A weak point on the Teenage Engineering OP-1 is the headphone jack becomes faulty.
It happened to mine. One channel became crackly and only worked after wiggling the connector.
I was advised to buy a whole new I/O connector board, which seemed like a waste.
Instead I found a bad solder joint that was causing one channel to be crackly and intermittent. Let me show you…
Remove the IO-board cover
Inspect the solder joints on the 3.5mm jacks
You might need to wiggle the jack to find the bad joint
Resolder any bad joints And that’s it
I resoldered the bad joint, adding a small amount of fresh solder. All working.
So if you have a faulty jack connector on your OP-1 it’s like just a bad solder joint on the connector PCB, which is easily be fixed with a soldering iron and much cheaper than buying a new board.