TurntablePSU.comReviews and Technical Analysis

Walker Precision Motor Controller

Overview
The $1650 Walker Precision Motor Controller (PMC) is a Variable Frequency Drive (VFD) that uses an XR2206 function generator IC to create the sinewave signal. There is a front panel mounted switch to go between 33/45 RPM and two recessed analog adjustments to alter each speed. There is a switch on the side of the unit that reverses the phase of the output signal and the newer units have a front panel On/Off switch (the unit we reviewed did not).

The case of the PMC is made from (heavy) folded steel, epoxy powder coated black. There is a heatsink on the right hand side of the unit. The PMC is powered by a 120VAC input via an IEC C13 socket and the unit has 1 output for 120VAC motors rated at 15W maximum. The output voltage is fixed by a single internal pot. Like most of the other PSUs on the market, the PMC uses 60 Hz for 33 RPM and 81 Hz for 45 RPM, so the turntable belt should be installed on the small spindle of the pulley (33 RPM). The motor we tested with would not start in 45 RPM mode so the user is advised to start in 33 RPM mode and switch to 45 RPM. The PMC does not work with 50Hz motors.

Circuit Description
The PMC uses an XR-2206 function geneator to create the sinewave. Exar, the sole manufacturer of the IC, has discontinued production of this part, although there continues to be a secondary market on e-Bay and other internet sources for the time being. The output of the function generator is amplified by an LM1875 single chip amp and directly coupled to one half of a 12VAC toroidal output transformer. The output is taken from the HV side of the transformer and is routed through a DPDT switch on the side to reverse polarity going to the motor. We could not detect any difference in operation or sound by switching this.

The unit we tested did not have an on/off switch so the output was live when ever the unit was plugged in. We also heard an audible oscillation in the output transformer when ever the unit was first powered up, but it only lasted for the first few seconds.

Measurements
The output frequency at 33.3 RPM measured 59.6 Hz and 80.2 Hz at 45 RPM. Adjustment range of the speed was approximately ±10 RPM for both 33 and 45 RPM modes. No load voltage was 112.3 VAC; the output dropped to 108.5 VAC at 5W, 103.6VAC at 10W and 98.7 VAC at 15W.

Specifications
Nominal Speeds...........................33 and 45 RPM .
Speed Stability.............................Not defined
Distortion (Freq Generator).............1.8%
Distortion (115VAC output).............3.52%
Pitch Control................................±10 RPM continously adjustable
Output Power...............................15W
Reduced Voltage..........................Not available
Turntable Connection....................120VAC on NEMA 5-15 3 prong socket
Dimensions..................................8"W x 17.5"L x 3.5"H
Weight........................................10.25 lbs.
Retail..........................................$1650-$1850