But how about the IIa?

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The Contax IIa/IIIa is a completely different mechanism from the prewar II/III and the Kiev, and it has different strengths and weaknesses.
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The Contax shutter is a removable module, but most of the works can be reached without removing it.
The gears & pulleys & stuff:

The two cams visible under the top cover (see the top sketch) are at the upper end of shafts that connect them, via bevel gears, to the actual working shafts which are horizontal (the length of the shafts is much exaggerated to make room for the other stuff in the sketch). Releasing the shutter starts the #1 curtain traveling, which turns the pulley which turns the bevel gear which turns the cam which kicks out the second curtain release lever (whew). The position of the release lever is controlled by the shutter speed dial, so that it gets kicked out quicker at higher speeds. At speeds of 1/250 and higher, this is the only speed control.

When the first-curtain cam hits the release lever, the second-curtain cam is free to turn, allowing the curtain to travel. At speeds up to 1/100 second, at some point the second curtain cam will strike the lever from the escapement - this will slow the travel of the second curtain momentarily while the cam pushes the lever out of the way; then the second curtain proceeds to close.

The IIa shutter is very different from the earlier Contax II: there are two separate sets of ribbons (one is actually a pair of cords), the curtains travel independently and travel speed is constant. It was not, however, a vertically running copy of the Leica design: the delay escapement is engaged at all speeds up through 1/100, there is no 'pin-in-hole' set of high speeds, and the controls operate through a set of cams and levers more akin to later SLR shutters than to an early postwar Leica.

The shutter's reliability problems, particularly high-end fade, derive from the complexity of the design. The elaborate train of perpendicular shafts and bevel gears, with attendant side forces on the shafts, results in unusually high friction in the bearings. The reliability of the slow speeds was probably not particularly worse than a contemporary Leica, but in the Leica this would only affect speeds below 1/30 second, while in the Contax pretty much every speed that wasn't fading was subject to the escapement hanging up.

The net result is to be left with two choices in using this very nice camera: Either use it regularly, or service it regularly.

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