Beogram 4002 (5513): Rebuilding the DC Motor and a Significant Improvement of the RPM Stability

One of the recently arriving Beogram 4002 (5513) has (had) strong RPM variations. This was shown by my initial RPM performance test. The frequent strong and erratic RPM drops indicated that the DC motor urgently needed cleaning and fresh lubrication. Consequently, I extracted the motor

and took it apart. This shows the motor taken out from its outer noise reduction enclosure:

The next step was to dismantle the motor to clean and lubricate the bearings:

After I reassembled it and installed it back into the Beogram I did another 24hrs RPM test with my BeoloverRPM device. This is the RPM spectrum I measured. It is shown in comparison with the original result of the 'entry RPM exam' of this Beogram:

The improvement was dramatic. As received this Beogram showed RPM variations as large as 5%, which are very noticeable. After the motor restoration (upper spectrum, shifted for clarity) the large negative spikes are all gone. What remains are smaller fluctuations that are related to issues in the electronic control system. Addressing these problems will be the next step in this restoration project. The remaining fluctuations are in the 0.2-0.3% range, which are already almost imperceivable for most people (of course this also depends on the type of music one listens to). But they are definitely outside the allowed spec for the 4002, which is 0.05%. Hopefully exchanging the RPM relay and installing encapsulated multi-turn trimmers will improve the situation. Stay tuned for further developments in this exciting story...;-)!