RB67 90mm NB Lens: Delayed Shutter Firing
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
This Mamiya RB67 90mm lens came into the shop with the very common issue of a delay firing of the shutter after being released. This normally entails disassembly of the lens to access the shutter and then removing the cocking ring, main cam, main spring and slow speeds escapement as a minimum. All of these parts are cleaned and then carefully lubricated (except the slow speeds escapement which runs dry) with a moly paste.
This example is the 90mm NB which was briefly produced between the original series and the C series. The name ring is where you start disassembly from the front and in this case, the name ring would not budge, even with the correct soft rubber tool. And, with this lens build, the filter ring turns but can not be removed until the name ring is removed. So, my secret weapon in this case was the so called “Liquid Teflon” dry film lubricant which I put around the name ring and let it soak in and dry. Then, using the 80mm flexiclamp wrench (the largest that I have) I was able to hold the filter ring while I removed the name ring with the rubber tool. I really recommend the Liquid Teflon for stuck threads.
After cleaning/lubricating all the necessary parts, the shutter still had a lag. After removing the main cam (shown in photo five) I noticed that the edge where the cam latches was worn (shown in photo six). This was just enough to cause a problem with the shutter releasing. Also, not related to shutter release but important for shutter speed control, I replaced the main drive spring. Photo seven shows the new drive spring on the left with the old compressed drive spring on the right. With the old spring, top speed of 1/400 would be around 1/150. And while there is a big tolerance for the 1/400 speed, 1/150 was a bit low (1/400 on this 1 Seiko has a 40% tolerance so 1/250 would be the bottom end.
