What to Look for in TV Fires

What to Look for in TV Fires

Part 1

Slotted openings in cover on the rear of television are provided to vent heat generated by tubes and transformer.But slots permit foreign material—pins, coins and even water from a flower vase—to fall into the enclosed intricate circuitry and cause shorts and possibly fires.

Photos by the author

Following World War II, the black and white television receiver arrived on the American scene. The first receivers were of the 10-inch variety, and in many cases were being produced by RCA, with the designation as the 630 chassis. In the early stages of the television receiver, other companies were using this chassis under license from RCA.

What made this chassis so unique? It was necessary, in the early stages of the television industry, to offer a receiver that would be able to deliver a clear picture and quality sound from the transmitter. At that time, the transmitters being used by the broadcasting companies were not as effective and powerful as the present installations. Also, the antennas for the receiver in the home were not as sophisticated as the present-day units. Again, the tube design was limited to certain characteristics, such as gain, efficiency, etc.

Design engineers had to take all of this into consideration when this chassis was designed. Because of this, a television receiver was created that was efficient and was designed to perform with a limited amount of service. The distribution of tubes limited the amount of heat that would be radiated throughout the set. The operating voltages were in the range of 300 to 400 volts for the receiving tubes, and the high voltage to supply the picture tube was in the area of 15,000 volts.

The replacement of tubes was the main service required on the early television receiver. This was followed by the cleaning of the tuner, adjustments, and in some cases, the replacement of resistors and capacitors that may have shorted out, causing a resistor or coil to burn. The earlier sets were all hand-wired, and the quality control was apparently very effective. On occasion, a flyback (high-voltage) transformer would short out and smoke. On one occasion the author had serviced a receiver that had good sound but no picture. When the trouble was located, it revealed that a mouse had nested under the high-voltage rectifier and was electrocuted. However, we never felt that it was necessary to install mousetraps in television receivers.

We noted that when a call was received that the set was smoking, and certain resistors were involved, in many cases the resistors may have acted as a fuse, although this was not the intent of the manufacturer. An example of this would be a 5000-ohm resistor burning from a shorted .05mfd capacitor. The capacitor would short out, the 1-watt carbon resistor would burn and there would be a slight odor. The service technician would repair it and solve the problem. Many of these services were performed in the home. This was not necessarily true of the printed board circuit, which will be discussed later in this article.

High-voltage transformer (32,000-volt quadrupler) was designed for color receiver.Damage to high-voltage rectifier tube, socket and coil was caused by a breakdown in the insulation surrounding these elements.

In the late 1950s and early 1960s, we started to see a change in the technology and development of the television receiver as we knew it. The manufacturer, because of cost factors and in some cases personnel problems, developed the multipurpose tube. This tube would consist of two or more (usually three) units in one glass envelope. This development allowed the manufacturer more leeway in smaller sets, reduced costs, and hopefully, more efficiency. This, of course, did create heat problems and cause the manufacturer to redesign some of the circuits and relocate certain components.

Tubes, circuits change

At the same time, the printed board circuit was making its entrance into the television receiver field, and with it came related problems. To the manufacturer, the printed board circuit was easier to assemble and the cost-saving factor was all important as it reduced the number of employees per receiver. The prefabricated circuits were to make an impact on the entire electronic industry.

However, to the service technician, new problems would arise that would have to be met. Our problems relating to fires in this type of receiver were confined mainly to the area of the tube sockets, where power tubes were employed and the heat dissipation was the greatest. It was noted, in some cases, that the printed board had cracked and the heat at this point had started ignition of some combustible material.

However, it is important to point out that this type of problem was in the minority, and when the fire did occur, it was never of any magnitude. It is my opinion that the main and related cause of these conditions was the fact that the tube, being mounted on the printed board, would cause a weakness in the printed circuit whenever the tube was removed or replaced.

During this change in technology with the redesigned television receiver, the technician was experiencing many changes, and we were not sure if it were for the better.

One large manufacturer of television receivers decided that it was a good idea to make all the tie points and connections on the top of the receiver chassis. This was done because certain types of soldering equipment would allow faster production. However, we’noted that with tie points on top of the chassis that are exposed to any foreign materials, such as a bare wire from the deflection yoke or power supply, a short could develop and possibly cause a fire. Also, over a period of time, again from the heat of certain power circuits, the insulation would break down on the printed board and two tie points would come together, creating a short circuit and causing a fire. As the receiver aged, this became quite evident.

I feel that another point that legislated against this type of construction was the danger of pencils, pins, metal buttons, coins, or water from a vase on top of the receiver, falling through the ventilation holes into the receiver. They would strike these tie points above the chassis and create the aforementioned problems. I had serviced one receiver where a coin had fallen across the b-power supply and a ground tie point, creating a lot of smoke and a small fire from a few resistors.

To my knowledge, I know of no fatalities from black and white television receivers in our area during this period of time.

We noted that the public was much more critical when the new color television receiver arrived on the market than they were with the introduction of the monochrome receiver. Obviously, they were looking for perfection, which was not being offered in the early stages of color. It was very apparent that when the set was moved from one end of the room to the other, the serviceman had to be called to neutralize the picture tube from the earth’s magnetic fields. If a vacuum cleaner was operated in front of the receiver when it was in use, this created a similar problem. However, the public accepted the problems and color television was here to stay.

By way of background for the color receiver, it is important to understand why we needed additional circuits and increased voltages and, ultimately, why this created some of our problems relating to fires in television receivers.

