Phonograph record eccentricity tester

Claim 1

In combination, a turntable having a center pin, a phonograph record carried by the turntable with the pin received in the record center pin hole, and a device for measuring the eccentricity of grooving in said phonograph record relative to the record center pin hole, said grooving including spiral grooving and a circular groove extending substantially concentrically but eccentrically about said hole and radially inwardly of said spiral grooving, said device comprising (a) a single arm having a pivot axis and carrying a stylus the tip of which is spaced from said axis and at all times confined only in said circular groove for tracking therein in response to rotation of the record on the turntable about a turntable axis in registration with said hole, the arm pivoting about said pivot axis to an extent determined by said eccentricity, (b) sensor means responsive to pivoting of said single arm to produce an AC output signal having an amplitude corresponding to the amplitude of said pivoting, and (c) circuit means including a digital display for processing said signal to produce a digital read-out corresponding to the peak amplitude of said signal, (d) said sensor means comprising a rotary variable differential transformer having a rotary core element coupled to said arm to pivot in response to said arm pivoting, said transformer having a primary coil receiving excitation at a carrier frequency and two secondary coils connected in series opposition so that the voltages induced therein are opposite in phase and said output signal represents the difference between said voltage induced in said secondary coils, said primary and secondary coils having fixed, non-rotating position so that only the core of said sensor means is rotatable, (e) said circuit means including a demodulator, amplifier, low-pass filter and high and low peak detector means electrically coupled in series relation between the output of said transformer and the input of said display, the detector means detecting high and low AC signal peaks and subtracting same to produce a difference voltage for display, and (f) there being means connected with the display to hold the display output at an input value for a selected time interval.