How to Dry Fire a Pentax 6X7 Camera
The Pentax 6x7 camera looks like a standard 35mm single-lens-reflex camera body that's been using steroids. This medium-format camera produces an image measuring 6 centimetres by 7 centimetres in size (hence its name), or roughly 2 1/4 inches by 2 3/4 inches.
The early models of the camera used electromechanical shutters; following a 1989 redesign, the shutters were fully electronically controlled. A safety interlock on the shutters prevents the camera from being fired if there is no film loaded. This makes it difficult to test-fire (or "dry-fire") the Pentax 6x7 for the purposes of checking shutter speeds or mirror operations. But there is a simple workaround.
Check for battery power. If the battery is weak, or there is no battery in the chamber, the mirror will stop part way up, blocking the viewfinder, and the shutter will not operate. The camera uses either a 6V alkaline battery (4LR44, A544) or a 6V silver-oxide battery (4SR44, 544, PX28). The battery is inserted in the battery well on the bottom of the camera, below the lens mount. You can check the battery strength with the battery check button on the back of the camera.
Make sure there really isn't any film in the camera before you start testing the shutter. Cock the camera with the wind lever. If there is no resistance, there is most likely no film loaded in the camera. If there is firm resistance, the camera is already cocked. Don't force the wind lever! Press the shutter release. If the camera doesn't cock and won't fire, there is no film in the camera. The shutter of the Pentax 6x7 camera is designed with a safety interlock to prevent firing when no film is loaded, presumably to avoid having the user taking images with no film in the camera.
Disable the shutter safety interlock with the shutter cocking key. The cameras initially came with a small coin-shaped device with a short nib on one edge, which allowed the user to operate the shutter without film in the camera. If you have the shutter cocking key, open the camera back by sliding the release catch down on the left side of the camera. Insert the nib of the key into the slot immediately below the wind lever. This disables the safety interlock, and you can cock and release the shutter without closing the camera back.
Use a workaround if you don't have the shutter cocking key. Open the camera back by sliding the release tab. Manually turn the exposure counter dial counterclockwise until the film counter indicates 1 or a higher frame count in the film advance window. Close the camera back while keeping your finger on the exposure counter dial. Rotate the film winding lever until the shutter is cocked. The camera shutter can now be fired, because you've fooled the safety interlock into thinking that there is film in the camera.