Beogram 4002: DC Motor Restoration and Dependence of RPM Performance on the Value of C10

I recently received two DC motors for restoration that were extracted from Beogram 4002 units that Sonavor is currently rebuilding. This shows one of the motors:

The first step for motor rejuvenation is performing an oil infusion of the brass sleeve bearings which are usually running try after 35+ years of operation. This requires a complete disassembly of the motor:

The bearings are on the black pad in the front. Their oil infusion requires placing them under vacuum while immersed in oil:

The air bubbles emerging from the pores of the bearings signal that the infusion process is progressing. As the vacuum draws the air out, oil replaces the air in the pores and the bearings regain their ability to form an oil film around the motor shaft. After 24-48 hours this bubbling typically stops and the bearings can be extracted for reinsertion into the motor. This shows the bearings in place:

Once the motor is back together it is time to give it a 24 hrs stability test with the BeoloverRPM device which allows the logging of the RPM over extended periods of time. This shows it in action:

The BeoloverRPM is available to other enthusiasts. Just send an email or use the contact form on the right if you are interested.

For a while now I am trying to figure out how to get these DC motors running with the highest stability. Since their control system is fully analog, factors like friction and temperature can significantly alter the stability of the system. Especially capacitor C10 which governs the feedback response of the system has a strong influence on the RPM stability. Over time B&O changed the C10 value between three values: 0.33uF, 0.47uF and 10uF. Our research has shown that most motors have a 'preferred' value for best performance. Which one can only be determined at this point by testing them with different values and then selecting the best. I did this for these two motors. This are the graphs that were measured:

The top two graphs show the RPM performance of both motors with 0.47uF installed for C10. Especially the red curve (motor #1) has some pretty big spikes to higher RPM, indicating that the feedback tends to over compensate the system and under certain conditions the RPM spikes. The same motor run with 10uF in place showed a perfect curve as shown below (light red). Motor #2 also benefitted from 10uF, but not very significantly. It already performed fairly well with the 0.47 uF value soldered in.

At this point it is a mystery why some motors react different from others. They show the same coil resistance and draw the same current etc...i.e. they are virtually identical in all measurable values. My speculation is that these performance variations may have to do with mechanical friction in the motors, which by itself is a damping factor that acts on the control system's transfer function. Until we understand this a bit better, I guess the best approach is to simply determine the best value empirically and enjoy some nice vinyl records!