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3X4 RB Auto repair

 
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Joined: 28 Sep 2001
Posts: 3196
Location: Mid Peninsula, Ca.

PostPosted: Thu Sep 15, 2011 6:50 pm    Post subject: 3X4 RB Auto repair Reply with quote

I have completed my first curtain replacement in a Graflex camera, a 3x4 RB Auto.

The camera arrived in the usual dried out condition with a torn curtain.

The rubber coating of the original curtain is easily removed by soaking the curtain in lacquer thinner for 1 to 3 minutes then scraping with a wide wallboard knife kept at a 90 degree angle to the material on a solid flat surface.

The copper aperture stays are removed by lifting up the ends on the rubber coating side then gently pulling them off the curtain. Once off use a 1 inch (25mm) wide wood chisel and gently push the chisel into the gap where the curtain was. Clean all residue from the curtain out prior to reuse.

After making a curtain, installing it, testing with the back off, putting the back on and testing at tensions 1 through 5 it tore on the 1/8 aperture 1st time on tension 6. Careful evaluation showed the mirror shaft and image box with the curtain guide rollers to be out of square to the body. The top guide roller was too high to allow a curtain stay to pass under the back's rotation ring without touching due to a side block screw walking outward when the camera was assembled because of hard grain in the wood.

The image is of the repaired measurements. Forcing the guide rollers square resulted in the bottom image box metal to bow inward.

The mirror shaft holes in the body were misdrilled by .002 inch. The old saying " a good carpender is able to compensate for his mistakes" applies here as the camera is repairable and functions correctly despite the errors in manufacture.



Next the spring gave trouble. Disassembling the lower roller revealed the spring to have been over tensioned and may have broken at one point in time as coils were interlocked.



After detangling the spring it would pull out of the retaining slot on the 2nd to 4th time of winding the curtain to 1/8 on tension 6. I drilled a small hole in the thicker section of the cap, made a loop from a thicker spring then attached the spring end to the loop.

The spring has 1 turn of inital tension adjustment latitude when meeting Graflex requirements of Curtain must close and lock from O on tension 1, Curtain must wind to 1/8 aperture on tension 6.

Using the speed tester I described in the accessory help section tension 1 speeds are 2/3 to 1 stop fast. Tension 2 through 6 speeds are within 20% for speeds 1/100 second and slower, within 30% for speeds over 1/100 second.

I shot some portraits with it using a Graphic 23 Roll Film Holder and TMY2 (T Max 400) and some still lifes on HP5+ using the camera table speeds with no compensation other than bellows extension on close ups and all exposures were within 1/3 stop of perfect. (see the 3x4 RB Auto set after clicking on my www button at the bottom of my posts).

Now on to making a new curtain for the 3A that is square.
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