Camera Repair is a Spiritual Exercise
One of my hobbies is to buy old, junk cameras to fix and take pictures with. I like to call it "The Camera Game." I don't try to work on the newfangled electronic auto-everything type of camera. Without the schematic and the proper test equipment you just can't do much with them. I like the old mechanical types. They whir, click, and make other neat noises. They are solidly built with all metal bodies, brass and steel winding mechanisms and, best of all, they don't need batteries to work. In many cases, these older cameras are far superior to newer models. "They just don't make 'em like that anymore!" If anyone made a fixed lens, leaf shutter, rangefinder camera like these today they would probably charge $500-1000 for it.
Unless it is mishandled or used for professional photography, a mechanical camera won't wear out in a person's lifetime. The shutters can be rated to up to 30,000 shots so you would need to shoot ten to twenty rolls of film a week to get even close to the camera's lifetime during your life. What happens is these cameras get neglected. They get dirt, sand and other environmental "gunk" in them. The lubricants dry out, migrate and jam up things. Generally, all it takes is an overall cleaning and slight adjustment to get the camera operational again.
There are some exceptions. If the camera has been dropped, gotten wet, or someone forced a wind lever on a stuck shutter, then parts can get bent, rusted or broken. The other major disaster is lens fungus. There is some type of fungus that likes the coatings on camera lenses. If the fungus grows long enough, it will etch the coating and ruin the lens. But, if you get to it soon enough, the fungus can be cleaned off and the lens will still take good pictures.
To fix one of these cameras, you first have to disassemble it. This can be harder than you think. The screws that hold a camera together are often hidden under coverings and can be hard to locate. Sometimes the covering can be almost impossible to remove, especially if the glue has dried out. You also find a wide variety of nuts and screw-like devices. Pin-faced screws, screw in cover plates, slotted and un-slotted retaining collars are just some of the ways cameras are held together. Without the right tools, you'll never get the thing open. Sometimes you encounter the dreaded "left-hand thread" type screw. These have to be turned clockwise to loosen. If you turn it the wrong way, you'll strip the head right off. Rusted or glued-in screws are even worse. In some cases, you just have to drill out the old screw and replace it. Only someone with a thorough knowledge of camera construction, the right tools and a feel for the materials should try camera repair. As many have found out, a first attempt at camera repair often ends up as a pile of parts in a plastic bag.
Once the camera is opened up you have to clean everything. Dirt and dried out lubricants are the major problem. It only takes a tiny amount of old oil in the wrong place to completely stop a mechanical shutter. It looks clean, but you eventually learn that if it doesn't work right, it's probably dirty. You dunk each part in solvent or the appropriate cleaner, scrub it clean and then dry it off. Once it's all back together it will usually work just fine.
However, sometimes you encounter a problem camera. Somewhere during it's life it has been mishandled and levers get bent, gears and springs get broken. For these cameras, special attention is needed. You have to bend, file, and sometime hammer the part back in place. For bent parts, you take the part and lay it on something straight, then give it a good whack with a small hammer. Every now and then a part is too far gone and you just have to replace it. But that, fortunately is rare. A good camera repairman can work with what's there, and usually make it work.
Once everything is clean, straight and back together in the right way, you make your adjustments. Lenses and rangefinders have to be tweaked to get perfect focus. Light meters need to be checked against a reference and adjusted to give the right exposure. But that's about it.
So you see, it's a spiritual exercise.