Beogram 4000: RPM Performance Test

I finally did a RPM performance measurement on the Beogram 4000 that I recently restored. I always wondered how the AC motor Beograms perform in comparison to the DC motors, and also the 8000/8002 models with their computer controlled linear motors. So I hooked up my BeoloverRPM device and let the 4000 run for about 12 hours:

This is the curve that I measured:

The AC motor is performing very nicely and the RPM is very stable, but there is some initial drift as the oscillator and the motor warm up. I measured 36C on the motor and 46C on the transformer after the 12 hour run. 

It is interesting to compare the three Beogram platter drives. This graph shows the performance of a Beogram 4002 with DC motor (blue), the 4000 (green) and a 8002 (red) that I recently fixed up:

It is clear that the DC motor 4002 has the largest fluctuations. Interesting is that the linear drive of the 8002 is not significantly better than the AC motor 4000. Of course the computer controlled linear drive does not have the initial temperature drift seen in the 4000. But these measurements show how well-behaved a synchronous AC motor drive can be. The great advantage of this concept is that there is no feedback loop that keeps the RPM constant over time. The AC motor rather follows the driving oscillator, i.e. as long as the oscillator is stable, the AC motor will perform very stable, too. While this is all very interesting, one should not forget that even the DC motor variations are very small, and  that they cannot be noticed while listening to records.