Reviving a vintage Transmission Electron Microscope - The Philips EM 420 Part 2, a short description of TEM hardware and electronics.

Figure 1. A simplified, cross sectional view of a TEM column as well as high tension tank , electronics and vacuum system.

Before I get carried away on the details of the inner workings of a transmission electron microscope, I should deliver a description of its basic sections and the reason why they are there. This will be brief, because I will go into more detail later.

1. Optics
2. Vacuum System
3. High Voltage System
4. Electronics
5. Cooling system

Optics (red, black and green)

The lenses in any electron microscope are electromagnetic. Magnetic fields can be shaped to direct electrons down a path essentially the same way glass, quartz or clear plastics do in a light microscope. The lenses (red) are made of copper wire wound around an iron core, with the lens coils essentially being shaped like donuts. Also included are apertures (black), some are fixed, and others are attached to mechanisms (green) for alignment and easy removal for replacement or cleaning.

Vacuum system (grey)

All transmission and scanning electron microscopes must have a vacuum system. Molecules and atoms in the air will obstruct and deflect the electrons as they fly thru through the scope near the speed of light. The EM420 like many scopes made during the time has a 3 stage pump system, each stage removes more atmosphere from the electron beam path than the previous. In this case the pumps being used are a rotary vain pump for low vacuum, oil diffusion for high vacuum and an ion getter pump for ultra high vacuum. Also included are a mess of valves (diamonds) involved in bringing the vacuum up to par sequentially, as well as isolating sections in cases of mechanical failures, high voltage discharge, troubleshooting, maintenance, specimen exchange in and out of the airlock among many other reasons. Then of course the various gauges for detecting vacuum levels (green hexagons).

The High Voltage system (purple)

This is what provides and emits the acceleration voltage, and is composed of a high tension tank (kV source), cable and the gun. The electron beam originates near the top in the gun, and travels down a narrow tube through the lenses, specimen, more lenses then creates an image on a phosphor screen at the bottom. Many TEMS made from 80s on came in varieties than can reach maximum high voltages of 80 to 300kV, depending on the model. There were some that went to 400kV or more, but they are less common, and I personally have never worked on one. Except...one time, I disassembled a massive JEOL 4000EX, that could easily fill a large living room with vaulted ceilings. It could reach 400kV. The interesting thing about electrons is the higher the acceleration voltage, the shorter their wavelength gets, which can improve resolution. There are other things involved that can limit resolving power tho, and those I plan to address in a later post.

Electronics

The electronics system is the whole mess of stuff that supplies different power needs for each system and its components, TEM control in general as well as the user interface. For example, each lens has its own regulators and controls. The microscope also has safety circuits that protect the operator from electrocution as well as circuits to protect the device itself from power outages, cooling system malfunction, operator error and more. Newer scopes than this, have computers the control most of everything, however old school dedicated logic chips can still be still integrated into the system that can make super fast decisions, for safety reasons.

Cooling system (not shown).

The cooling system circulates water or refrigerant through the lenses, oil diffusion pump, and all supplies, and regulators that require it. It is obviously a very important component, but I have seen many neglected filthy chillers that were causing a plethora or problems. Most scopes I have worked just have distilled water circulating thru the system, however some use a substance I believe to be just ethylene glycol. Without a good chiller, your scope can go into thermal overload, and you do not want that. Walking into a lab with a smoking lens is not something anyone wants to experience!

Lastly,

The orange circle in the diagram represents the specimen airlock and goniometer, which is for manipulating the specimen. The viewing chamber window and phosphor screen I colored yellow.

Next part, I will discuss the vacuum gauges and even tear some apart so yall can see their insides. They are the next thing I need to attend to on the 420. I am actually almost done with writing that section, so it will be up...soonish.

Reference:

Agar, A.W. Principles and Practice of Electron Microscope Operation. 8th Edition. North-Holland. 1991