Microscopes 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.

Maybe there is some wear at the thread where the stage fits, causing that the stage cannot hold.

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.

The knob you are talking about is called the x-axis control knob. It does not directly affect the focus. What affects the focus is the slight manipulation of the stage and or slide while you are adjusting the x-axis control knob. It only takes the slightest pressure to throw the focus off very far. Theoretically you could completely remove the specimen holder and move the slide around with your fingers.
The specimen holder may need to be replaced but without further more detailed knowledge of the mechanical problem I cannot instruct further.

There are two reasons. One reason is that when on high power you are working so close to the glass slide that it is easy to misjudge how much you are moving the specimen toward the lens that you can break a slide before you realize it. The coarse focus moves the stage with the specimen on it very fast and you only have a very very short distance within the focus plane before you run the objective lens into the slide.
Secondly, it is just harder to control the minute adjustments needed at the higher powers with the "coarse" focus knob. If you start at the low magnifications find what you want to concentrate on with the coarse focus knobs and then work your way up to the higher powers, you will have very little trouble moving to the fine focus controls at 40x and 100x while still having control of your image.

There is what is called a "stage stop" screw. Its purpose is to limit the specimen from rising high enough to contact the objective lens. The "stage stop" screw is directly behind the black stage platform in plain sight and very easy to access. It is a thumb screw and sits vertically. Simply loosen it a bit to allow the specimen to rise high enough to focus.

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.

Try holding the Right knob so it does not move, whilst firmly moving the Left knob towards you (clockwise), this is the adjusting method used on Zeiss Jena 1960's High End microscopes, e.g Eduaval & Ergaval.
Good luck,
Alan Mowle