Over the past month, Ken and I have been troubleshooting the Cambridge Stereoscan 200 scanning electron microscope that was donated to the lab. The machine was sitting in a barn for a while and was quite dirty when we received it. After vacuuming out dust and rodent debris we noticed that the power distribution chassis was behaving very erratically. Several rotten capacitors were replaced and some connector terminals required attention due to corrosion from exposure to uric excretia. Replacement or refurbishment of those parts solved the chassis's stability problems and allowed us to examine other parts of the microscope.
The embedded display screen on the control desk unfortunately does not power on yet, but a BNC jack located at the rear of the desk is able to output a video feed to an external television. A primitive computer located in the control desk adds an overlay to the video feed which displays microscope settings such as magnification and working distance, permits simple linear distance measurement, and allows the user to input annotations for recorded micrographs. The computer is constructed from several 7400 series integrated circuits, some of which contained corroded pins and sockets which were replaced. Now that the computer appears to be fully functional, it will be helpful when making sure the rest of the controls are functioning properly -- for instance, right now we can tell that the spot size selector switch needs to be cleaned because the computer doesn’t react properly to some selections.
While we have made some significant progress in our repairs and understanding of how the microscope electronics work, we have not given much attention to the vacuum mechanism. The primary vacuum pump is a turbomolecular pump and there is a rotary vane pump backing it. Both pumps need at least an oil change before they can be earnestly tried out because they were left unused for an extended period of time without being properly prepared for storage. We will also likely need to disassemble the rotary pump to get an assessment of its condition since it did not spin during a cursory test. None of the high voltage supplies (for the electron beam filament, display CRT, etc.) will activate before a reasonable vacuum is achieved. There are also some minor electrical problems remaining that are observable in the current state. In addition to cleaning some of the front panel controls, we should evaluate the electron optics and deflection circuits and be sure that they look okay.
We’ve had a blast working on the microscope so far and look forward to the day that we achieve beam!