SAFE servicing is desirable in all troubleshooting operations, not merely in the localization of tube troubles. It is relevant be cause accidents delay servicing and tend to increase the expenses associated with it. A man who spends a month in the hospital after an explosive session with a picture tube can hardly be said to have repaired a tube trouble quickly.
Delay and inefficiency may be associated with shocks and burns as well as implosions. The author has known technicians to stop working after a severe shock or to work at considerably reduced ability due to the emotional upset they have gone through. Safety is thus definitely linked with faster tube troubleshooting, just as it is with the troubleshooting of any other component.
Safety considerations with respect to picture tubes
There are three dangers associated with cathode-ray tubes that merit consideration. These are implosion, shock and X-rays.
Implosion of a cathode-ray tube can cause severe injury or death.
Shock from the high voltage at the anode cap of a glass tube or the cone section of a metal type is generally more unpleasant than dangerous; secondary effects may, however, be fairly serious for some individuals. Shock suffered during the handling of a C-R tube may result in the dropping of the tube, followed by an implosion. All the more reason, then, for reducing the chances of getting shocked.
How great is implosion risk?
The occurrence of implosions without apparent cause has made some technicians a bit apprehensive while working near picture tubes. However, it's safer to handle a cathode-ray tube-provided proper precautions are observed-than it is to cross a city street.
A rectangular tube is more likely to implode than a round one.
The glass thickness in rectangular tubes is greater at the corners of the screen than in the center (the corners represent areas of poor pressure distribution). As a result, heat and cold produce un even expansions and contractions at different parts of the screen, weakening the tube's resistance to stresses. Greater care in the handling of rectangular C-R tubes is therefore necessary.
Metal tubes offer considerably less implosion hazards than all glass types.
How implosions occur
Since the picture tube is highly evacuated and its surface is large, the external atmospheric pressure brought to bear on it is very great. The resultant stresses set up are maximum at the tube's front rim. A blow, bump or even a scratch can raise the stress to the breaking point, causing first an inward movement or cave-in of the glass, called an implosion, followed by an out ward movement or explosion of the broken glass sections.
Many service technicians handle picture tubes with a gross lack of respect. The author has seen technicians unimaginatively manipulating screwdrivers and other tools near picture-tube screens in such a way as inevitably to hit them if the tool slipped by accident. Other service technicians place tubes face down, on workbenches littered with little sharp metal particles, and violate elementary rules of safety in numerous other ways. Quite often nothing happens-C-R tubes hold up under substantial amounts of abuse; sometimes a bad accident, however, punishes the technician for his negligence.
It is important to realize that glass has a substantial degree of strength until its skin tension is broken. A skin fracture, even one that is microscopically small, may severely weaken the opposition offered by the glass to the heavy pressure of the atmosphere. In one case known to the author, an implosion resulted from the accidental brushing of a technician's ring against the tube's front surface. Even a relatively mild bump of one tube against another or of a tube against a metal surface can lay the groundwork for a subsequent implosion.
How to avoid causing an implosion
Handle a picture tube gently while replacing it. Don't let it strike the chassis or metal units on the chassis. Get someone to help you carry the tube if it is a large one.
Don't set a cathode-ray tube down in any area where people can conceivably stumble over it or where other kinds of undesired collisions can occur. Be particularly careful not to let one tube bump into another; two implosions are worse than one.
Never set a picture tube face down on any surface other than soft cloth. Be particularly careful not to put the tube down on a concrete floor-its surface will inevitably be scratched if you do, weakening the glass.
Never let a picture tube act as a support for the yoke or focus coil. Another crime against safety is to let the tube support the chassis when the latter is turned over for servicing.
Watch out for gritty particles under ion magnets; they can cause scratches on the tube neck. Wipe them off before moving the ion magnet back and forth along the tube neck. If the yoke and focus coil do not slide smoothly, dust a little talcum powder between the contacting surfaces, to reduce friction.
Make sure that protective rubber or felt strips (see Fig. 101) are reinserted between the metal support band and the tube when the support band is replaced after having been disassembled. These protective strips prevent possible breakage of the tube due to excessive tightening of the band. When they are out of position, put them back into place.
Don't lift or support a tube by its neck. Most of the tube's weight is concentrated at its broad or bulb end. The tube should therefore be supported largely at that end when it is being lifted; very little pressure should be placed on the neck.
