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The ribbon vibrator works when you press 'shift' but does not otherwise. What could be the problem, I wonder?
And where to look.
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There is a high chance that the ribbon vibrator is bent, and operating the shift is allowing the distorted vibrator a little more room to move. Take the ribbon out and examine the vibrator to see where it might be catching. My educated guess is that it will be bent towards the keyboard.
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Thanks, but I found the problem. It was related to the carriage shift mechanism, part of which was disconnected. Once I reconnected it, the vibrator began working fine.