Praktica "B"Main modules: the body with the cocking and film-advance mechanism, the shutter which comes to be fixed inside, the mirror-pentaprism block which comes in front. Miss the electronics which comes to equip all that (including the selectors for shutter speed and film speed), and the selftimer mechanism, which is set by 2 screws on the right of mirror-block, in front of the shutter mechanism. The frontage of the body comes to equip all that, with the lens mount, then come the covers and sleeving.
BX20Only two main modules remain; the hull is very close to that of “B”, but the exposure-window is more on the right.The shutter is fixed behind the mirror-pentaprism block, which directly carries the support of the lens mount, and electronics covers all that, no more wire to connect when this block joins the body (what also misses in the top image is the forest of electric wires!)
The covering of BX20 body is made with moulded plastic, it directly covers the assembly. The lens mount holds the frontal part of the caps.The advantage is that the BX20s has a different look, only by modifying the moulding of the covers.
On the right of the mirror-block of the "B", you can see the photodiode; the mirror is semi-transparent (you can guess the semi-transparent zone in rhombus shape) and the light is returned on the side towards the sensor. A mask (interdependent of the mirror support) covers the sensor during the exposure, when the mirror is raised.For the BX20, they returned to a very traditional system, with a sensor fixed behind the pentaprism, over the eyesight (plus another under the mirror, looking towards the film, for the real-time exposure with the flash.In short, even if the BX20 is more recent, its predecessor was more innovating and clever.
You perhaps noticed that my “skeletons” still have their belt eyelets: they are crimped and not screwed, they cannot be re-used on a Zenit body!Small easy way: the switch that you can guess on the left of the battery-housing is not highly reliable and the "B" series tend to empty their batteries when switched off. To avoid that, set the shutter speed setting knob to "B" when not using the camera, this switch is then out of the circuit and the batteries have a normal lifespan!