Beogram 4002 DC Motor Restoration

A Beogram 4002 DC motor arrived from Norway for restoration. This shows the motor as received:

I did the usual initial bench test by applying ~4V between the blue and the red leads. This motor did not turn at all, but the bench supply showed a short circuit. Not a great sign. I removed the outer enclosure and found that the tabs that hold the upper bearing in place were bent up. A sign of previous human interaction with this motor:

I opened it up and this impression was fortified by the bent commutator brushes:

I extracted the bearings (the two small donuts on the black pad):

and immersed them in motor oil and pulled a vacuum:

Immediately, vigorous bubbling started, indicating that the vacuum pulled air from the empty pores of the Oilite bearing material. This process makes room for fresh oil to diffuse into the bearing, refreshing its ability to lubricate the shaft.

After ~72 hrs the bubbling stopped and I extracted the bearings and re-assembled the motor. I tried to run it again from the bench supply, but it still showed a short circuit.

This indicated that at least one of the three motor coils was short circuited by its spark snubber. I took the rotor back out

to replace the snubbers. They are the three devices (one has a red dot) connected with wires between the rotor poles.

I removed the snubbers. This shows them next to the modern TVS replacements:

I soldered the TVS device into place:

If you try this at home, make sure you press the TVS units into the rotor windings until they are above the commutator. Otherwise they will interfere with the brushes potentially stopping the motor rotation.

I put the motor back together and now it ran nicely from the bench supply. Time to do a 24 hrs RPM stability test with the BeoloverRPM device in one of my Beogram 4002s:

The BeoloverRPM allows logging the RPM in 10sec intervals for extended periods of time. Perfect for catching that intermittent RPM variation that otherwise might go unnoticed. This is the curve I measured:

This is pretty much as good as it gets with the Beogram DC motors. This motor is back in business!