Beogram 4004 (5526): Restoration of the DC Platter Motor and RPM Stability Test

After replacing the RPM trimmers, the RPM relay and the light bulbs in the RPM trimmer panel with LEDs, it was finally time to restore the DC platter motor of this Beogram 4004 (5526). This sequence is important, since one can only test the RPM stability performance of the motor properly after the entire RPM control circuit has been rebuilt. This shows the extracted motor:

The motor needs to be completely disassembled to get to the dried out Oilite bearings:

The bearings are the two small donuts on the black pad. I infused them with motor oil under vacuum:

The raising bubbles indicate that air is drawn from the porous brass bearing material. This creates space for oil to enter the bearing. After about 12-24 hours the process usually stops, at which point the bearings are again full of oil.

This shows the bearings after the infusion process:

After reassembling the motor I put it back into the deck and adjusted the RPM with the BeoloverRPM device:

Then I used the device to measure a RPM stability curve over 18 hours:

This motor showed an exceptionally stable RPM performance and can be considered ready for duty.

Beogram 4002 (5514): RPM Performance Measurement after Restoration of DC Motor and Control System

After rebuilding the DC motor and the control system of the Beogram 4002 (5514) that I am restoring right now, it was time to do a RPM performance test. I set up my BeoloverRPM device that allows the long-term logging of the platter RPM, and then did a 24 hrs test:

After the data was logged on my computer, I graphed it relative to a measurement that I took when I initially received the unit for restoration. This shows the two curves in comparison:

The red curve shows the performance before the restoration. The typical telltale sudden negative RPM spikes are visible that indicate failing brass sleeve bearings in the DC motor. This is very common for DC motor Beograms. In this case the spikes are still fairly moderate, but would have been audible since they are in the 1% range. The black curve shows the performance after rebuilding the motor and the installation of new RPM trimmers and RPM relay. The large spikes are gone and the variations are much smaller. There are still some variations, which are related to the intrinsically slightly erratic performance of the analog motor control system, which is affected by temperature changes and probably the moon phase (that was a joke...;-). Some Beograms show them others do not. At this point I do not have an answer why they sometimes occur. The good news is that they are fairly slow over time and also small enough that one cannot hear them. The short term 'wow and flutter' RPM changes (the high frequency 'noise' on these curves) are within the spec given in the service manual (0.05%).  So this Beogram is back in business.

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.

Beogram 4002 (5513): More RPM Measurements

This is a follow up on yesterday's post about this Beogram 4002 (5513). I did an overnight RPM performance measurement with my BeoloverRPM device. This is the curve that I measured:

This result is pretty much as good as it gets for the DC motor versions. Minimal <0.05% variations over time satisfying the specifications stated in the service manual. The initial slope during the first couple hours is usually observed in the 551x and 552x models. I currently think it is a result of D13 (the Zener diode that stabilizes the DC motor supply) and R12 with which it forms a pretty stiff (i.e. high current) voltage divider to set the base voltage of TR2 heating up. After a while the temperature stabilizes at a certain value in equilibrium with the environment. The band gap change in the semiconductor of the Zener changes its Zener voltage and with that the motor slowly gets a bit more power until a sable condition is achieved. This 'ancient' voltage regulator design is a bit of a drawback of the DC motor versions of the 4002. But one should not get to worked up about it since the change is pretty slow, and only represents a total ~0.3% change, hardly noticeable by most listeners. I am thinking this phenomenon could possibly be alleviated via the adaption of the circuit to use a modern voltage regulator. 

Beogram 4002 (5513): Initial RPM Performance Characterization

This is a follow up to my initial post about this Beogram 4002 (5513). In the meantime I did a RPM performance evaluation of the unit using my recently developed BeoloverRPM device.

This measurement clearly demonstrated the suspected sudden speed variations. This graph shows the RPM vs. time over an approximate 24 hrs period:

The sudden negative spikes suggest that the DC motor control system and the motor itself needs an overhaul.