International projectionist (Jan-Dec 1945)

Proper Operation of Projection Arc Lamps By R. CTTOOLE RCA SERVICE COMPANY The author of this article was associated a number of years with a manufacturer of projection arc lamps. His duties, which consisted of the installation and servicing of this equipment, afforded him excellent opportunities to check on the various causes of faulty operation of arc lamps. He sets forth some of his observations about the subject and offers sound practical suggestions for eliminating many causes of trouble resulting from projectionists' lack of understanding of proper arc lamp operation. WE HAVE three electrical conditions in a lamp which governs its regulation. In their order of importance they are (1) the arc gap; (2) the voltage, and (3) the amperage. We know that if we spread the gap the voltage goes up and the amperage goes down. If the gap is too close, the amperage goes up and the voltage goes down. Therefore, the gap is the guardian over the amperage and voltage and keeps them in the right proportions. We also know that if the gap is too close we get a smaller gas ball and as we cannot fill the mirror with a smaller gas ball we will not have proper screen coverage. In such cases we crowd the carbon, which, in turn, means carbon waste and poor feeding. Low voltage makes the feed motor sluggish. The arc voltage is what holds the arc in the first place, and a low voltage means more carbon loss or more variation with normal and short trim. It also means low motor voltage. Amperage is a matter of choice as lamps run at different amperages and they all work. As for actual lamp troubles. I find the feed probably is the most complained of item. Most of the later lamps are of the constant feed type and constant feeding cannot be obtained with a radical motor. On the relay types this is taken care of in the relay. On large motors running at full load we undercut the mica between the commutator segments. We do this because when the copper gets hot and expands the mica is forced up from between the segments and holds the brush off the copper. A good example of this is the concrete highway. In the summertime when the slab gets hot the tar comes up and forms ridges. In the winter the slab contracts and we have a cutter between slabs. For the purposes of insulation we put the mica between the segments, but to undercut it on a large motor is the lesser of two evils. If we leave it flush, the motor will not run; if we undercut it, it will run. If carbon and copper dust get between the segments causing a leakage, it is of small importance as it is not a precision motor doing a precision job. What is medicine for one motor may be poison for the other.' On the arc lamp motor we are not running the motor at full load, nor does it draw a lot of current. Therefore, we do not have the copper or segment expansion to contend with in the first place. If we do not have the first evil why create the second one? We are primarily interested in precision and a motor with a leaky commutator will be erratic in operation. I have had considerable success by turning the copper down flush with the mica and not undercutting the mica. The motor runs with less trouble and over a longer period of time without attention. On precision motors, such as are used on arc lamps operating at low voltage, the commutator must be clean. Many projectionists have found it necessary to wipe the commutator every few reels. On high speed applications we do not run like materials together due to freezing: on these motors we run a graphite brush on a copper commutator. We use graphite because it lubricates the commutator and prevents unnecessary wear. Too much graphite, however, blackens the commutator with a graphite deposit, which, when it comes in contact with the graphite brush, causes brush chatter or squeaks. If we use a carbon brush, it will cause unnecessary wear of the commutator. However, should we use a graphite brush on one side and a carbon brush on the other side, the graphite will lubricate the carbon and prevent it from wearing; and the carbon brush on the other side will remove the graphite and keep the commutator clean. Any deposit left by one brush is followed by another material. I have been following this method for years with excellent results — in fact, this method will eliminate most motor troubles. Another common trouble is the lamp wiring. Most of the installations are replacements of low intensity lamps which run at 25 amps, and 55 volts. When this setup is used the clamp on the lugs of the d.c. line causes very little trouble. But at 40 amps and 27 volts these lugs are quite troublesome. On rectifiers I find on most jobs that if the line voltage is 230 the rectifier will be set on the 250 volt tap. This means a low filament voltage and to get the amperage the secondary high voltage taps will have to be set up throwing the tungar tubes out of balance. For a better and closer contact, I file the tit off the base of the tungar bulb. After many years of servicing projection room equipment, I would say that most causes of breakdowns are due to projectionists' lack of thoroughly understanding the proper care and operation of arc lamps. I hope this article will help I. P. readers to gain a clearer and more thorough understanding of their projection-room equipment. NEW ALTEC APPOINTMENTS Altec Service Corporation has announced the following new appointments: Frank B. Evans, who has been with the company since its formation in 1937, to branch manager of the Philadelphia office; H. S. Morris, as Eastern representative of Altec Lansing Corporation, in addition to his duties with Altec Service; Carleton S. Perkins, who has been manager of the electronic division, to manager of commercial engineering, and Robert D. Fairbanks, supervisor of the Boston district, to branch manager of the district. OPERATION 16-nim PROJECTOR (Continued from page 9) FIGURE 8 lamp, or one not properly aligned as described in connection with Figure 8, may cause noise. This source of trouble can be checked by removing the film, running without it, and observing whether the volume of the noise increases when this is done. Troubles not known in the theatre include low volume caused by accidental displacement of the speaker or speakers after they have been correctly positioned. Other sound troubles, if any, will be mostly of routine variety. 12 INTER1VATIONAL PROJECTIONIST