Following the replacement of the RPM relay and trimmers in the Beogram 4002 (5513) with updated parts, the DC motor required attention. The DC motor often proves to be the most challenging aspect of restoring a 4002 and nearly always demands attention. The primary hurdle lies in restoring the motor’s Oilite oil-infused brass sleeve bearings. These bearings are “pre-lubricated” in a new motor. From a typical consumer perspective, with a lifespan of 10-15 years, these bearings might be considered “maintenance-free.” However, on a longer timeline, this duration falls short, necessitating oil replenishment. This is achieved by removing the bearings and submerging them in oil within a vacuum environment. The vacuum expands the air that has gradually replaced the oil within the bearings, drawing it out and allowing fresh oil to take its place.
The initial step involves disassembling the motor, as depicted below:
With the motor open, the bearings must be extracted, which essentially means taking the motor apart completely:
Once the bearings are removed, the vacuum infusion process can begin. The image below shows the bearings submerged in SAE30 oil within a mason jar, right after a vacuum has been applied using a FoodSaver pump:
Air bubbles will emerge from the bearings. The larger top bearing (left) typically releases more air than the bottom bearing (right). This is likely because the top bearing experiences greater torque during its operational life due to the belt bearing directly on it. This likely leads to more oil being forced out of the bearing due to the increased contact.
After approximately 12-24 hours, this process largely comes to a halt, as seen here:
This indicates that the oil replenishment is complete, and the bearings can be reinserted:
The top bearing is secured using a lock ring that presses the bearing mount against the motor housing. This ring requires firm pressure against the housing to reshape the tabs that hold it in place. A 3D-printed tool was created, enabling the use of adjustable pliers to apply uniform pressure across the ring while simultaneously bending each tab back into position.
This image displays the top bearing reinserted with the tabs bent back:
The bearing needs to be firmly seated. Testing it with a toothpick can help determine if it’s installed correctly; if it moves with some resistance, then it’s properly installed.
The next stage involves reassembling the motor, ensuring it rotates freely and in the correct direction. The orientation of the bottom plate determines the motor’s direction. The screws securing the bottom plate to the main enclosure offer three possible orientations, with only one being correct. Marking the motor before disassembly can be helpful here. Once the motor spins in the right direction, ensure it does so with the lowest possible friction. Measuring the current during testing is a reliable method to confirm that everything is in order. The ideal scenario is achieving the lowest possible current, indicating optimal alignment between the motor housing and the rotor. Typically, a current of around 20-30 mA at 5V is achievable when the motor spins without resistance:
If the current is higher, the bottom plate needs adjustment, achieved by manipulating the three mounting screws. Tighten them gradually in sequence, making multiple passes, similar to tightening a vacuum flange.
With the screws tightened, the second enclosure can be replaced. Finally, test the motor’s performance over a 24-hour period using the BeoloverRPM device to ensure optimal functionality:
The test results for this motor are shown below:
The black curve represents measurements taken before the motor rebuild. For an original DC motor of its age, it’s in relatively good condition. The average listener might not even detect the RPM drops. Typically, these drops are far more noticeable when people report inconsistent RPM in Beogram 4002s. However, their presence indicates that the bearings are nearing the end of their lifespan, and larger drops would likely soon occur.
The red curve represents measurements taken after servicing the bearings and installing the new RPM relay and trimmers. The drops have been eliminated. While some variation over time remains, this is largely attributed to temperature drift—a characteristic of 1970s analog control systems.