Why does the mechanical stage for my microscope keep falling to the bottom-most position?

There is a tension adjustment just inside the coarse focus knob. (same side as the switch) It looks like a chrome ring with 2 or 3 holes in it. You may have an odd looking tool that came with your microscope that is used to make this adjustment. Tighten it just a bit until the stage stops falling. You don't want it too tight or it will begin to make it difficult to adjust your coarse focus.

Just inside the coarse focus knob you will see a "chrome" ring with holes it it. That ring needs to be tightened just a bit. It will make the coarse focus knob stiffer so tighten only enough to stop the downward drift of the stage. A funny looking tool should have come with the scope. It is used to adjust this "tension" ring.

Just inside the coarse focus knob you will see a "chrome" ring with holes it it. That ring needs to be tightened just a bit. It will make the coarse focus knob stiffer so tighten only enough to stop the downward drift of the stage. A funny looking tool should have come with the scope. It is used to adjust this "tension" ring.

There is a tension adjustment for this. It is located on the coarse focus knob which is on the same side as the switch. It looks like a chrome ring with about 3 holes in it. A strange looking tool should have come with the scope. It is designed to fit into these holes and allow you to adjust the tension on the coarse focus knob. You need to tighten it just a little bit. Not too much or you will have trouble operating the coarse focus knobs smoothly.

First, a scope of this grade will not be completely in focus as you move from one magnification to the next. But it should be close enough that you do not loose your point of interest.
Be sure you are not pressing down on the stage specimen platform as you change magnifications. It is very sensitive to pressure. Also, be sure that the coarse focus tension is tight enough that the platform is not drifting down imperceptibly as switch magnifications. Look through the scope and watch if the image goes out of focus while you are watching it. If so, you have what is called "stage drift". This is corrected by tightening the tension on the coarse focus knob.
The tension adjustment is on the coase focus shaft. It looks like a chrome ring with about 3 holes in it. There should have been a strange looking tool that came with your scope. It is used to adjust the tension. If your specimen is "drifting" out of focus, simply tighten the tension ring a little bit at a time until the specimen no longer goes out of focus. Do not get it so tight that it is not easy to operate the coarse focus knob.

You need to tighten the coarse adjustment so it generates enough friction to keep the stage in place. How to tighten it will vary from scope to scope, but most just have the screws on the knob (metal circle inside the knob with two holes in it). To adjust this type, you'll need a special key wrench.

Tighten the screws of the coarse adjustment. You must disasseble the coarse adjustment knob (i.e. remove the plastic caps) in order to do this.

The problem is that the stage is heavy and the adjustment screws are loose. You need to tighten the coarse adjustment knob screws in order to create more friction so that the stage stays put. That, or don't adjust anything and keep holding on to the coarse adjustment while you're using it.

You need to tighten the coarse adjustment screws to provide more tension so it sticks. Leave the fine adjustment as it is, it works on a different system. It's the coarse adjustment crank that is slipping due to the weight of the stage and lack of pressure from the screws.

This might be sign of trouble, because the Nylon gears inside might be worn out. I did not noticed a tension ring on Optiphot 1 or 2. The Nikon S and Metaphot from 1960s to 1976 have Nylon gears with tension ring adjustments.