Protecting the circuits

The design engineers, and eventually the manufacturers, knew that in order for receivers to operate efficiently and effectively, they would have to add new circuits and create some higher voltages. As we stated before, the monochrome receiver needed a 15,000-volt supply to operate the picture tube and offer a good quality picture. However, in the color receiver, we are looking at a power supply that is rated at 25,000 volts and has to be well regulated. In the absence of regulation, we have a picture that is out of focus, not sharp, fuzzy and, in some cases, all over the screen. In effect, this means that additional tubes had to be added, and voltages increased while still maintaining important high-voltage regulations. When this was done, added heat and its possible problems were created. Again, the circuits are more critical because of the frequencies involved and the necessary stability required. Recently, the high-voltage supplies have been increased to 32,000 volts, which will be discussed later.

At this point, I would like to digress and discuss the methods employed to protect certain circuits in television receivers.

Failure to replace cover on high-voltage cage can permit any fire caused by insulation breakdown to spread to cabinet or other combustible material in set.

When a television receiver is designed by the engineers, they calculate or compute how much current will pass through a certain circuit and what type of protection may be needed in the event of a short circuit in the line. This is excellent and certainly needed for adequate protection in the event of a short circuit. However, with the abundance of television receivers on the market and each engineer figuring certain calculations differently, we wind up with a lot of different fuses. This certainly created a problem for the serviceman. Not only were the fuses of different values, but they were of different sizes, shapes, forms and mountings. The poor technician had to carry many different fuses. In many instances, he may have been in a position to alter a circuit slightly and still have the proper fusing. Again, with the owner of the television receiver turning technician, we would experience some problems with overfusing. It had been noted in some cases that wrong fuses were forced into holders and mountings for which they were not designed. The classical example is where the wire is wrapped around both fuse terminals.

Along came the circuit breakers for the television receivers to reduce the fuse problem and offer a more positive protection for the homeowner. However, some homeowners were not going to allow this diabolical plot by the manufacturers to protect them in the event of malfunction of some circuitry. A piece of wire across the two terminals was very effective, as well as jamming the circuit breaker reset button against the back cover.

Fuses and circuit breakers are needed, should be there to provide protection and, above all, should not be tampered with.

Cause of fires

I believe that it would be helpful to point out some case histories of television fires and what actually happened.

A fire department responds on a box alarm for a residential fire. Upon arrival of the first-due companies, smoke is seen coming from a living room. The entrance is ‘made, the sizeup completed, and a television receiver is removed to the front lawn of the home. The living room is filled with smoke and smoke ejectors are used to clear the room. The neighbor calls the service company and reports the fire. The serviceman arrives and the owner says, “Take the receiver away as we do not want it repaired and we do not want any youngsters playing with it on the lawn.” Upon removal to the shop and an investigation into the cause of the fire, it was noted that a filament choke ($1.50) was burned, because it was too close to the chassis. A new unit was installed and the receiver works fine.

The question here is why so much smoke and why did the filament choke burn? Obviously, a redesign of the choke would eliminate the smoke, and closer quality control might have eliminated the fire.

Approximately two years ago, our department responded to a fire involving a color television receiver at 2 p.m. I happened to be in the area of the fire and responded also. The firemen carried the receiver from the living room to the front yard and extinguished the fire with a carbon dioxide extinguisher. A close inspection revealed that the top of the wooden cabinet had burned through and some damage was noted inside near the high-voltage tubes. This particular model had apparently experienced a build-up of heat over a period of time, and ignition occurred. The set, which was in operation for a period of time, had apparently ignited an accumulation of dust, or throughout the life of the set some pyrophoric carbon may have developed around the cabinet. This, in turn, had reduced the ignition temperature of the wood.

In my opinion and experience, the problems involving the high-voltage section of the color receiver are centered around the high-voltage transformer. Specifically, this transformer, which is enclosed in a cage, does develop some heat during the operation of the receiver. The heat is confined because it is necessary to provide radiation protection from this unit. The ventilation openings on the top and bottom do not always allow for adequate air movement.

Due to the high voltage employed and heat in the cage, you can experience a breakdown in the transformer windings. They will short out internally and burn or short against the cage, causing ignition of any nearby combustible materials. I wish to make a very important point: if the cage is closed, there is really no combustible material except the base of the high-voltage rectifier, which is usually plastic and can create some acrid smoke. However, if the cover of the cage is left off, there can be some exposure to the rear cardboard cover.

Recently, manufacturers are using a quadrupler, which now offers 32,000 volts for the operation of the color picture tube. This transformer is completely sealed in plastic (maybe PVC) and houses the diodes inside. However, when this shorts out, the plastic does decompose and possibly liberates some gases.

We have experienced fires involving high-voltage sockets which are adjacent to the high-voltage transformer. Specifically, the high-voltage sockets are made out of polyethylene. There are other plastic materials that are used that can burn and create an unpleasant smoke condition in the home. It can be noted that we still have a manufacturer who, with every new set that leaves the plant, inserts a cardboard tag in the high-voltage cage, and it has been our experience that over a period of time, this cardboard tag will burn, with the possibility of its falling through the bottom of the cabinet onto any combustible material that would be under the set. The obvious answer here would be for the manufacturer to leave the tag out of the high-voltage cage.

Ventilation is all-important to a television receiver. Tubes create heat that has to be dissipated. This is why it is necessary to keep all the openings in the back of the cabinet free from any obstructions. If a television receiver is to be mounted in the wall of a den or recreation room, it is important that ventilation be seriously considered.

To be continued

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