Why I made this conversion? Because even after cleaning the film counter mechanism of my "Iskra", the camera couldn't sense the beginning of the film correctly. Trying to adjust it didn't seem to work. So I started to think about bypassing that fragile film thickness sensor while retaining all the other features of the mechanism. What I came up with seems to work and isn't irreversible, as the "red window" conversion seems to be...
To make the conversion, the film counter needs to engage right after camera's back is closed, but the dial must start turning from a position "below zero". The position to what the dial resets is determined by part "A" in the first image. If the protrusion on it is shifted to the left, the counter dial resets to an earlier position. I calculated the approximate amount of the shift needed by the distance between the teeth of sprocketwheels under the dial, the distance between the two frame positions on the film and the length of the film leader from the "start" arrow to the first frame. The shift was ca 5 mm. I then made a new part and fixed it in place with a drop of superglue. I planned to position the aligning marks at the center of the film gate, but trial with an empty backing paper put these marks near the film aligning screws at the right side of film channel. So I painted them red.
Next thing to do now was to replace the spring "B" with a weaker one. The arm under what it lies is also responsible for pushing the film sensing axis against the film, but its strength prevents the film counting mechanism from engaging when there is only backing paper in film gate.
Last, I marked a new black dot on the film counter dial to mark the new start position.
I have tried the new mechanism with some dummy films, and no problems so far. I'll update this post if something unexpected happens...
The mechanism before conversion (and cleaning):
New spring at left. No, I didn't make it from safety pin, I just found something similar to the old one in my drawer:
After the conversion:
Addendum (march 27th): Here's an image of the first two rolls with a modified "Iskra":
Dear Tõnu, thank you for this great tutorial. May I ask you just a pleasure? Could you please upload a front view of your film counter? It will help me understating my camera. All the best.
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