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.
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dont know the model but it sounds like you have a problem with the light source, Is there a sub-stage condenser?. Try googling
"set up microscope lighting" If the mirror has 2 sides, concave and flat use the flat-plane side. dont point the mirror at an outside sky but rather at a lit pale coloured wall, piece of paper etc.
as a rough start try a piece of white tissue/toilet paper under the
slide and investigate full focus movement ...carefully!!
be careful the the objective stays well clear of the slide.
start with the lowest magnification first. good luck
You will need to focus in on the slide. There should be 2 1" diameter knobs on either side of the stand, and just to the rear. Those knobs control the height of the stage (where you slide sits). Slowly raise or lower while looking into the eyepiece. At some point something should be see, if the microscope is working properly.
Sometimes it is easiest to start with the lowest magnification. On the outside of the objective (little lenses hanging down that can be rotated) there should be some numbers written on them (usually 4, 10, 40). Those numbers reference your magnification. If you multiply that number by the eyepiece (10) magnification, you will get your total magnification, so start out with your 4x. It will give you the largest field of view, so it is great for finding whatever it is that you are looking for on your slide. It is also the easiest one to get focused. The higher you go in magnification, the smaller the field of view, thereby the more difficult it is focus in on your sample.
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.
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.
I think your problem lies in the way the stage is limited upwards. On top of the stage, right behind it where it slides against the arm, there should be a little screw. This screw limits the movement of the stage to a certain point upwards. If the limit exceeds the adjustment gear and its opposing track, then the stage just goes up one notch and then it clunks down. If you screw it down the stage is limited further down, if you screw it up it's limited a bit higher. The point of this screw is to prevent objective and slide damage. Try adjusting this little screw a bit further down and see if it still 'clunks' when you turn it up. After that we may work on properly adjusting it. If not, please post again with whatever detail you can provide to aid you with this problem further.