Slide projector lens focusing problem

I have a carousel 4200 projector with an Ecktagraph 100-150mm zoom
ff3.5 lens. The only way to get a slide in focus is to hold the lens
abount 3-4 inches away from the projector, so the light goes thru it
onto the wall. When I put the lens in the projecotr, it doesn't focus
at all (I've tried from distance of 10 to 20 feet away from the wall).
I get no image other than different color lights until I take the lens
out of the projector - and then I get a clear picture. What could
cause this? Before I spend money on a new lens (shorter and wide
angle) or take the projector in to be fixed, I could use any
suggestions. Thanks for your help!

 

This is basically a guess but a fairly educated one. The lens might be
designed for a long throw - 100 and more feet from the projector. By moving
it further from the slide you are essentially forcing it to focus closer
than it was designed to focus.
Whether it was designed to do this or something happened to it (for
example it might have been assembled wrong) I can't think of anything simple
that would fix your problem.

 

I had the same lens have that problem
and an element in it had fallen out some
how. I had to replace it but took it apart
and used the back half as a nice loupe
for my lightbox. Try another lens and
see what happens.

 

I took the lens to a camera store and they told me it was probably
broken, too. I just got another lens - a 5 inch and it works great!
Thanks for your help.