Before handling a (disconnected) glass tube that has an Aqua dag coating, discharge it by touching a grounded wire to its anode receptacle. More than one discharge or a prolonged short may be required to discharge the picture tube completely. Glass tubes that have been separated from their chassis can retain a charge for 24 hours, possibly longer. The author has, on more than one occasion, seen technicians jump noticeably due to the unexpected shock experienced when an un-discharged C-R tube was picked up from its resting place on the floor. Breakage of the tube is more than a remote possibility in cases of this sort.
Avoid using tools in any way that make striking the tube possible. When some tool must be used in such a manner, place one hand between the tool and the tube to prevent the tube from being struck if the tool should slip. (The result of an implosion is shown in Fig. 102.) ..,
METAL SUPPORT BAND RUBBER OR FELT STRIP
Fig. 101. Position of rubber or felt strip under metal band holding cathode-ray
tube in place.
Don't use ordinary C-R tubes for display purposes. Use one that is no longer a vacuum type. Distributors can supply such tubes or the dealer himself can let air into a tube, using the technique described later in this section.
Some older types, such as 17CP4's, tend to develop visible cracks. Several inches long, they extend horizontally along the tube and generally appear near its lower surface. The cause is faulty construction, not improper handling. Cracked tubes should be replaced to avoid possible trouble.
When a picture tube is to be removed, follow the procedure recommended in the set manufacturer's service notes (unless you are so familiar with the mechanics of the job that you can dispense with such guidance). Cautions for minimizing implosion injuries While the risk of an implosion is remote under ordinary circumstances, precautions should nevertheless be taken to minimize injuries in case of an accident.
Fig. 102. Result of a cathode-ray implosion due to inadequate safety precautions.
If you are working in a customer's home, keep other people at a safe distance from the tube when it has been exposed and is being handled. For maximum safety, send them into another room.
Wear goggles when handling a C-R tube. This is a safety measure widely neglected by technicians. It is, however, uniformly recommended by all TV set manufacturers. The eyes are very fragile, highly important parts of the body-protect them. Some manufacturers recommend the wearing of goggles, not only when tubes are being handled, but when any top chassis work is being done. The wearing of gloves is also recommended to prevent scratching the tube with rings and also to protect the hands from possible injury. Don't leave old-or new-tubes lying around the shop. Put them into suitable cartons and store in an out-of-the way place.
Fig. 103. Deactivating a picture tube. Drilling a hole in the key way of
the tube exposes the exhaust tip, which can then be broken.
Disposing of unwanted tubes
If a defective tube is not wanted, don't simply throw it into a garbage can-a subsequent implosion is possible. Neither the garbage collector nor others who may handle the can are likely to treat the tube with the care necessary to prevent an accident.
Bear in mind that you or your employer can be held legally responsible for injuries suffered by other people in the event of an implosion resulting from improper disposal of a C-R tube.
To make a discarded tube safe:
1. Put the tube you want to dispose of into a carton designed to hold a unit of its size. Enough soft cloth should be placed under the face of the tube to make its base stick out at the top, outside the fold-over flaps of the carton (Fig. 103).
2. Drill a 1/4-inch hole in the center of the keyway or locating lug.
3. Now insert a small file or similar tool into the hole and break off the exhaust tip at which the tube is sealed. Shatter only the point if the tube is to be returned for credit; if a larger area is broken, the too-rapid intake of air will blow off the screen coating and reduce the tube's trade-in value. When a metal exhaust tip is present, a small three-cornered file or similar tool can be used to create the small hole needed.
4. To insure that the tube is perfectly safe, break the tip completely later on if you haven't done so right at the outset. The seal may be recessed below the base,, hence should be inspected to see if it is broken.
Instead of going through the procedure just outlined, some technicians simply break off the keyway at the tube base with a pair of cutters or pliers. The exhaust tip is automatically shattered during this process. After the tube has had its vacuum filled with air, it can be placed into a sealed carton and thrown out or returned for credit on a trade-in.
