I recently obtained two DC motors from Beogram 4002 turntables that are being refurbished. One of the motors is pictured here.

The first step in restoring these motors is addressing the brass sleeve bearings, which are often dry after more than 35 years of use. I do this through an oil infusion process, which requires complete disassembly, as shown.

The bearings, visible on the black pad in the front, are submerged in oil and placed under vacuum.

Air bubbles escaping the bearings indicate that the oil is being drawn into the porous material, restoring their ability to lubricate the motor shaft. This process usually takes 24-48 hours. The photo shows the bearings reinstalled in the motor.

After reassembly, the motor undergoes a 24-hour stability test using a BeoloverRPM device, which records the revolutions per minute (RPM).

The BeoloverRPM device is available to other enthusiasts - feel free to reach out via email or the contact form if interested.

Achieving optimal speed stability in these DC motors has been a focus of my research. Their analog control system is sensitive to friction and temperature variations, with capacitor C10, responsible for feedback response, significantly impacting RPM stability. Over time, B&O used three different C10 values: 0.33uF, 0.47uF, and 10uF. My findings indicate that each motor tends to have a “preferred” value. Currently, determining the optimal value requires testing with each capacitor and analyzing the results. The graphs below show the results of this testing.

The top graphs illustrate the RPM performance with a 0.47uF C10 capacitor. The red line, representing motor #1, reveals significant spikes, suggesting the feedback system is overcompensating. However, with a 10uF capacitor (light red line), the same motor stabilizes significantly. Motor #2 also benefits from the 10uF capacitor but shows decent performance even with the 0.47uF value.

The reason for these variations remains unclear, as the motors share identical electrical characteristics. I suspect mechanical friction within each motor influences the control system’s response. Until further research provides more clarity, the most effective approach is to determine the ideal capacitor value empirically and enjoy the music!