Beogram 4004: DC Motor Restoration and RPM Stability Test

A Beogram 4004 DC motor arrived from France for restoration. This shows the motor as received:

I disassembled it to extract the Oilite bearings:

The bearings are the two small donuts on the black pad. This motor had a seized mounting screw on the inner enclosure, which I isolated in the small plastic bag for the time being.

I submerged the bearings in motor oil and pulled a vacuum:

Immediately, vigorous bubbling started, indicating the removal of air from the porous Oilite bearing material. Removal of the air from the pores enables diffusion of fresh oil into the bearing. Oilite bearings lubricate the shaft by diffusion of oil from the bearing. At some point the oil runs out and the bearings run dry. This vacuum infusion procedure restores the oil and the process can start anew.

After about 48 hours the bubbling stopped and that was the sign that these bearings are 'full' again. I extracted them from the oil

and assembled the motor. This shows the bearings re-installed:

At this point it is usually a good idea to check if the very fine wires to the feedback coils survived the surgery. This can be done by measuring the resistance across the two coils, which are connected in series:

If everything went well the resistance should come in at about 42-46 Ohms. If open circuit is found, some microsurgery is necessary to reconnect the coils.

I was lucky with this motor and the measurement checked out. I put the rotor back in and installed the motor in one of my Beogram 4002s for performing a 24 hrs RPM stability test with my BeoloverRPM device, which allows logging the RPM in 10s intervals for extended periods of time:

This is the curve I measured:

This is as good as it gets with the Beogram 4004 DC motor. This motor is ready for its journey home to France!