X-ray radiation Picture tubes tend to produce significant X-ray radiation at anode voltages in excess of 16 kv. Since tubes 21 inches or larger in diameter operate at voltages of 16,000 or more, the X-ray radiation produced is worth considering. It does not have sufficient intensity to penetrate the protective faceplate and is there fore not a danger to the set owner. With the protective glass absent, however, some danger of excessive exposure exists for the technician working on the receiver. Most X-rays are given off toward the rear of the tube and come out that way since the glass is thinner there. Keeping the chassis between yourself and the tube is a good rule. How much danger is actually present is not yet certain; it is most likely very small. Nevertheless, when chassis using picture tubes with second anode potentials in excess of 16 kilovolts are worked on, it would seem wise to avoid long or repeated periods of exposure while the tube is in operation, and to keep out of the path of the beam as much as possible.
Don't look at or stand in front of the tube face any longer than you can possibly help.
Make sure that the high-voltage cage is in place since a high voltage rectifier tube operated at potentials above 16 kv will also give off X-rays.
Avoiding TV shock
A certain number of electrical shocks are probably unavoidable in servicing a TV set: they can, however, be kept to a minimum.
The first precaution to take when a C-R tube is to be substituted in a TV set is to check whether the grounding clip or strap (if the tube is a glass type) is making contact with the Aquadag coating (Fig. 104). If it isn't, the proper adjustment should be made to remedy the condition and insure that no residual charge is left on the tube when the power is turned off and tubes are interchanged. The receptacle or cup of a disconnected glass C-R tube should be discharged to ground before the tube is handled.
GROUND STRAP
Fig. 104. Sketch showing ground strap on picture tube. Hairpin type clips are also used to place the glass envelope at ground potential.
Avoid changing metal tubes with the power on. Shock is likely if this precaution is not observed. Discharge a metal type tube's anode connector (and receptacle) to ground before putting your fingers near the metal cone section. (Power is, of course, assumed to be off.) Watch out for shock hazards when handling or adjusting controls near metal tubes (preliminary to changing tubes or after tubes have been changed). A rapid movement of the hand is likely to result in brushing against the metal surface of the C-R tube, causing a shock.
When the set is on the bench, clear the area around the set.
The author still remembers an unexpected shock he got when a stray wire hanging from a shelf over his bench contacted a metal tube at one end and touched his ear with the other. The wire, incidentally, made contact with both tube and ear through its insulation.
LINE TERMINALS AT THE ELECTRIC OUTLET (A) (B) GOOD CONNECTION TO EARTH
Fig. 105. When a technician (drawing A) puts two fingers of one hand across
the line terminals, current flows through the fingers and hand only and does
not pass through vital organs. Also, the current is limited by the resistance
of the wiring in each branch of the line (RI and R2 represent' this resistance).
When the technician (drawing B) puts one hand between the "hot" side
of the line and a good external ground, current flows from one hand to the
other through his body. The current will be greater than in A because R2 is
short-circuited.
Don't attribute undeserved virtues to plastic sleeves covering the metal section of a metal type picture tube. An acquaintance of the author's who inadvertently touched such a plastic covering with one hand while contacting the chassis with the other got blisters that remained on his hand for two days. Interestingly enough, he felt no shock at the time of contact; neither did a skeptical friend who subjected himself to the same experience a short time later.
When working at the underside of a TV chassis (an unfortunate necessity in the case of some tube replacements), discharge the high voltage by shorting the high-voltage capacitor to ground if (power-off) tests or substitutions that involve the handling of components in or near the high-voltage section are to be made.
Don't get your fingers too close to high-voltage terminals on any set-they may draw an arc.
Radio shock hazards
The possibility of getting electrocuted while working on a radio must not be ignored. While the risk is not great, it does exist and is something which we must concern ourselves since it is present even during tube substitutions.
You can, while changing tubes in a little innocuous-looking ac-dc set, get a jolt considerably more dangerous than the high voltage present in a television receiver. The author will never forget the "kick" he got when his slightly damp hair came in contact with the metal shade of a lamp while his hand was resting on the chassis of an ac-dc set. The metal shade had shorted internally to the socket and was making contact with one side of the line; the chassis was connected to the other side of the line.
The resultant setup was a good one for an electronic murder.
Novices who see old-timers testing for line "juice" by putting their fingers across the ac line terminals should not confuse this with getting their body between one side of the line and a good ground. (See Fig. 105-a,-b. RI and R2 represent the resistance of the line.) The current-delivering capacity of a line generator is somewhat less in the first case than it is in the second, making an electric shock less dangerous; more important, putting two fingers (of the same hand) across the line limits the path of current to the hand, leaving vital organs unaffected. Actually, the current going through the fingers is greater than that going through the body, but is not as dangerous.
