"Mac" writes:

The image of our Amray SEM has a horizontal band across the center
third of the image which is slightly out of focus. The top and bottom
thirds are fine. This machine, while being fairly old, is new to our
institution and I believe some of the maintenance practices have
lapsed. I was wondering if it was possible that a weak filament or
contaminated optical column or dirty apertures could cause a problem
like this.

Is it affected by settings like magnification? Have you ruled out stupid
stuff like the video monitor itself? Is the change in focus sudden
(with respect to vertical location) or gradual? Have you looked at any
of the signals that control focus?

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.