Tension on upper thread brother galaxie 2100

If you are using a standard tension of 4 or 5 and this problem has developed over time, the most likely cause is lint deposited between the tension disks.......
Raise the presser foot and with a length of scrap fabric, use an action like flossing your teeth to get between the disks......in extreme cases a probe (old needle) may be used very gently to remove thread and lint, but be VERY careful not to scratch the polished surfaces.
I have also written a tutorial on tension balance which may be of further assistance:

Please TRY the solution BEFORE giving your considered rating.

Ensure that all is clean and free of lint and jams, this is the most likely cause....now for tension troubleshooting .......

This solution is for tension problems...if you cannot form any sort of stitch, the issue is quite different, so please let me know if you need a different problem solved.....

It is quite long, but just work through each section in order.
The "knotting up" can reveal a lot. If you have loose threads on one side or the other, the tension on the opposite side will be the culprit.

QUICK SUMMARY FIRST:
Ensure sharp new needle, 
Thread guides and Bobbin are Clean & Clear of lint
Set Top Tesion to 4 ....then....
Balance Bobbin to suit.

TOP THREAD TENSION:
If the looping threads are on the underside as you sew, it is the top tension. Top tension ought to be between 4 & 6 (this variation to allow for the different weights of fabric in your 
projects). 

IS YOUR NEEDLE SHARP ?
If you are using a needle that has seen quite a deal of work, or you suspect it may be blunt, change it for a new one !

TOP TENSION & GUIDES:
Make sure that when you thread the machine the presser foot is up so the thread goes between the discs and not to one side, top tension between 4 and 6, and that you have threaded through all the guides, including the last one, usually on the needle arm, just above the needle clamp.

It may be there is lint trapped between the discs, this will keep them slightly apart and reduce the actual tension, sometimes dramatically.

If tensions appear correct, and the thread is definitely in the channel between the discs, but still too loose and looping, try raising presser foot and remove your thread.

Now, with a 2" (50mm) wide strip piece of fabric 8 - 10" (20 - 25cm) moistened with methylated or denatured spirit, gently insert the fabric strip and clean between the discs with 
a see saw / to and fro action.

In the worst cases, gentle use of a needle to pick & remove the jam may be necessary, but be very gentle and make sure the tension is set at Zero and the presser foot is raised, (to 
disengage tension plates).... do not gouge or score the plates, they need a polished surface to work correctly.

BOBBIN TENSION:
Far less common, but if the loose threads are on the top, it is bobbin tension that is loose, it too may have lint in the spring and be giving a "false" tension. 

I would not recommend fiddling with bobbin tension without good reason, it may end up with missing small screws and spring pieces, however, you can take the needle plate off to clean 
the hook race area (where bobbin case sits) 

...this is just good housekeeping, my wife does this every time she replaces the bobbin....

just take it out and clean the bobbin case and the fixed metal hook race with a small brush to remove lint. If there is a significant amount of lint, use a vacuum and small brush to get the worst.

Then wipe all this area with a cloth or cotton bud (Q tip) moistened (not soaked) with methylated spirit, especially if there appears to be fine dirty deposits....oil and lint combine to conspire against you.

If it seems likely that you ......really ....do .....actually .....need .....to adjust the bobbin case, first check there is no lint trapped in the metal spring where the thread is tensioned. 

TOP LOADER:
Drop-in Bobbin case will look similar to this image with the tension screw in the middle of the metalwork....

 ...the other screw at one end is holding it all together, so beware....it is not a tragedy to undo the whole lot and clean it, but very gingerly and lay the bits out in sequence and orientation, or you risk tearing your hair out !

FRONT LOADER:
....this is a bobbin case from a front loading machine and works in a very similar fashion to the top loader with drop in bobbin, again, if you dismantle it, take care so you can put it all 
back properly. FINISHING UP
GETTING THE BALANCE RIGHT:
When you are certain there's no trapped lint in top tension or bobbin, set the top tension to 4 and the bobbin tension to a point where you just begin to feel resistance.

Try using good quality thread of contrasting colours so you can more easily spot the changes.

Set your zigzag to one width less than maximum (eg. 5 of 6 ...or... 4 of 5 etc) and sew a sample for a few inches and check the result.... adjust the bobbin tension screw very little at 
a time, perhaps 1/16 of a turn.

You may find you are playing with this balance for some little while and if you are putting the needleplate on and off each time begin to think it cannot be correct to do this.....BUT....it is, 
and eventually, you do get a "feel" for the correct tension and then it happens quite quickly.....as a user you won't be doing it very often unless there is lint built up (or are there small hands at work around the house !?!?!)

OTHER ISSUES:
If you live near the ocean as we do, salt air can play havoc with metalwork inside and out, so to help minimise this, keep a few small packets of dessicant (silica gel) in your machine 
case....no case ? then make some sort of cover ! 

Same applies in any damp or humid environment, keep your machine dry and dust free.

Budget for a proper full service every couple of years (more often if heavily used) and if you don't use your machine for a few years, be aware that old oil will dry out and combining with 
dust and form a "clag" like glue (another reason for some sort of cover, even a teatowel !)

FINALLY, A WORD ON THREAD:
If it is worth spending the time, energy and money on making something that you would like to give lasting enjoyment......use quality thread, .......it may seem to cost a little more at the 
time, but the results, ease of use and added longevity will be worth the extra, and as a bonus, your tension troubles may be fewer and further between, because there is a more consistent diameter with good thread, and less compensating to be done by your tension plates and less thread breaks ! 
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