You would have to look at the relationship between the slow speeds timer and the speed selection mechanism.
The speed selector wheel operates a blade, that inserts into a bronze disk with slots. When you lift the selector, you can move the blade from one slot to another. Each slot = one speed.
Past the 1/15 mark, the selector begins to engage the slow speed timer.
I believe that yours is engaging the slow speed timer in every position, i.e. it somehow passed the bump that limits movement of the selector wheel further than the 1 second position, which is the slowest speed (all the way counterclockwise). If so, moving the selector from one speed to another in your camera would make the buzzing sound characteristic of winding the slow speeds timer. The bump is visible between two slots, at about 7 o'clock in the bronze disk in the picture below.
If this is what happened in your camera, it should be possible to make the speed selector go again over the limit bump in the opposite (clockwise) direction, to recover normal operation, without having to open the camera top.
This photo is from a previous thread in which I asked for help about a Fed3B (please ignore the arrows). Hope you can identify the bronze disk with slots and the blade in the picture.
In this camera, the slow speeds timer has been removed and the slot is in the 1/125 mark. You might have to remove the slow speeds timer in order to fix yours. Good luck.
