Beogram 4002 DC Motor Restoration: Repair of a Damaged Pickup Coil and Oil Infusion

I recently received a Beogram 4002 DC platter motor from Australia for repair and restoration. This shows the motor as received:

I was told that the motor would always run at a high speed. This immediately pointed towards a damaged RPM feedback coil. If there is no feedback signal back into the circuit that tells it how fast the motor is running, the circuit puts the pedal to the metal, so to speak, and the motor runs as fast as the circuit can accelerate it.

I took the motor apart, and it became immediately clear that 'human creativity' had occurred. This shows the bush carrier of the motor. The brushes were installed wrongly. Their contact terminals need to be underneath the contact past and not above like I found it in this motor. Also there is some solder burn next to one of the terminals.

The two small square objects are the pickup coils. They send an AC voltage back to the circuit via the white lead. This AC signal is generated by energized rotor coils passing across the coils, which causes the induction of a current due to the changing magnetic field. The frequency of the current is proportional to the RPM of the motor (see oscilloscope trace below).

One can measure the resistance through these coils, which are connected in series, by measuring between the white and blue jumpers. When everything is o.k., about 41 Ohm should be seen. This motor yielded open circuit, indicating that the connections of one or both coils were ripped out. This happens very easily while taking these motors apart, since the magnet wire used on these coils is of a very thin gauge.

I completely disassembled th motor to get to the bearings for infusion. This shows the parts spread out:

I immersed the bearings (two small donuts on black pad) in motor oil, and pulled a vacuum. Immediately vigorous bubbling started:

This indicated that the vacuum was drawing air from the porous bearing material, creating space for oil to diffuse into the bearing instead.

While the infusion process took its course, I looked into the pickup coils. One of them was still o.k. and I was able to measure 20.5 Ohm across it. The other, however was open circuit. I looked at ti under the microscope trying to identify the loose ends where the wire was broken. This is a  sisyphean task even if nothing happened but pulling a bit too hard on one of the leads on the coil. In this case, a repair effort seemed to have made things worse, and several windings had been cut with a tool, probably while trying to get some loose ends for splicing together again. I had to unwind several loose ends from the coil until I finally found an end that would connect to the other contact lead of the coil. I measured about 15 Ohms:

Luckily, one of the disconnected parts of the cut windings was a bit longer, yielding close to 3 Ohms:

I connected the main winding and the fragment and this resulted in a 18.5 Ohms coil:

I decided that this would be enough windings for getting a decent feedback signal and I fixated the mess in place with some soft double sided tape to insulate the solder spot from the motor housing and to keep things neatly together:

At this point I let things be until the bubbling of the bearings stopped after about 48 hours. I extracted the bearings from the oil:

Then I put the coils and the lower bearing back into the brush carrier:

The top bearing was installed using a 3D printed tool for pressing the bearing retainer tabs back into the original shape:

Then I put the motor back together and measured the feedback signal while the motor ran freely at 5V:

This looked like a healthy feedback signal at about 150mV amplitude, enough to satisfy the motor control chip. This indicated that the coil repair had been successful.

Encouraged by this result I put the motor in one of my Beogram 4002s and ran a 24 hrs RPM test with the BeoloverRPM device:

The BeoloverRPM allows the logic of the RPM for extended periods of time, a great feature for pinpointing RPM stability issues. After about 24 hrs I extracted this curve:

This is a pretty good result for a Beogram 4002 DC platter motor. The small peaks are a result of minute friction changes, while the bearings get used to their new alignment etc...This usually goes away after a playing the deck for a while. At any rate these fluctuations are much smaller than what human can identify, i.e. this motor is back in business.