Damage can arrive from a quite minimal impact, lets say..even the platter edge being minimally slapped by a 5 year old (playing DJ, like they saw on TV) can ruin one of these less expensive turntables or even easily ruin a more expensive one.

The aluminum is soft, the lever is ridiculously long, the mount itself has very little cross section to help spread the PSI (pressure per square inch-on the platter mount), the tolerances required are tight, etc.

As I’ve got one beside me..lets say.. an empty not too thick walled ceramic coffee cup, from about 2-4 inches height, dropped near or on the edge of the platter - of almost any inexpensive technics turntable (or sony, jvc, akai, nec, sansui, you name it)... would be enough to make for at least some to possibly major permanent platter wobble.
Anyone else notice that the OP apparently still hasn’t laid the offending platter(s) on a flat surface to determine if they’re the problem rather than something more serious? I’m not sensing a lot of commitment here. 
Lol @jeffreylee thanks for your input here. Very helpful!

Thats the very first thing I did. If you look at the actual platter in question, it has several levels of surface with a recessed outer ring that’s doesn’t actually touch the table when you place the platter down. But great idea!! 

Anyone else notice this is @jeffreylee’s 21st post? I’m not sensing a lot of commitment here...
I’ll post more often when you promise to give up on the idea of buying aftermarket platters for a $75 turntable with a plug-in cartridge. 
IMHO spending money does not equal commitment.

Thanks for your opinion and very constructive additions though!

peace and love!
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. 

Thank you @millercarbon and @teo_audio. I've done this and gotten as flat as possible I think.  I think this is the best solution.  The wobble has been minimized. Thanks all.