NOTE: Please disregard this post. After reading RichC's responses below, I'm convinced by his point of view and no longer think that failure to reseal the adjusting screws could be a cause of backlash. (I still think it's a good idea to reseal the screws, but only to keep them from vibrating out of position.) Originally I was going to delete this post, but decided to leave it in case anyone still had use for the illustrations and descriptions.
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QUOTE=jim_buchanan]jlw, after this last operation, any thoughts on rangefinder backlash improvement?[/QUOTE]
I do have some thoughts, but please understand that they are very
speculative! I'm going to list some guesses at things that
might cause RF backlash, based on my casual look at the mechanism, but I don't
know that they cause backlash. So, see what you think, but don't necessarily take any action based on my speculations!
For those coming late to this discussion, "lash" in general is the clearance between moving parts in a mechanism. For example, I have an old 1970s sports car, and one of the things I have to do every spring is set the "valve lash" -- the amount of gap between the top of the valve and the rocker arm that opens it. Too much lash makes the mechanism sloppy, and not enough binds it up so it can't operate freely.
"Backlash," specifically, is the lash that occurs when a mechanism reverses its direction, and this is what people worry about on the R-D 1 (and other RF cameras, for that matter.) The way it manifests itself is this: You're trying to focus on something, so you turn the focusing ring in one direction (say, from infinity toward close) watching the RF-patch image move smoothly as you do so. Then, you overshoot the focus point a bit, so you turn the focusing ring the opposite way. Instead of immediately reversing direction, the RF patch image stops moving briefly; then, after you've turned the ring enough to "take up the lash," it starts moving in the opposite direction as it should. You can see that if there's enough backlash, you'll find yourself in a situation in which the rangefinder focuses accurately when you turn the focusing ring in ONE direction (say, from infinity to close) but not when you turn it in the other direction (from close to infinity.)
A certain amount of backlash isn't uncommon in RF cameras, and it's probably almost impossible to eliminate it completely. How it got into our collection of R-D 1 neuroses, if I recall correctly, is that when people started sending R-D 1s to technician Don Goldberg for rangefinder adjustments, he noted that a certain fraction of them seemed to have enough backlash to affect accuracy.
So, now you know what backlash is and why I was asked if I saw any obvious causes and cures when looking at the R-D 1's RF mechanism. Again, remember that all this is
speculative, but I'm going to group my suspicions into three categories:
1 - CAUSES OF BACKLASH THAT HAVE NOTHING TO DO WITH THE RF
This mostly would involve lenses; some lenses have a certain amount of lash in the focus-cam mechanism, so if you've got a backlash problem, you can't rule this out as a possible cause. Obviously, if that's where the problem lies, you can adjust your R-D 1 until you're blue in the face without resolving it.
2 - CAUSES THAT INVOLVE THE RF, BUT YOU CAN'T DO ANYTHING ABOUT THEM
I can think of one area that seems as if it could be a
possible cause of backlash; again, remember that I didn't actually
observe any in this specific area. But when trying to turn the close-range adjustment screw (seen in the following picture from Rich C's website) I noticed that the lever on which the screw rides flexed noticeably when I pressed down on the screw. (It helped to support the lever from underneath with the barrel of a screwdriver.)
The lever connected to this screw is one of several long levers that connect the coupling arm in the lens throat to the RF module. They're underneath the module and mostly invisible. Again, note that I
did not observe any flex in these levers along the direction they move as you focus, so there is no reason to assume they ARE an inherent cause of backlash. But I could easily imagine that if one of these levers
did flex or twist during focusing, it would cause backlash. And if it did, there basically wouldn't be anything you could do about it.
3 - BACKLASH YOU MIGHT BE ABLE TO TREAT
This one is not only speculative, but potentially controversial, because I'm about to contradict something on Rich C's website, and he knows a lot more about this stuff than I do. But to understand my speculations, first look at the following picture of the adjusting mechanism for the RF module:
In this picture you see the now-famous screws for adjusting the RF image focus, infinity position, and vertical alignment. They work by changing the position of the swinging lens that moves the RF image. To make this a bit clearer, I drew a not-very-accurate schematic of the swinging lens and its adjustments:
What I want you to notice is how the RF-focus and infinity screws (the two left ones) work. They are NOT micrometer-type adjustments that work by moving something along their threads. Instead, they are
eccentrics -- the screw head turns an off-center portion of the shaft, which rides in a slot in the adjustment plate. As the off-center portion turns, it presses against the slot and causes the plate to move from side to side.
When turning these screws in my R-D 1, I noticed that there was a small amount of free movement before the plate started moving. Obviously, this meant that there was a bit of clearance between the slot and the eccentric section; there'd almost have to be, or the mechanism would have a lot of friction and would wear more quickly.
So, I could at least
imagine the possibility that when you reversed focusing direction, there'd be some wasted motion in the adjustment plate until it "took up the slack" in the slots --
unless the screws were sealed to the plate.
In other words -- and here comes the controversial part -- I suspect that
the sealer used on the RF adjusting screws is necessary to reduce backlash.
Now, Rich says on his site that the screws have enough friction that they don't require sealer, and for all I know he may be right. As I said, this is speculation.
But my speculation is that if you've removed the sealer from your R-D 1's adjusting screws with solvent, or if you've loosened its grip through too many attempts at adjustment, this
MIGHT (again, only "might") be a
potential cause of backlash. Without any sealer, reversing the focusing direction would cause the adjustment plate to shift around to the extent of the clearance in the screw slots, until they pressed up against the eccentric screws from the opposite direction.
I just want to emphasize one more time that I am only GUESSING, that Rich apparently thinks differently, and he knows a lot more about camera innards than I do! In other words, I wouldn't recommend delving into your R-D 1 just to re-seal the sealer.
And I also want to emphasize that the RF almost undoubtedly was designed to reduce the risk of this. I didn't take the R-D 1 apart far enough to see, but on other cameras I've examined, the design precludes the possibility of the adjustments producing focusing lash. Typically the whole moving portion -- the adjustment plate, the eccentric screws, and the moving lens frame -- sits on a movable base, and it's this base that turns the moving lens in response to the action of the focus coupler in the camera throat. The adjustment screws move the adjustment plate only relative to the movable base plate -- there's no drag on them during focusing, so there's no reason they should be subject to backlash.
Again, that's the way it
should be, and I'm pretty sure based on what little I could see of the R-D 1's insides that its rangefinder is designed to be this way. But I could also imagine that extra drag on the moving lens (not sure where it would come from, but it's a possibility)
might get transferred to the adjustment plate and cause it to wiggle enough to produce backlash.
At any rate, after I did mine, I did re-seal the screws (using automotive "thread locker") and if you think my speculations are plausible, you might want to do the same if you have to do a top-off RF adjustment for some other reason. After all, Cosina did put sealer on the screws in the first place, so it's not as if you're doing anything "foreign" to the camera by re-sealing them.