When focused on a slide the stage drops to become out of focus. I have taken the focus knobs apart and played with the tension adjustment but nothing works. You can see the fine focus knob turn when the stage is slowly dropping out of focus.
It sounds like you do have a pre-tension problem, and if the fine focus is slowly turning as the stage desends, that is where your problem is. If you could tell me make and model, fixing it should be no problem.
Regards.....Dave
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