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(e) Poor ground connection on the projectors. Clean and tighten.

(f) Noisy photo-cell polarizing battery (in SPU equipment). Check for low voltage.

(g) Noisy or old "B" batteries in the voltage amplifier circuits (in SPU equipments). Check for low voltage.

(h) Optical system out of adjustment in such a way that the light ray passes through the sprocket holes of the film, or through the frame lines of the picture.

(i) Defective "C" batteries. Check for low voltage.

(j) Guide rollers out of adjustment. The guide rollers in the sound gate shoe should rotate freely. There should not be any side play in the outside roller, but it should not bind on the gate shoe. If this guide roller is loose or out of position, the film will weave in and out through the gate, thereby causing "motor-boating" (frame line or sprocket hole noise), and the reproduction will be very poor when using sound film.

251. Whistling Sound When Using Either Sound Film or Discs. -When an unusual whistle occurs in the PG-13 and SPU equipments, it is usually a sign that one of the UX-250's has become defective-not burnt out, but the filament emission may have dropped. When the UX250's are greatly unbalanced in plate current, it is usually indicated by a whistle. If this occurs, it is necessary to determine which power amplifier it is that is whistling. Insert the monitor plug in each monitor jack until the whistle is heard in the monitor speaker. This will indicate that it is the amplifier into which the speaker is plugged that is whistling. The UX-250 tubes should be removed from this amplifier, and replaced with a set of matched tubes from the theatre's spare supply. Of course, in the case of a PG-4, PG-8, PG-10 or PG-13 there is only one power amplifier, rendering it unnecessary to go through this procedure to determine which amplifier is whistling. If replacing the UX-250's does not stop the whistle, put in a new set of UX-281 rectifier tubes. In most cases, replacing the tubes in the power amplifier will stop this whistling, but if the whistle continues after all the above mentioned things have been done, it is possible that the whistle may come from the voltage amplifier. Tap gently each tube in the voltage amplifier, one at a time, to ascertain if any are excessively microphonic. If any of the tubes are abnormal in this respect, replace both tubes in the stages containing a defective tube with two new tubes. A whistle may be caused by unusually low "B" batteries.

252. Test Lamps. -It is very handy to have a test lamp in the booth. Where there is 220 volts as well as 110 it is well to have two testers, one with a single bulb of 25 watts capacity for testing fuses and speaker field continuity. The other tester, with two 110 volt lamps wired in series, is used for testing 220 volt circuits.

To test for blown fuses with the lamp tester start at the main power switch. Hold the leads one on each side of the line-if the bulb lights, all fuses, etc., from that point to the power house are O.K. If it does not light, it is a sign that power is not reaching that point. If O.K. test at the next fuse block. Test on the power side first then on the load side. If light is obtained at the load side of the fuses it is a sign they are not blown. Continue this procedure until the "open" in the line is found.

Do not attempt to test fuses in this manner in circuits rated at more than 220 volts.


Chapter Fourteen Pages
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