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Electric Reproducing Pianos (The Duo-Art)
(Tuners' Journal - Cont. from November, 1928)

By Wilberton Gould, Member N. A. of P. T., New York City

Service, as defined by Webster: "An act of one who serves."

It is one of God's greatest gifts to mankind, for who is happy who does not serve? Hence it follows that service requires sacrifice. It is the foundation stone of every enterprise. Whatever it may be, its success or failure depends upon whole-hearted co-operation.

And may service and sacrifice continue to be the keynote of our progressive Association.—The Author.

The Duo-Art

In the servicing of the Duo-Art mechanism it is absolutely necessary that the technician understand the expression system and the principle under which it functions. The only tools required are a screw driver, a Duo-Art test roll and a musical ear. The mention of this last item might seem out of place in this series, but facts have proved otherwise.

As the expression box is the heart of the Duo-Art, through which it is possible to reproduce the artist's musical thoughts, this phase will be gone into quite thoroughly in order that the principle on which the Duo- Art operates may be understood in its simplicity. All references made to the expression box will apply both to the grand and upright models, with one exception, which may be referred to in the Duo-Art 1927 service manual, and which will be taken up in its order later in this series.

The adjustment of the various degrees of shading in the Duo-Art must be approached from a musical standpoint in order to achieve effective results. Assuming, of course, that mechanical conditions are equal, it is possible through the unique dynamic control to control the entire register of the instrument either by the accompaniment regulator or the solo regulator, as will be proved as this series advances.

The keyboard of the instrument is divided musically into theme, or solo, and accompaniment in the same manner as if it were played manually. For example, the matter of carrying the melody from the bass to the treble, or crossing the hands, or carrying the melody from the treble into the bass, is effected through the accompaniment regulator or the theme regulator, as the case may be. There is no duplicate control system in the Duo-Art, one for the bass and one for the treble. On the contrary, one regulator, either the accompaniment or the theme in conjunction with the theme valves, may control the entire register. In order to understand how this is accomplished it is necessary to know the travel of atmosphere from the pneumatic stack to and through the expression box, and also to know what is taking place during the operation.

Referring to the phantom view on page 17 of the service manual, it will be noted that figures 19 and 25 are the main supply from the pneumatic stack to the expression box, bass and treble ends. Under normal conditions, that is, when the theme secondary valves (figures 16 and 27) are up against their top seats, they cut off air channels from chambers 20 and 24. This causes the atmosphere entering the nipples (figures 19 and 25) from the pneumatic stack to enter the accompaniment chamber ( figure 22) and pass down, as indicated by the arrows, into the accompaniment regulator, through the knife valve port and thence to the pump. By this it will be seen that air travel is through the accompaniment regulator as long as the theme secondary valves remain against their upper seats, thus proving that normally the instrument is under accompaniment control.

The degree to which the strength of the blow is governed by the movement of the knife valve is controlled by means of the accordion pneumatics 6, 5, 4, 3, 31, 32, 33 and 34 in conjunction with the theme secondary valves 16 and 27. The collapsing travel of the accordion pneumatics being 1/16th, 1/8th, 1/4th and 1/2 inches, as these accordion pneumatics collapse they control the opening of the knife valve port. The greater the area of the knife valve port the stronger the blow given.

It will be noted that there are adjusting screws on the cross bars of the accordion pneumatics. These should not be disturbed, as they are set correctly at the factory and should he adjusted only by a set of accurate gauge blocks that are made for that purpose. It is also necessary that there be just enough tension on the springs to bring the accordion pneumatics back to rest snappily, with not too much or too little tension. Care should be exercised in this operation ; if there is too little tension the accordions will not come to rest quickly enough, and if there is too much tension the accordions will lag in their collapsing, with the result that the knife valve will not open enough, as it should. Likewise, the tension springs on the regulators should not he drawn down to a greater degree than is necessary, but should be just taut enough that they do not rattle. Pulling down the tension springs on the regulators is frequently clone because of lack of knowledge. When this is done it upsets the zero setting of the regulators, and if this operation is carried to the extreme it will ruin the springs for the fine setting of the accompaniment or the theme and it will be impossible to bring down the tone to that softness so much desired by real music lovers. If the springs have been ruined the only remedy is new springs. Therefore, it is suggested that a tuner tread very softly along this path.

