Machine tension knob does not work.

With threading tension issues, first you start by checking the bobbin tension and once that is set correctly, everything is done using the top tension dial. This is explains in text article

What is Perfect Sewing Machine Tension? Perfect sewing machine tension occurs when the top and bottom threads are perfectly balanced. That means you shouldn't see any little loops on either the top or bottom of your fabric and the stitching should look even on both sides.

Sewing Machine Tension - Top 10 TIPS - Treasurie blog

The two top videos are excellent at explaining how to set bobbin tension, and how to diagnose where the issue is, top or bottom?

https://www.google.com/search?q=sewing+machine+tension

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Hi there Maybe a Manual will help you just click on the link and you can read it or download it if want bet i can only leave it there for a short time all best Clive
Just click on blue link.

https://my.pcloud.com/publink/show?code=XZhOOx7ZifJC4gDMBCSdqzecQD8Jf4MmrghV

The tension control has a screw on the inside wall of the sewing machine. It probably came loose and will not allow the tension to pull taught. If you take the top off you can look down to where the tension knob is coming from. There is a screw that you can tighten while you push forward on the tension knob while set to zero. Keep the presser foot up.

.... if it was sewing fine before the bit 'dropped off' it probably doesnt need the other thing done - they just are standard type check / adjust things done with a service. Weigh up how much a new machine will cost you v the repair. How happy are you with it? How often do you use it?
I am not familiar with this model, but the 'clutch' is generally a very simple setup. If you feel confident and can get the end cover off , have a peek - find the bit that dropped off ......
The only tension you should need to adjust is the bobbin tension - which I am sure you already know how to. Top tension is normally just a clean out of fluff etc.

there could be something wrong with the cam that does the decorative stitches, you may want to have the machine checked..

here is a free manual for the machine

http://www.brother-usa.com/ModelDocuments/Consumer/Users%20Manual/UM_LS2250PRW_ENES_2733.PDF

the machine may need to be checked to see if there is something
wrong with the tension knob

try changing the needles
is the thread thickness the same for all the spools or cones?
it might be a good idea to have the machine checked to see if
there is something wrong with the tension knobs

Make sure your needle and tread match your project. There simply isn't such a thing as a "universal needle." Also try lengthening your stitch. Heavier fabrics typically need a slightly longer stitch. Upon commencing to sew always grasp the 2 threads and slightly pull as you begin your seam.

I assume your garage sale find has a manual. Be sure your bobbin is unspooling in the correct direction. If it didn't come with a manual.. http://www.singerco.com/uploads/download/430_8280_es.pdf

Good luck.

there are generally two variable controls .. one adjusts the stitch length (distance between stitches measured in stitches per inch) ... the second is stitch width .. that adjusts how far the zigzag moves left and right .. thats usually just a number from 0 to 10 or so .. 10 would be maximum width .. 0 would be used for straight stitch ..
so you need to adjust the stitch width control to make the stitching narrower .. the controls are called different things on different brand sewing machines .. if you have the brand and model number then there may be instruction booklets available ...

I'm guessing you would have loosened the tension? Is there grit or something sticky (dirty oil) stuck between the tension discs?