To restore the platter drive system of a Beogram 4002 (5513) DC motor, several upgrades are necessary. These include installing modern multi-turn RPM trimmers, a new RPM relay, replacing the RPM trimmer backlights with LEDs, and refreshing the motor bearings with oil. Testing the RPM stability after each step reveals their individual impact on performance.
This particular Beogram 4002 had already undergone most of these improvements, with the exception of the motor restoration. The following details the motor restoration process.
[Image of the extracted DC motor]
The motor needs to be completely disassembled to access the bearings.
[Image of the disassembled DC motor]
The bearings are the small, donut-shaped components on the black pad.
Once removed, the bearings need to be infused with oil. They are made of porous “Oilite” brass, which is pre-filled with oil at the factory. Over time, this oil depletes and requires replacement. This is done under vacuum to remove air from the pores, which gradually replaces the oil.
[Image of the bearings in a mason jar undergoing oil infusion]
The presence of air bubbles indicates that the oil infusion is working. The process is complete when the bubbling stops, typically after 12 to 24 hours.
Next, the bearings are placed back into the motor housing.
[Image of the lower bearing reinstalled in the brush carrier plate]
[Image of the process of installing the top bearing]
Custom 3D-printed fixtures are used to press a tabbed ring back onto the bearing, securing it in place.
[Image of the reassembled motor bottom plate]
After reassembly, the motor undergoes an initial test. It is run at approximately 5V, and the current is measured. It should be less than 30mA. If higher, the brush carrier plate is loosened and retightened until the current is sufficiently low. This ensures smooth motor spinning for optimal long-term stability.
The motor is then reinstalled for a multi-hour RPM characterization, typically lasting 12-24 hours. This helps identify any remaining issues or confirm the motor is functioning correctly.
[Image of the RPM characterization setup using the Beolover RPM device]
The Beolover RPM device clamps onto the Beogram frame and transmits RPM measurements to a computer, allowing for graphical analysis of RPM stability over time.
[Graph showing RPM performance, with the blue curve representing the initial measurement]
Unfortunately, the initial results were not as expected. Significant RPM spikes were observed at higher RPMs, indicating an unsatisfactory performance.
After further investigation, the motor was tested in a different Beogram 4002 5513, where it performed flawlessly. The same positive result occurred when tested with the main PCB from the working Beogram installed. This narrowed down the problem to an issue with the original Beogram’s main board.
A comparison of the two boards revealed that C10 on the working board had a 10 uF capacitor, while the problematic board had a 0.47 uF capacitor. Previous observations had shown that some Beograms were equipped with 10 uF capacitors, while others had 0.47 uF or even 0.33 uF capacitors installed as C10.
Replacing the 0.47 uF capacitor with a 10 uF capacitor on the problematic board successfully resolved the issue.
[Graph showing RPM performance, with the red curve representing the measurement after replacing C10]
This suggests that replacing C10 with a 10 uF capacitor is recommended if RPM issues persist after implementing the other stabilization measures.
Further analysis explored the reason behind the RPM fluctuations caused by the smaller C10 capacitor.
[Image of the DC motor control circuit on the PCB]
[Image of the motor control circuit diagram]
The motor’s feedback signal, generated by coils in the brush carrier plate, is amplified and converted into a square wave. This square wave’s duty cycle, influenced by the motor RPM, determines the charge on C8, ultimately controlling the motor’s current.
The sensitivity of this control loop is influenced by the feedback provided through the C10/C9/R20 voltage divider. A larger C10 capacitor results in a lower impedance, leading to a more responsive system that dampens voltage spikes across the motor more effectively.
Essentially, a 10 uF capacitor for C10 makes the system less sensitive to RPM changes, as evidenced by the red curve in the graph. The 0.47 uF capacitor seems to cause an overreaction, leading to unstable oscillatory states. This is supported by the blue curve’s tendency to jump between specific RPMs in a seemingly random fashion.
The fact that some Beogram 4002 models were originally equipped with 10 uF capacitors, despite the service manual specifying 0.47 uF, suggests that B&O recognized this issue and implemented a solution. This highlights the challenge of diagnosing and addressing long-term RPM instability without access to modern digital tools in the 1970s.