International projectionist (Jan 1941-Dec 1942)

former secondary. The negative (or upper) terminal of the same voltage source must connect to the grids of V-3 and V-4, and may be traced to them as follows: up, left a little to the first junction point; up to the center tap of the secondary of T-2; through that secondary to the two grids. The circuit is complete. It is very similar to the circuit previously traced, in which R-7 was regarded as a voltage source, but a little less simple to see by a glance at the diagram; some slight checking of connections is needed to uncover it. For the plate supply to V-3 and V-4 it may be enough for a given purpose to locate some convenient distribution point as the positive terminal of the source of power. An easy one is the center tap of the primary of T-3. We know power must reach the plates of those tubes, and the center tap of T-3 primary is the only external connection to their circuit. Here, then, is where plate power must come in, there is no other junction through which it can enter. Call the center tap of T-3 primary the positive terminal of the plate power source; call the upper end of R-4, as just traced, the negative source terminal; call the plate and filament binding posts of V-3 and V-4 the positive and negative terminals of the load. The circuit is complete. It may be necessary to find out how power gets to the center tap of T-3 secondary. Perhaps a voltmeter check has shown there is no voltage there. It is not hard to trace back from that point to the positive side of the power source. That's like tracing back from the projection room switchboard to the meter board in the cellar. From the center tap in question a black and white wire runs downward. From this wire three lines branch to the right; those are the only connections to it. Two of them, however, are blocked by condensers, through which d.c. won't flow; the third, the top connection, must be the one that runs back to the positive side of the power source. It can be traced back right, up and right to a jumper in receptacle J-2. There a dotted line branches off; the drawing says it goes to the field coil of a loud speaker; trying the other line first, this is found to run left, down and left to the top of L-l, and through that coil to the center tap of the rectifier filament secondary, which is the positive terminal of the ultimate source of power. The negative terminal of this source is of course the center tap of the plate secondary of the power transformer, and this tap is easily traced back to the top of R-4; also it can be traced (straight up as far as possible, and then right) to ground. Three of the seven circuits of the S.M.P.E. Meets-Elects Griifin President HERBERT griffin, vice-president of the International Projector Corporation, is now President of the Society of Motion Picture Engineers, having been elected, at the October convention at the Hotel Pennsylvania, New York, to succeed Emery Huse. Griffin has been a pioneer in the development and improvement of projectors since 1907. Elected with him, as Vice President, was Loren L. Ryder, director of recording for Paramount. The usual Spring convention of the S.M.P.E. will not be held in 1943, it was decided, because of war conditions. The award for the best original paper Herbert Griffin printed in the Society's "Journal" for I ''I I was won |)y Donald Mackenzie and Walter .1. Ubersheim, both ol Bell Telephone Laboratories, for their joint paper, "Analysis of Sound Film Drives.'1 In this contribution, which appeared in the November, 1941, issue of the "Journal", stabilizers for controlling film motion are analyzed mathematically in great detail, as if they were electrical filter <ircuits, and as if the vibrations to be removed were electrical frequencies to be filtered out of direct current — the direct current representing the steady driving force. The practical result of Mackenzie and Albersheim's mathematical analysis was the production of film drives with less flutter than any drive previously known to the art, according to measurements taken with a flutter bridge. Because of the war the progress medal was not awarded this year. The three-day convention was marked by the reading of many papers on 16miliimeter equipment and procedures and by strong emphasis on wartime uses of motion pictures. Film development techniques also received a large share of the interest of the members attending the meeting. Highlighting the discussions relating to 35-mm projection were two papers by E. K. Carver, R. H. Talbot and H. A. Loomis, all of Eastman Kodak Company, which were read in sequence by Dr. (Continued on page 18) output stage have now been traced; the filament, the grid bias and the plate power circuits; and in addition the plate power circuit has been run back beyond convenient distribution points to its ultimate source. The other four circuits are those for speech input and speech output. One speech input circuit may be traced from one load terminal (the filament of the lower tube) down, through the transformer winding, up through R-4, left to the next junction and up to center tap of T-2 secondary, a source terminal. From the other load terminal, the grid of the lower tube, trace straight left to the other source terminal. Similarly with V-3, its load terminals for speech input are its filament and grid connections; the source terminals of its speech input are the top and center tap of T-2 secondary. These circuits share all the wiring of the grid bias circuits; grid bias and speech input being in series. The source terminals of the speech output circuits are the plate and filament of each tube; the load terminals are the two ends of T-3 primary, and the center tap of that wiring — which is the common load terminal connecting to both filaments as its sources. The connection is readily traced: down the black and white wire as before, and right through C-4 and C-5 to ground. The filaments also are grounded (as repeatedly traced) through R-4. The fact that there is no by-pass condenser around R-4 to carry the current of the speech output circuit introduces a certain amount of reverse feedback into this stage of amplification which acts to improve sound quality. None of the circuits of V-2 has been traced for the reader, and the plate circuit of V-l has not been traced further back than C-6. taken as a distribution point. The reader may wish to locate and identify those circuits of the diagram for himself, following the procedure used above. That procedure is: in the case of any load, find its two terminals. Locate two terminals of a source supplying that load: connect source terminals with load terminals. For instance, in the case of speech input to V-l, the secondary of T-l was taken as the source. It was not necessary to go further back to the primary of T-l, and thence to the incoming fine — that is another circuit. NOVEMBER 1943