Beogram 4004: DC Motor Restoration and RPM Stability Test

A Bang & Olufsen enthusiast from Texas sent me the DC motor from his Beogram 4004 turntable for restoration. The deck had developed the usual dramatic RPM fluctuations that go along with dried out Oilite shaft bearings. Most of the Beogram 400x motors from the 1970s have now dry bearings that need to be re-infused with oil under vacuum to give these motors another lease of life. 

The Beolover restoration process involves disassembling the motor, extracting the bearings, placing them in motor oil under vacuum, reassembly and then a 24 hrs RPM stability test to make sure that the motor performs again. Here are a few impressions from this particular motor. The motor is very easy to extract. Removal of four screws and unplugging is all it takes:

This shows the bottom part of the motor that houses the brushes and the lower bearing. It also supports the two small coils that are responsible for generating the tacho feedback that allows the control electronics in the Beogram to keep the RPM constant:

This shows the motor taken apart:

The two bearings are on the black pad to the right.

Then it was time to put the bearings into the oil and pull the vacuum:

This picture is a beautiful example of the process. Both bearings are very thirsty and a lot of air is being pulled out by the vacuum to make room for the oil to enter the porous brass material.

After about 24 hrs the bubbling ceased and the bearings were ready for reinstallation:

This shows the 3D printed tools that I use to install the top bearing back into the motor housing.

Once the motor was back together, I did a 24 hrs RPM stability test with my BeoloverRPM device that allows the monitoring of the RPM over extended periods of time. See here for more information - it is available to other enthusiasts for occasional RPM checks and motor diagnosis.

This shows the tool in action:

And this is the RPM graph that I measured on this motor:

The small fluctuations are well below the audible threshold and are a result of the control system interacting with the motor to keep the RPM constant...somewhat like driving along a straight road, one steers a bit left, a bit right...and so on. The initial slight drop of the RPM is normal and a result of the motor and the electronics warming up and then reaching a steady state. This motor is good for another few decades of service in its Beogram 4004.