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The Thorens TD124 Eddy Current Brake
An eddy current is a swirl, like a whirlpool, of current that is induced into a solid conducting mass. By moving the reflector into the field of the magnet, the turning pulley, between the magnet and the reflector, encounters the eddy effect from the magnet. These swirling eddy currents act to oppose the forces being exerted, by the driving belt. The position of the reflector determines how much of an eddy effect is exerted into the spinning pulley.
The star shaped knob in the center turns independently of the speed switch lever. It can turn left or right to operate the eddy brake and thusly affect a fine pitch adjustment to platter speed. When you see that the strobe locks in stationary, your adjustment is right.
Turning the pitch control knob for least eddy effect.
The steel flange, a magnetically permeable reflector, presents the least amount
of material within the magnetic field for minimum eddy effect.
Turning the pitch control knob for maximum eddy effect.
In this position the reflector presents the most amount of material within the
magnetic field for maximum eddy effect.
Adjusting the gap distance between the magnet and the driven pulley.
Above photo: Measuring gap thickness at one side of the magnet using an automotive feeler gage. Gap is .031 this side.
Above photo: measuring gap thickness at the other side of the magnet. Gap is .021.
Above photo: Method of correction. Set the speed calibration control knob for max brake. A pair of paper shims is placed between magnet and drum and at either end of the magnet. The locking screw is loosened. The magnet is allowed to draw itself tight against both shims. The locking screw is retightened. The gap distance is set according to the shim thickness and is uniform for the length of the gap. I have my gap set to .010 inches. This setting, for me, puts the correct pitch within the middle of the braking range.