[EXPERT TIPS][MR BOB]
Scheimpfluge on RPTVs
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"Focus is off from one side to the other - I can get it right on the left 2/3 of
the screen but the right side still blooms, or on the right 2/3 of the screen but the left
side still blooms. I can't get it to correctly focus all over. And where it decides
to focus CORRECTLY changes with the turning of the lens barrel. What to do?"
What you are talking about is Scheimpfluge, or the angle at which the CRT hits the
lens, then how the lens hits the screen. After all, in an RPTV, those angles will NEVER be
parallel. So the lens is angled to compensate, and not always correctly, when they set
everything in stone inside your RPTV for mass production, including the angles of the lens
mounts. The big ceiling projectors have highly precision spring loaded adjustment setting
screws for this purpose - the purpose of highly precision changing of these lens angles.
Using washers under the mounts where the screws hold your your RPTV lenses in place can
make it all symmetrical, tho you may still have problems with center vs. side equality. Be
sure to tape these washers in before securing the lens on top of them, so they don't fall
out later and into the works down below, when it is time down the line for coolant cover
cleaning, under the lenses.
I just had to do that on a Pioneer 510 yesterday - the blue had exactly the same
differing focus response, left to right. Putting in 1 washer each under the mounts where
the screws are, at the 2 nearest to center screws holding the blue lens in, did the trick,
by angling the blue lens OUT just a little bit more than it was designed to do. But then
it was obviously designed incorrectly, which is not uncommon in RPTV production units.
You may have to recenter your image just a bit after this op, of course, but just
changing the angle involved with shims does not really impact the direction of the image
beam much at all, does not really tow it off its mark at all. The difference it makes in
getting the focus CORRECT, on the other hand, is phenomenal.
In my case, it still wasn't perfect, center to sides, but now the error at each side
was identical to the other side's error. The Mit lenses have much better center to side
design than the Elites do, and I think this is because of the slight dip they have in the
center of the curvature of each lens.
And it may be just the opposite on your set, as to where exactly to put the washers.
The best way to accomplish knowing exactly where to place your shims is to defocus the
optical so far that the grid lines are a half-inch to an inch thick, at your screen. That
will tell you much more graphically what to do. On one side of your screen they will be
wider/thicker than on the other side, when there is Scheimpfluge error.
Then when these thick lines are balanced, left to right, refocus for real and you're
balanced, left to right, like it should be.