The big spring is the tensioner. Rotating it increases/decreases the tension. Somewhere on the length of the bar and spring mechanism, perhaps at the centre, there will be a series of holes which you can put strong allan key or similar into, which will allow you to turn the spring, or hold it in place. I must stress strongly that you need to be very careful tho, cos if it slips it'll be off with your fingers. The amount of tension is balanced against the weight of your door. To set it properly you need to undo the blue spindles at each end, adjust the spring then put the spindles back on and connect the cables.
What i suspect has happened to yours is that the door has been jarred and one of the cables has simply jumped off. The original tension will still be on the spring tho. The easy fix is to close the door, insert the allan key or similar and release the tension a little, and refit the offending cable. You will probably need to remove the blue spindle to ensure you get rid of any twists in the cable.
The tension should be such that pushing lightly with one hand will open or close the door from roughly its central position. The door should move to the same central position by itself when you let it go. If it's not doing this then check that the rollers and tracks are ok and lubricate them with WD40 or similar. Don't use grease as it goes thick and clogs the rollers.
It sounds difficult but it's not a bad job for a fairly competent DIY er.
Sorry the above post should say that letting the door go from below the half way point it should go to half way by itself. Obviously when put up it should stay up.
Ok tar its spilt pins I think that hold the blue bit on. I think it might need professional help? Anyone have any ideas on how much and can recommend any one in north side of Wakefield.
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