Chris B wrote:

Now the door worked fine for 20+ years until the first failure of this
type, but having reassembled and re-tensioned it about 5 years ago it
has now failed again in exactly the same manner.

Here's another thought. The spring is simply shagged. It can no longer
maintain enough tension over its full range of movement to reliably grip
the anchor spiral when it's at its lowest tension.

Let's face it, after all these years it doesn't really owe you anything.

Tim
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Please don't feed the trolls