Here are some instances which illustrate the possibilities that lie in wait for the unwary technician:
A service technician working on an ac-dc set asked another technician, also servicing an ac-dc receiver, to pass him a hacksaw.
The sets were connected to opposite sides of the line, as they realized very shortly. When the men made contact with the hacksaw, the circuit was completed. Both shook like St. Vitus' dancers until one of the radios fell off the bench and put an end to the shocking business.
Another case involved a man who was cautious enough to wear crepe-rubber-soled shoes (to insulate him from ground). He was holding an ac-dc chassis when his wife came in out of the rain, walked up quietly behind him and kissed him on the back of the neck. The chassis was "hot" and the wife was well grounded because of her wet shoes. The result was a shock that raised the hair on the back of the technician's neck.
Many other cases could be cited, in some of which fire as well as shock occurred. Fatalities, while infrequent, do take place. A voltage as low as 40 at 60 cycles has caused death. Caution is therefore necessary. The hazards are chiefly associated with the servicing of ac-dc sets. Use of an isolation transformer between line and set will eliminate most of the risks.
Service technicians should keep in mind that shock from the same source may be merely unpleasant in one case, fatal in an other. Four factors are important in determining the degree of shock:
(1) nature of the voltage (ac or dc); (2) resistance of the skin; (3) pathway of the current through the body; (4) duration of the shock. (We are ignoring the amount of voltage as a factor, since a 110-volt source is assumed.)
Under the conditions present in the typical service shop, 110 volts ac is likely to be far more dangerous than 110 volts dc.
Dc voltage does not produce the strong contractions of the heart muscles that ac does.
The internal resistance of the body is relatively small. The skin, however, has a comparatively high resistance--about 100,000 ohms per square centimeter when dry. If it is thoroughly wet, the skin's resistance may drop to as low as 1/1,000 of its dry value.
Prolonged contact with an applied voltage causes the protection offered by the skin to be largely lost, due to the formation of blisters. With high voltage the protection given by skin resistance is lost because of puncture of the skin by a high-voltage arc.
The pathway of current through the body is vitally important.
If no vital organs are in the path of the current, the danger is small. When contact is made between two points on the same arm, no current passes through the body at all; when the path of the current is from head to foot, however, the danger is considerable, since the electron flow passes through the heart. The chances of a fatality vary directly with the duration of contact.
The shorter the contact, the less the risk.
Avoiding tube-caused burns
When a receiver has been in operation for some time, most of its tubes will become quite hot. High-current tubes (rectifiers and output tubes) become extremely hot. Caution should be used in withdrawing and replacing tubes to avoid getting a burn. Tentatively feeling a tube before withdrawing it is not always a burn preventive-an adjacent tube that has heated up greatly may bite the technician's hand as he maneuvers the cool tube.
To avoid trouble, remove tubes with the power off. Use a cloth, gloves or a tube puller to protect the hand when a hot tube, or a tube in the vicinity of a hot one, is to be withdrawn or inserted.
Safety procedures.
It is possible and necessary to increase safety during TV and radio servicing by following a few simple precautions: Make sure that any insulating materials provided by the manufacturer are not removed or damaged during servicing. Replace any worn or damaged insulation.
After the set has been installed in the cabinet, make a visual check, to see if there is anything visibly wrong; then check from chassis to external ground with an ac voltmeter. Reverse the line cord plug in the power outlet, and repeat the test. Make the volt age test with the power switch on and off. In either case, no ac voltage should be measured between chassis and ground.
Make sure that isolating networks, especially in antenna circuits where they are more likely to be disturbed are intact.
Sets should be placed away from grounded objects like radiators, bathtubs or pipes. They should also be situated well away from windows, where rain may drive in the back and temporarily short insulating boards.
Sets should be inspected immediately for damage, when they have been subjected to conditions likely to cause damage-the tip ping over of a portable TV stand, for example.
Don't overlook conventional but necessary safety inspections of the line cord; it may be frayed or exposed at some point. Never ignore a possible fire hazard. Check for, and replace, oversize fuses and warn against installations in poorly ventilated places.
The technician may also perform another service of consider able psychological value. On completing work on a receiver, he can give the owner a statement that the equipment has been tested and found free from shock hazard. This may be typed out on the bill. Such a statement will carry considerable reassurance to the owner and may even be of value if an accident should occur some time after the equipment has been serviced.