Mamiya 6: Wrong Shutter Speeds from Spring Lodged in Speed Selector
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
This Mamiya 6 arrived with a common issue, a broken viewfinder window. But, it also had a problem that I have never seen before, which was various shutter speeds (but not all) being incorrect. Here is a breakdown:
1/500 selected actually delivered 4 seconds
1/250 selected actually delivered 2 seconds
1/125 selected actually delivered 1 second
1/30 selected actually delivered 1/4 second
1/15 selected actually delivered 1/8 second
Quick explanation to explain this: while some cameras use variable resistance for inputing shutter speeds, the Mamiya 6 uses a four bit Grey code. And, all of these shutter speeds have one thing in common in that they should have a logical zero as the final bit. But, from measurement on the circuit board all of these speeds had a logical one (1.2V) as the final bit and this was changing the input to provide incorrect speed input to the processor.
Some disassembly of the speed control unit showed something very surprising: a camera part (flat spring, shown near were it belongs in photo six) stuck inside the speed control ring. Photo four shows the view that I was greeted with and you can see the flat spring stuck in the plate through the transparant plastic cover. This was shorting 1.2 volts to the G3 bit and giving incorrect speed input. I have attached the Grey code chart for the shutter speeds in photo five and it is obvious that all the slower speeds that have a final bit of zero will give an incorrect speed when forced high. The speeds slower than 1/15 would not be affected since they all have a final bit of one already.
I doubt that this info will be useful in the future as the odds of this happening again are probably very slim. I don’t think I would be able to force this part into the speeds selector without disassembly of the parts to recreate this.
