Beogram 4002: Restoration of DC Motor - Oil Infusion of the Bearings and Installation of New Spark Snubbers

Recently, a Beogram 4002 DC motor arrived from New Zealand for restoration. As usual, I first did the oil infusion procedure of the dried out Oilite bearings. This fixes the RPM instability in many cases. This shows the motor:

The upside-down installed pulley indicated that it had been 'worked on' before. I opened it up and extracted the bearings. What I found confirmed this initial impression:

One of the brushes was bent badly. Probably one of the reasons this motor did not perform well anymore. This shows it completely disassembled:

The bearings are the two small 'donuts' on the black pad. I immersed them in motor oil and pulled a vacuum:

Immediately a burst of air bubbles emerged from both bearings indicating a thirst for oil. The vacuum pulls out the air and oil diffuses into the bearings as 'nature abhors vacuum'...;-). After 24 hrs the bubbling had stopped and I extracted the bearings and put them on a paper towel to absorb the excess oil:

I straightened out the bent bearing: Before:

and after:

Then I assembled the motor and installed it in one of my Beogram 4002s for the obligatory 24 hrs RPM performance test. This shows the BeoloverRPM device in action. It can log the RPM performance for extended periods of time.

BeoloverRPM is available to other B&O enthusiasts. A great way to ensure the RPM is accurately adjusted, or to track down RPM stability issues. If you are interested, send me an email, or use the contact form on the right.

This graph shows the curves I measured during the restoration of this motor.

Unfortunately, the red curve was measured after the oil infusion. Clearly, this did not fix this motor. Usually, this indicates the the spark snubbers of the motor are on their way out. They seem to die by occasionally shorting out one or more of the coils on the rotor. So I opened up the motor again to replace them with modern TVS diodes. This shows the rotor with the original snubbers:

The snubber diodes are the three plastic enclosed reddish devices arranged in a circle around the commutator. They can be removed by sucking the solder from the three terminals that connect the windings to the commutator:

After removal of the original snubbers, I soldered three TVS diodes in place:

Care needs to be taken that they are soldered as close to the coils as possible to prevent mechanical interaction with the brushes on the commutator. After this procedure, I put the motor back together and installed it for another 24 hrs run. The blue curve in the graph above resulted. Much better! this motor seems to be ready for duty again. It is time to travel back the approx. 8000 miles to New Zealand!