The front derailleur on the cellist's bike suddenly stranded the chain on the big ring and refused to bring it back. It's a problem I've seen several times before with Shimano front derailleurs.
One end of the return spring is held by a tiny aluminum nub. After a while, the aluminum can't hold back the spring steel and simply wipes away. This usually happens when the rider has shifted the derailleur out as far as it will go, putting maximum strain on the return spring against its inadequate stop.
The Sora derailleur I just got to replace the cellist's broken Tiagra has just as tiny a bit of aluminum holding the return spring. It's pretty, but no better than the strength of its designed-in fatal flaw.
Usually I scavenge parts off a broken derailleur and throw the rest in the recycling. But I might try to bend the spring on this latest casualty to see if I can get it to bear on the thickest part of the arm. It could go forever like that. More likely the spring will snap. But there's nothing to lose. It's scrap metal as it stands.
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