The control knob is stuck on my D40

make sure flash is on your flash on cam is sticking.

take a wooden stick rape a cotton deep in alcohol try to clean it see that the alcohol should not' go inside or go to repair

The auto focus on modern cameras is pretty intense, I would recommend taking it in for repair.

The D40 Dynamic area focus starts on the centre AF area then moves the focus to follow the subject.
The camera has followed something to the right and selected the right focus area.
This is normal.

You could select Single Area in the menu and move the area back to the middle using the rear AF Sensor Selector.

I have similar problem on my D40. I think there is something mechanical with the camera's shutter button, some connections, I guess... The only solution I've found is to go to Menu > Custom Settings Menu > "AE-L/AF-L" and set to "AF". Now you can focus by AF-L button. Focusing works then perfectly. The WORST thing is that I can't lock exposure now and that's sad :( Maybe I can try to fix this in the camera center later, we'll see. But it takes months in Ukraine to do the fixing... Hate that!

You would be best served if you bought and optical slave trigger or a radio trigger for that camera. If the Novatron unit is old enough, the voltage may be too high to use with the Nikon D40. Once you have a trigger, the camera can be used in the Manual mode like your F3HP. the synch would be 1/60th and you set the camera aperture through the control dial.

The D40 doesn't have a Live View capability. You have to use the viewfinder.

If you were seeing the image before you shoot, there was definitely something wrong with your camera.

The D40 does not have a Live View feature. The display is for menus, shooting information, and image review. Not preview.

Hey mcheng,
Since I am assuming you have opened the D40 the store can charge you a 15% restocking fee, but with an exchange they rarely do. Without the restocking fee you would just pay the difference between the two cameras’ prices. Remember the return/exchange period on digital cameras' is 14 days. I hope this helps!

Sincerely,
Allan
Go Ahead. Use Us.

Are you sure it's the zoom ring that's stuck, and not the focus ring? The focus ring doesn't move when the camera is in autofocus.
If it's indeed the zoom ring that's stuck, you can spend more money than the lens is worth in repairs, or buy a replacement (circa 35 dollars on ebay) for dreadfully cheap. Good luck.