CRT Repair: Risks and Safety

As we look to discuss CRT Repair Risks and Safety, it is helpful to have some descriptive anatomy. There is obviously the screen of the CRT but three other areas should be noted. In the photo below we have marked three important places:

• The Funnel - The area marked by the red segment
• The Anode Connection - The object marked by the red box
• The Neck - The area marked by the red arrow.

The hazards fall into four main categories.

1. Implosion
2. Weight
3. High Voltages
4. Toxic Materials

You have probably heard about the hazard of implosion with a CRT. It is real. A CRT is a large vacuum tube. It is evacuated to a near perfect vacuum, and consequently there is a great deal of force being resisted by the various parts.

The screen of a 27" CRT has about 800 pounds of force being resisted on its surface, resulting from atmospheric pressure. it is almost flat or is flat, and needs to be very thick to deal with this force. The curved portions of the CRT (like the neck and funnel) are better able to resist this pressure and can be made of thinner material. This is very significant in the case of the neck, because while it can deal with the atmosphere, it can't deal with the weight of the CRT. Do not handle a CRT by its neck. It will break and could implode. Do not subject a CRT to physical blows anywhere on its surface. If a CRT breaks, it can do so violently.

If you must disconnect a CRT, use care and pull straight back on the CRT connection board. Don't twist or jerk it off. A CRT can be depressurized (and destroyed as a working unit), by removing a small glass bubble at the end of the neck of the tube. In general this isn't something you should attempt and removing the connector located at this point carelessly, can break it and ruin the CRT.

Implosion was a greater hazard in the early days of the CRT, but since the 1950's , the implosion protection band has come into use. This is a strong metal strap that is installed around the CRT near the screen area of the tube to strengthen the screen. CRT's without this band will implode with amazing violence.

Weight constitutes a hazard because CRT devices are so heavy and awkward. You can easily hurt yourself moving them or drop them on yourself (see Implosion). The weight distribution is also awkward with most of the weight on the screen side (thick glass). Also the out of balance nature of the weight can make it do unanticipated things.

Moving an older large CRT television often required more than one person, especially as the available sizes grew. The largest manufactured was by Sony, with a 43" screen and weight topping 400 pounds. A 27" television could weigh between 60 and 90 pounds with some up to 100 pounds.

Interestingly, larger CRT monitors are having a mini renaissance for retrogaming setups. The larger monitors (as opposed to TVs) topped out at about 21" and weighed in the 50-60 pound range. All of these are much more weighty than an LCD or OLED monitor of equivalent size and very awkward. So watch your back!

CRTs are vacuum tubes. Really big vacuum tubes that need really big voltages to operate. A 27 inch color TV will use voltages in the 27kV range. (A good rule of thumb is 1+ kV operating voltage per inch of diagonal measurement.) And the available current, while not necessarily lethal every time, will make the experience of being the human discharge device very unpleasant. The structure of a CRT makes it like a giant high voltage capacitor (and it functions that way), so it is necessary to discharge it before any kind of work is done on it. The dangerous anode connector is generally located on the side or top of the funnel of the CRT (see picture above). More recent CRT devices will incorporate a discharge resistor in the flyback transformer, so letting them sit for a day before opening will greatly reduce the hazard. It is still important to discharge the CRT even after this wait. Once discharged the anode connector should be connected to the ground on the device to prevent charge buildup.

CRTs contain a variety of toxic materials. Old CRTs are far more of a problem in this regard than the newer CRTs. Beryllium and Cadmium salts were used in the early phosphors, and since the phosphors are not "glued down", if a tube lets go they can be blown about. The actual glass of most CRT's is leaded glass to cut down on the emission of x-rays from the tube. Other elements are now incorporated into the glass, but it should not be mixed in with normal recyclable glass, it is considered a hazardous waste.

• The most important safety device you have is your brain. Use it. Do not take chances, a few minutes of care will save months or years of remorse.
• Think before acting around a CRT until it has been discharged, and keep in mind the fragile nature of the neck and funnel.
• When working around a undischarged system use one hand only and keep the other in your pocket. It is worth assembling a discharge tool in advance and connecting it to the ground point before probing under the rubber cap located over the anode connection.
• A face shield is cheap insurance against mishap. It may seem like overkill, but it doesn't impede your ability to see the way glass shards can.
• The safest position for a CRT device that needs to be serviced is face down on a soft surface. This lessens the chance of tipping with the neck and funnel exposed to potential breakage. There likely has never been a shoe made that could break the front screen of a modern CRT.
• Do not treat a CRT as an exciting destruction project. It can go bad fairly quickly, especially older CRTs.

For a "contrarian view" try: https://lowendmac.com/2007/the-truth-abo...

For a view that points to the hazards, and advocates caution from hard experience, try How dangerous is it to work on a CRT display?

For a well balanced, conventional view of the hazards try this excerpt of a book from Peachpit Press