Before going further into the expression box, the expression control ports on the tracker bar should be explained, so that they may be better understood when referring to the expression box.

Referring to the illustration "E" at the bottom of page 17 of the manual, we find the following:

Bass End Treble End
Re-roll Soft Pedal
Sustaining Pedal Electric Cut-Off
Bass Theme Treble Theme
No. 1 Dynamic AccompanimentNo. 1 Treble Theme
No. 2 Dynamic AccompanimentNo. 2 Treble Theme
No. 4 Dynamic AccompanimentNo. 4 Treble Theme
No. 8 Dynamic AccompanimentNo. 8 Treble Theme

It will be noted that the note holes start at the fourth hole from the bass end and the fourth hole from the treble end, reading from the outer edge toward the middle; that is, the first speaking note is No. 5 and the last No. 84, making eighty speaking notes. This applies to the modern type Duo-Art. In the older models the speaking scale is full eighty-eight notes on straight eighty-eight note rolls, but still remains eighty speaking notes on the Duo-Art reproduction. Note holes 1, 2, 3 and 4 at the bass end and 85, 86, 87 and 88 at the treble end have each directly above them a perpendicular oblong port. These oblong ports are the accordion dynamic control ports and are connected by tubing to the Duo-Art cut-out pouch blocks directly behind the spool box. These blocks contain a series of four pouches on either side of the blocks, one on the bass end and the other on the treble end, and are connected to the supply from the Duo-Art cut-out block on the left-hand end of the spool box, being marked "Duo-Art On" and "Duo-Art Off." With the Duo-Art lever at the "On" position, atmosphere is admitted to top side of the note pouches on the blocks marked 1, 2, 3 and 4 notes and 85, 86, 87 and 88 notes, which inflates the pouches and cuts off the note holes from the tracker bar. With the Duo-Art switch at the "Off" position, the process is reversed, and air is admitted to the underneath side of the accordion port pouches. Now with the Duo-Art switch at the "On" position, atmosphere is admitted through any one of the accordion dynamic control ports above the note ports, passes through the pouch cut-out block, thence to the accordion primary valve box, from there to each of its respective primary valves, and in turn to each of the accordion dynamic pneumatics on the expression box. In the upright model the dynamic valve box is as sembled on the expression box frame, and in the grand model it is attached to the frame of the case as closely as possible.

The theme valve ports on the tracker bar lead to the theme primary valve box (valves Nos. 29 and 30) and thence to the theme secondaries in the expression box. (See figures 17 and 18 in illustration "E.")

The re-roll port and the port in the takeup spool lead directly to the repeat and reroll primary valve box underneath the key bed, which valves control the re-roll and repeat pneumatics. In the upright model, the electric cut-out switch port on the tracker bar leads directly to the electric switch pneumatic, on the right-hand side of the case, and in the grand model to the switch primary valve box underneath the key bed on the left-hand side of the expression box. (See Correction Concerning the Electric cut-out switch in the Upright Model -click here.)

The sustaining pedal port on the tracker bar leads directly to the sustaining pneumatic in the grand type, and to the pedal regulator in the upright model. The soft pedal port leads to the soft pedal primary valve box in the grand model and to the pedal regulator in the upright model. In the later types of grands, the soft pedal not only raises the hammer rail but also shifts the action, thus giving extremely fine shading in the soft passages of music. (See Correction Concerning the Soft Pedal -click here.)

Explanation of the Duo-Art expression box will be continued in the next installment.

(To be continued)

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