This is a LONG OVERDUE hit on my question about 3M Overhead going dark.
My original question:
I have a 3M overhead that is going dark around the edges. It doesn't
seem that the dust is enough to be causing this dimming. The fresnel lens
seems a bit concave but as I remember - it's supposed to be that
way. Anyone know what I might look into next?
There seem to be a lot of possibilities for the culprit. I'm embarrassed
to admit I haven't done the trouble-shooting yet to see who was "right!"
Basically, it could be
1. the fresnel lens upside down or warped or improperly cleaned
2. the wrong bulb or mis-seated
3. too close to the screen
4. a variety of other things as described below in the verbatim responses.
Guess it's time for me to bone-up on my Overhead Projector repair and
maintenance. Anyone joining my club?
Thanks everyone,
Marcia Dressel, Librarian K-5
Verbatim responses:
have you looked at the reflecting mirror and head? There might be a dust or
dirt build up there.
**************
Is the OHP old? It may be that the lens needs replacement.
I found a place over here that repairs and services overheads. Just a
one-man company really, but he was wonderful at fixing all the odd problems!
I'm sure you'd find someone in your part of the world who provides a similar
service.
We had a strange problem last year. Brand new OHP. The children removed
the small mirror in the angled fitting at the top. When the teacher
switched it on, the heat generated by the concave lenses, etc, burned
through the plastic. Only took a minute or two.
***************
It is my understanding that the Fresnel lens on the overhead projectors
have to be flat to work, they cannot be concave. The amplification is
caused by the cuts and ridges on it. I do not remember seeing a Fresnel
lens that is "slightly concave." They are also supposed to correct the path
of light. If the fresnel is bent or "concave," it is sending light to
different places.
Of course, I may be mistaken. You can test it by using a different model, I
am sure any school may have two or three models lying around. If you find
other models in a classroom and their fresnels are "slightly concave," then
that lens is probably not the problem. If those are not concave and project
clearly, then you have the culprit.
***************
You may have the fresnel upside down. I had a teacher who cleaned one and
then put it in upside down, and the result was that the field of view got a
whole lot smaller with a dark edge.
My A-V repairman also told me that if the fresnel is cleaned with anything
but water it can be damaged. He said that you shouldn't even use Windex or
alcohol wipes.
*******************
On some models of overheads if you get too close to the screen the
edges may turn darker. This was usually in older Bell and Howel
models.
******************
The fresnel lens shouldn't be concave. If it gets too much heat it may
warp, which can explain a darker edge.
********************
If you try to adjust the focus the whole image should be fine. If the
center is in focus and the edges aren't (or the edge in focus and the
center isn't), then your fresnel lens may be warped.
******************
If the thermostat on the overhead goes bad, the plastic fresnel lens
gets too hot. You would need to have the overhead repaired.
*******************
If you have done maintenance and cleaning of the overhead lately, it is
also possible that the fresnel lens got put back in upside down.
******************
On some older overheads, there was a metal housing with the lens that
drops down over the bulb. If the bulb was replaced and the metal
housing not slid back in place then you can also get a dark edge. I
don't recall our 3M models having this - some of our older Bell & Howel
(& Eiki) models did.
*********************
Is the bulb cover closed completely?
********************
I've had the same problem. I think it has something to do with the angle
of the light being reflected from below. I remember I had the guy come
work on it and it was a simple thing like realigning the reflector or the
glass & mirror
*********************
We had that, and it turned out that the lamp switcher was not switching
totally--and only part of the lamp ws actually shining. We had the 3M
9110 or something like that. If yours has the dual lamp housing, you
might want to check that!
************************
Not sure if you have the same type as our school, but as I recall, there
is a slide-type button that sometimes gets inadvertently slid over a bit
and this causes a darkening around the edges. If button is in its correct
position, you should have full screen illumination.
******************************
Is the lens inserted properly? Try turning it over. Also, have you
been cleaning it with any cleaning products? According to my assistant,
you're supposed to us only a clean, soft cloth.
Marcia Dressel, Librarian
Osceola Elementary and Intermediate Schools
250 Tenth Ave E 949 Education Ave
Osceola WI 54020
dressel@osceola.k12.wi.us
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