A few years ago, I bought an 8 mm home movie camera in a second hand store. I did a teardown on it here and pulled out for your pleasure those parts of it which I considered interesting. My vague plan was to put a Raspberry Pi in it, but instead it provided a gateway into the world of 8mm film technology. Since then I’ve recreated its Single 8 cartridge as a 3D printable model, produced a digital Super 8 cartridge, and had a movie camera with me at summer hacker camps.
When I tore down that Single 8 camera though, I don’t feel I did the subject justice. I concentrated on the lens, light metering, and viewfinder parts of the system, and didn’t bring you the shutter and film advance mechanism. That camera also lacked a couple of common 8 mm camera features; its light metering wasn’t through the lens, and its zoom lens was entirely manual. It’s time to dig out another 8 mm camera for a further teardown.
A Different Camera To Tear Down
My test camera is a battered and scuffed Minolta XL-250 that I found in a second hand store for not a lot. It takes Super 8 cartridges, of which I have an expired Kodachrome example for the pictures, and it has the advantage of an extremely well-thought-out design that makes dismantling it very easy. So out it comes to be laid bare for Hackaday.
Once the sides have come off the camera, immediately you can see a set of very early-70s-analogue PCBs containing the light metering circuitry. Typically this would involve a CdS cell and a simple transistor circuit, and the aperture is controlled via a moving coil meter mechanism. This camera also has a large mostly-unpopulated PCB, giving a clue to some of the higher-end features found on its more expensive sibling.
Turning our attention inside the camera to the film gate, we can see the casting the film cartridge engages with, and the frame opening for the shutter To the left of that opening is a metal claw that engages with the sprocket holes in the film, thus providing the primary film advancement. The metal claw is attached to a slider on the back of the film gate, which in turn is operated by the rotation of the shutter, which is the next object of our attention.
The shutter is a disc that spins at the frame rate, in this case 18 frames per second. It sits in the light path between the back of the lens system and the film gate. It has a segment cut out of the disc to let light through for part of the rotation, this is how it operates as a shutter. On its reverse is the cam which operates the slider for the film advancement claw, while its front is mirrored. This forms part of the through-the-lens light metering system which we’ll come to next.
The mirror on the front of the shutter is angled, which means that when the shutter is closed, the light is instead reflected upwards at right angles into a prism, which in turn directs the light to the light meter cell. The PCB on the other side must have a charge pump which takes this 18 Hz interrupted analogue signal and turns it into a DC to drive the moving coil mechanism. There’s a 10 uF capacitor which may be part of this circuit.
Finally, we come to the powered zoom feature that was missing from the previous camera. On the top of the camera is a W/T rocker, for Wide/Telephoto. that operates the zoom. It is connected to a set of levers inside the case, which emerge as a pin at the front of the camera below the lens. This engages with a small gearbox that drives a knurled ring on the lens body, and selects forward and reverse to turn the ring. It’s driven by the same motor as the shutter, so it only works when the camera is operating.
I hope this look at my Minolta has filled in some of the gaps left by the previous article, and maybe revealed that there’s more than meets the eye when it comes to 8 mm movies. Careful though. If you dip a toe into this particular puddle it may suck you in head first!
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