Another Beogram 4002 (5513) DC motor in need of repair recently came to my workbench. During my standard pre-restoration test using a Beogram 4002, this motor was unusually sluggish, only operating with assistance. This contrasted with the typical DC motors I encounter, which generally run powerfully but exhibit RPM fluctuations and often produce high-pitched sounds due to dried-out bearings. This particular motor was noticeably underpowered.
I disassembled the motor and performed my usual bearing oil infusion to ensure the mechanical components were within specifications before addressing the sluggishness. The process of re-lubricating the Oilite bearings provided some insights. Here are some observations of the motor’s condition upon arrival:
[Image of Beogram 4002 DC motor]
And here it is disassembled:
[Image of disassembled Beogram 4002 DC motor]
Here are the bearings submerged in motor oil under a vacuum:
[Image of Beogram 4002 DC motor bearings in oil infusion process]
The smaller (bottom) bearing shows bubbles, indicating the release of air from the porous “Oilite” material as it absorbs oil. The top bearing released fewer bubbles, suggesting it retained some oil.
After a 24-hour period, the bubbling ceased, and I reassembled the motor. However, the sluggishness persisted, indicating a problem beyond dry bearings.
I disassembled the motor again, recalling a similar issue from the past. In that instance, the motor resumed functioning after removing the spark snubbers connected across the rotor’s three coils. These snubbers suppress the electromotive force (EMF) generated during coil switching, preventing sparks and potential damage to the driver circuitry. The image below shows the rotor connected to a multimeter set to resistance mode:
[Image of Beogram 4002 DC motor rotor connected to multimeter]
The three blue components encircling the commutator are the snubbers. They typically act as insulators at low voltages used in resistance measurements. The displayed 16.8 Ohms represent the resistance of the rotor coil connected in parallel to the snubber. I then measured the adjacent snubber:
[Image of Beogram 4002 DC motor with multimeter probe on a snubber]
The reading for the next coil was 22.9 Ohms. Moving on to the third snubber:
[Image of Beogram 4002 DC motor with multimeter probe on a faulty snubber]
This measurement revealed a short circuit, effectively disabling the connected coil and explaining the motor’s lack of power. It was operating on two out of three coils. Fortunately, I had a three-spark-snubber assembly from a previous repair in my B&O parts collection, shown here in front of the rotor:
[Image of Beogram 4002 DC motor rotor with a replacement set of snubbers]
After removing the shorted snubber from the rotor (they are spot welded), I installed a functioning one from the salvaged assembly:
[Image of Beogram 4002 DC motor with a new snubber installed]
A subsequent multimeter test confirmed a restored resistance of approximately 20 Ohms for this coil.
I reassembled the motor once more and installed it in the Beogram 4002. Using my BeoloverRPM device, I ran a 12-hour test, yielding the following performance curve:
[Graph of Beogram 4002 DC motor RPM performance after repair]
The motor’s performance appears to be back to specifications.
I researched motor spark snubbers and found various types. A basic test using my bench power supply showed that the functional snubbers began conducting at approximately ±10V. This suggests they are likely bidirectional transient voltage suppressor (TVS) devices with an onset voltage around 10V. Replacing them with modern through-hole TVS components of suitable size might be possible. Alternatively, removing the snubbers is an option, but it risks reducing brush lifespan due to sparking and potentially impacting the Beogram’s motor control circuit. My previous tests showed the circuit could tolerate a snubber-less motor for a limited time. Nevertheless, I plan to explore replacing the faulty snubbers with new TVS units when I encounter this issue in the future.