In the Shop with Bob Moog:
A Personal Account, continued

The major activity in building the MTS keyboards was the fabrication, wiring, and installation of the keytop sensors. The keytops for the naturals were cut out, oversized, in the shapes of their respective keys from thin sheets of epoxy-glass circuit-board material, each containing several octaves’ worth of keytops. On one side of each keytop had been laid a conductive pattern, leading to terminals at the four corners. The keytops for the sharps (piano technicians call all black keys “sharps”) were small rectangles of such material, also cut from sheets. On each keytop, on the side opposite the conductive pattern, after I carefully masked off areas not to be painted, Moog screened a thin film of black “resistive” paint (i.e., paint that conducts electricity but with some resistance). The paint was cured under heat lamps, then sprayed with a thin coat of urethane. When everything was dry, I soldered a multi-wire ribbon cable to the terminals at the back end of each keytop, and then a connector plug, for connection to scanning circuitry, to the other end of each cable.

The way the keytop sensors worked was described to me this way: The resistive paint surface and the player’s finger form two plates of a capacitor, the finger being considered grounded at high frequencies by virtue of its connection to the rest of the body. The urethane coating over the resistive surface is the insulating dielectric of the capacitor. A high-frequency alternating voltage is applied to the four corner terminals and the painted surface via the ribbon cable and the conductive pattern on the back of the keytop, and the resulting current at each terminal is measured. The proportion of the total film current measured at each corner terminal indicates the position of the finger relative to the corners. Each key is electronically scanned 200 times per second to give a continuous reading of these values.

After wiring up the keytop sensors, the next step was to glue them to the keys. On the Yamaha we decided to glue them directly to the plastic keytops already on the keys. Removing the plastic seemed unnecessary and difficult, and these being one-piece tops and fronts, would have left us without key fronts as well. First we had to trim a small piece off the back ends of the Yamaha keytops to accommodate the ribbon-cable connections. Each sensor was then glued to its appropriate key with five-minute epoxy and clamped until dry.

Following the gluing came the most tedious and time-consuming job of the entire project: trimming and filing the oversize sensors to exactly match the shapes of the keytops to which they were glued, as well as to eliminate any sharp edges and create a uniform appearance from note to note. Despite the tedium and epoxy dust, I found this job strangely satisfying, probably because some semblance of art was involved. After testing for good connections, the black-painted keytop sensors on the natural keys were painted with white epoxy to once again resemble a piano. The bottom edge of the fallboard was trimmed to accommodate the now slightly higher keytops.
 

Gregory Kramer's MTS Keyboard, made from a Yamaha CP-80 Electric Grand. Photo Credit: Gregory Kramer

The 88 ribbon cables now trailing from the keys had to be connected to the scanning circuitry, which consisted of 11 circuit boards. The question was where to put all this stuff without fouling up the movement of the keys and action — one reason Moog had hired a piano technician to assist him. Fortunately, almost as if anticipating our need, Yamaha had thoughtfully provided a rather large, empty space beneath the keyboard of the CP-80. Moog cut out a portion of the instrument bottom, hung a couple of hinged trap doors, and installed the circuit boards inside. A shallow slot was cut down the side of each key, and each ribbon cable was dressed down the slot into the cavity below. The cables were carefully routed so that the collective mass of wires would not push up on the keys above, thus limiting their movement. The 11 scanning circuit boards were all wired together and connected to still other circuit boards that made sense of their data.
 

Closeup of Yamaha MTS. Photo Credit: Bob Moog Foundation

As mentioned, Moog’s other innovation was to add a sensor for the vertical position of the key in its stroke. This sensor, another variable-capacitance device, consists of an aluminum vane attached to the bottom of the key and a pattern on a circuit board mounted on the keybed below. In this case, air is the insulating dielectric and the capacitor’s output depends on the distance between the vane and the circuit board. The vanes — small rectangles of thin aluminum — were stuck to pieces of foam rubber and attached to the bottoms of the keys, carefully positioned over the circuit-board patterns. The boards were spring loaded so that their distance from the vanes could be finely adjusted. As with the keytop sensors, these 11 circuit boards were also wired together and connected to other circuit boards in the cavity below.