International projectionist (Jan-Dec 1938)

inefficient operation. The same is not true, however, of transformers that carry speech current only, which can readily become noisy. Complete replacement can often be avoided: in many cases the bad connection will be found, or associated with, the external terminal board of the winding. • Tube, Socket Troubles Noisy tubes are commonplace: the cause may be imperfect welding of the internal units or faulty soldering at the base prong or grid prong. In the latter case, the tube can usually be repaired, if it is of a size and cost to justify the trouble, which most of the more modern tubes are not. There still are many sound systems that utilize expensive tubes, costing more than ten dollars per unit; but the majority of present-day equipments are designed for the two-dollar tube, even in the output stage. Sockets also are less likely to set up noise. The older type, with flat contact leaves pressing upward against the bottom of the tube prong, still need watching and cleaning; the newer kind, in which the contact clips embrace the sides of the tube prong, are more reliable and need not be suspected until more probable causes have been thoroughly examined. Ground connections still can be as troublesome as ever, especially waterpipe grounds, which are subject to loosening and corrosion. Ground contact noise is one that may seem to originate at every point in the system, and should be suspected when locating the point of noise origin proves exceptionally difficult. The defective contact, as stated, may be outside the sound system entirely: thus sparking at the arc feed motor relay points is a familiar source of noise which can be picked up by the sound equipment. The remedy in this case may involve external units, such as cleaning and adjusting the relay contacts, or bridging them with a condenser to reduce sparking; or it may involve investigation of the sound head shielding and grounding, or all these steps. All the imperfect contacts mentioned thus far, both in apparatus and in wiring, are series contacts. Exactly similar symptoms are produced by an intermittent short-circuit or ground. A well-known type of short of this character occurs in lead-shielded cable when the sheath has not been carefully stripped. A jagged edge piercing the underlying rubber insulation produces an intermittently noisy ground connection. Coaxial cable, as used for modern photo-cell connections, picks up projector oil, which accumulates in the in sulation and .eventually causes noise. The only remedy is replacement. Causes of this form of trouble are so numerous (as has been seen) and symptoms so uniform regardless of cause, that general rules for finding it are quite useless. Some hints may have value. Thus, in the case of Fig. 1 it has been noted that the .01-mfd. coupling condenser is more likely to be at fault in this way than any of the other condensers. Similarly the 500,000-ohm potentiometer should be the first resistor investigated. It carries no appreciable current — none strong enough to break down a small flaw and make the unit entirely inoperative. Moreover, it embodies a moveable contact, always a possible source of noise. The same considerations, except for the moveable contact, indicate the 150,000-ohm resistor at the extreme left as the next most likely suspect. Headphone tests are useful only occasionally. Thus, if the left-hand 150,000ohm unit in Fig. 1 were at fault, noise might very possibly not be heard there (owing to its low level) and still be heard almost everywhere else in the unit diagrammed. Or noise originating in the 15,000-ohm voltage divider unit would be heard throughout Fig. 1; and the same might be true of an imperfect ground connection at "A." When, as often happens, the trouble is intermittent, headphone tests become still more difficult and more exasperating. One of the best remedies is to wait until the show breaks and then overload, shock, shake, pull, rattle and tear through all suspicious components or wiring until the merely imperfect contact is broken down to a thorough open (or thorough short, as the case may be) — a condition that can readily be reached through ordinary troubleshooting. Headphone and battery tests of the components — disconnected from the nor ERRATUM 'Variable Area' is Academy Standard Nomenclature for Sound Track In the June issue of I. P., the article by John K. Hilliard entitled "Academy R. C. Nomenclature for Release Print Sound-Tracks," beginning on page 22: In reprinting this article, an official release by the Academy of Motion Picture Arts and Sciences, I. P. substituted throughout, including illustration captions, the term "variable width" for the term "variable area". The Academy urgently requests that emphatic notice be given of its recognition of the term "variable area" as standard nomenclature. mal circuit for that purpose — are useful only if the test battery voltage is equal to that under which the component normally operates. A resistor or condenser noisy at 200 volts may be silent at 90. A remedy of desperation, after soldered contacts have been gone over and all ordinary remedies have been exhausted, is wholesale replacement. Thus, if noise is known with certainty to exist in Fig. 1 but is difficult to find, every resistor and condenser in that unit can be replaced, at present prices, at a cost of about two dollars for materials alone. Amplifier replacement parts were far more expensive a few years ago, thus this method of quick treatment never gained much favor. Under modern conditions it is worth consideration, as a somewhat sloppy procedure and not to be recommended as good engineering practice, yet preferable to allowing an obstinate noise to disturb the audience for days or weeks at a time. • Various Popping Noises A different type of noise having a somewhat similar cause is a pop-gun sound, which may appear in volleys or only occasionally as a single discharge. The latter resembles somewhat the sound of an imperfect patch passing through the exciting light; the former, a quick succession of such patches. Where patches and similar film faults may be eliminated as causes, the condition is almost invariably due to a sudden rush of current through a reasonably good but highly temporary connection. The symptom is not only trouble in itself but quite often a warning of impending breakdown, thus requiring unusually prompt attention. This form of trouble, if it constitutes a surge of material dimensions, can often be discovered by watching meter fluctuations either in the power supply line meters normally installed or in analyzer instruments temporarily connected for the purpose. The plate current meter, however, is an unreliable indicator in this case, since it may respond with a slight flicker to the change in sound current representative of the noise rather than of the basic cause. Headphone tests and elimination via block schematic are more useful in this case than with other noises. • Special Precautions When a noise of any description is highly irregular in its appearance and (however annoying) does not persist for long, special steps out of the line of ordinary trouble-shooting are likely to be essential. Consider a noise more or less continuously present, and assume that Test No. 1 has failed to show (Continued, Col. 1, page 13) JULY 19 3 8 II