A friend of mine bought a lot of ten Pentax K1000 cameras in an auction in Canada for $10 and was kind enough to send me one. It mostly worked but the slow shutter speeds were a bit sticky. I figured the old lubrication was probably on it's last legs so I opened it up and sprayed some tri-flow on the mechanisms. This was probably a mistake.
Not only did I manage to get tri-flow on the shutter curtain, but the fast shutter speeds are now wrong. Judging by my most recent almost completely blank roll of film, I'd say they're too fast. If I was going to fix it, I would need a way of measuring it, so that's what this post is about.
The K1000 shutter has two curtains: the opening curtain and closing curtain. Both move horizontally. The opening one starts out covering the frame and moves out of the way to expose the film. Conversely, the closing one starts not covering the frame and moves to cover it. At speeds at or below 1/60th of a second the opening curtain should completely uncover the frame before the closing curtain starts to cover it. This is important when using a flash; if the entire frame were not exposed when the flash went off then only a portion of the frame would receive the light from the flash. At speeds faster than 1/60th of a second the two curtains form a slit which moves across the frame.
The measurement rig has two jobs: measuring the exposure time and measuring the exposure evenness. The first is pretty straightforward; just measure the exposture time a given spot. The latter needs to measure the exposure time at multiple locations on the shutter to ensure that each part of the frame receives the same amount of light.
To measure the light at a given point I use a reverse biased LED hooked up to a transimpedance amplifier. Since an LED's reverse leakage current increases when light hits it, they can be used as a light detector. The tranimpedance amplifier then converts and buffers the tiny current (~200nA in bright light) to a voltage. Each output is then hooked up to a channel on my oscilloscope to measure the timing.
Granted, using actual phototransistors would have been easier and worked better, but I had the LEDs on hand, so that's what I used. The MCP6294 is overkill and I'm fairly certain any old op amp will work just fine.
To use the circuit I take the lens of the camera and shine a desk light at the shutter. I position the LEDs so they are almost touching the shutter, set the scope to single shot mode and fire the shutter.
The setup works really well, unfortunately the camera does not. Not only are all of the timings too fast, but the closing curtain is moving faster than the opening curtain, which causes uneven exposure.
The above scope traces show the shutter at 1/60s. The timing isn't bad but it's really uneven. Using the scope's measurement abilities I can measure the pulse widths to determine the shutter speed. Note that channel 2 is not measuring correctly, probably because the change in voltage is not great enough. Looking at the pulse widths you can see that one side of the frame gets significantly more light (22.0ms) than the other (17.2ms).
At 1/125s the problem is worse. The timing is way to fast (5.5ms instead of 8ms) and the exposure is more uneven. One side barely gets any exposure at all at 0.7ms. This unevenness is from the closing curtain moving faster than the opening curtain.
Next I need to buy the proper oil to relubricate the camera and give it a proper CLA (clean, lubricate and adjust). Luckily I found a copy of the repair guide which should make the process a lot easier.