Pentax MX Troubleshooting

The LEDs in the viewfinder doesn't light up when the meter is activated.

Install new batteries in the camera.

Check the contacts in the battery compartment. Old leaky batteries are a common problem and the acid can corrode battery contacts. Vinegar can be used to clean the corrosion. Isopropyl alcohol or a contact cleaner can be used to clean oxidation.

There is a contact located under the bottom cover (red) that turns on the light meter when the shutter button is pressed. If the contacts are dirty or misshapen, they may not close properly. Inspect them and make sure continuity is achieved during a shutter button press.

The LEDs in the viewfinder turn on but don't turn off when the advance lever is pushed in.

Inspect the meter switch contact mentioned in the previous section. They should be open when the shutter button is in the home position. Reshape as necessary.

When there is a small tab that interacts with a post on the shutter release plate. Its purpose is to keep the shutter release rod slightly depressed when the advance lever is in the 'ready' position (sticking out from the body). This keeps the light meter on for extended periods of time, until the user pushes the advance lever in, allowing the shutter release rod to return all the way. If the post is not adjusted properly, it may keep the release rod depressed even when the advance lever is pushed in. Adjust the position of the post by turning it with a slotted screwdriver.

The reflex mirror in the body of the camera is stuck up in the taking position blocking the viewfinder. The shutter won't fire and the advance lever is locked.

When the shutter completes the exposure the closing curtain trips a latch (red), which releases the mirror and lets it return to the viewing position. If the mirror return latches on the chassis or mirror box are dirty, they may not be releasing properly.

It's also possible that the closing curtain is not traveling fast enough or with enough force to trip the mirror return latch. Getting the curtain back to a proper operating speed requires a more thorough shutter service.

The shutter won't fire and the advance lever is stuck. It won't do anything.

This, unfortunately, could be cause by a number of things. Instead of looking for one or two typical failure points, the recommendation is more general. Remove the Top Cover, Bottom Cover and Front Lens Board. Then try to cycle the camera while inspecting the mechanisms and see which part isn't functioning as it should. Consult the wiki How the Pentax MX Works for detailed descriptions of how the individual mechanisms work.

The number on the frame counter doesn't increment when the film door is closed and the camera is wound.

The frame counter doesn't return to zero when the film door is opened.

A hot-shoe mounted flash will not fire when the shutter is fired.

The connection between the flash sync circuit in the camera and the hot shoe is done through flexible contacts on top of the eye piece (red). If the contacts are too bent or otherwise misshapen, they may not be making contact properly.

The X-sync contact that triggers the flash on the hot shoe is located up under the wind plate (red). The contact may be dirty and not closing properly. This can be accessed by removing the Front Lens Board.

Film advances normally for several frames but then stops part way through the roll. Evidenced by the rewind knob no longer turning or half of the frames being blank after development.