Beogram 4002: Restoration of DC Motor Video Published - Check It Out!

By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demonstrates how to take the motor apart and how to restore the Oilite brass bearings with an oil infusion under vacuum. If your Beogram has RPM fluctuations this is the video for you! Check it out:

BeoloverRPM: RPM Measurements on a Beogram 8002 and Comparison to a DC Motor Beogram 4002 (5513)

My recently developed BeoloverRPM device determines the measured RPM via the time it takes between the 'ribs' on the platter to cross the sensor field of vision. It is simple math to determine the RPM from that, and I was pretty confident that my measured absolute RPM numbers were accurate. However, there is always the possibility of human error, and so I was itching to test the BeoloverRPM on a microprocessor controlled Beogram 8002, which should have a very good RPM accuracy and stability.

So the Beogram 8002 that I just resurrected was in the 'right place at the right time' for this! I used my most recent version of the BeoloverRPM that can be set up on top of the aluminum plates, i.e. the Beogram does not need to be put into service position for a RPM measurement. I did two measurements, for 33.33 RPM

and for 45 RPM:

And it appears the Beogram 8002 and the BeoloverRPM microcontrollers are in happy agreement! The Beogram is spot on, and the BeoloverRPM appears to deliver a precise RPM measurement.

I measured a RPM performance curve for a few minutes while having my 7" test single (Donald Byrd: Black Byrd) on it. This graph shows the curve in comparison with a DC motor Beogram 4002 (5513) in perfect condition:

The red curve on top was measured on the Beogram 8002, while the blue curve below represents the performance of the 4002. We see that both units have a periodicity in their RPM, but the 8002 has a noticeably smaller variation than the 4002. The periodicity itself is unavoidable in a feedback-based control system since a PID mechanism needs some change to be able to operate. The art of designing such systems is to get the change as small as possible, while still being able to run the mechanism in a stable way.

But we clearly see here that a microcontroller based feedback system is superior to an analog system that depends on temperature, component tolerances and probably the moon phase (that was a joke...;-). In practical terms this difference is probably not perceivable by a human ear, but the 8002 definitely represents progress. I am curious to see how the AC motor Beograms perform in comparison...stay tuned for the next episode of this exciting unfolding story...;-)

Beogram 4002 (5513): Sudden RPM Changes

I thought I was pretty much done with the Beogram 4002 (5513) that I recently restored and I started testing it by listening to my favorite vinyls. After a while I realized that the RPM drifted over time. This was cured by replacing the original RPM relay with a drop-in replacement based on a modern encapsulated signal relay and exchanging the original principal 5k RPM trimmers on the circuit board with modern encapsulated multi-turn precision trimmers. Here are a few impressions:

This shows the original RPM relay on the main PCB:

This relay is the same as used in the Beogram 4000, so my recent design of a replacement of the 4000 relay could be fitted here, too. This shows the drop-in replacement board:

And installed:

Above-left of the relay are the original 5k trimmers. I replaced them with these:

The lead-extensions were needed since I had to install them upside down to make sure that the set screws are accessible through the PCB from the solder side. Otherwise, it would be impossible to tune the RPM while the deck is running:

Here is a top-view:

This solved the RPM drift issue. However after a lot of listening to vinyls, most notably an original first pressing of a 12" single of Grace Jones' "Slave to the Rhythm", it became apparent that in addition to the drift phenomenon there was an other, more serious issue: Sudden sharp drops in RPM that would only occur on relatively rare but completely random occasions. Not so great! I decided to get to the bottom of this and I connect my oscilloscope to the signals around IC3 that controls the DC motor feedback control loop. To observe these signals while a record is playing one needs to solder jumper wires to the interesting signals of IC3:

This enabled to hook up my 4 channel oscilloscope:

Here are the traces that I measured during the occurrence of the phenomenon. Luckily once the issue started to happen it reoccurred a few times in a row, allowing me to capture traces with the single shot button. This shows the phenomenon at 33 RPM

and the much more frequently occurring analog at 45 RPM:

The traces are assigned as follows (see circuit diagram below): blue pin 1 (this is the AC induction signal from the DC motor feedback coils), yellow: pin 6 (this is the pulse width modulated output from the Schmitt trigger that corresponds to the amount of power the motor receives, green pin 7 (this is the RC network that determines the actual pulse width that the Schmitt trigger puts out, and red pin 4 (this voltage relative to the TR2 stabilized 8.5V output corresponds to the voltage the motor receives).

All this was pretty interesting: The DC motor of the Beogram 4002 (5513) is a 3-pole motor. This means that each of the three coils on the rotor is fully powered up every six commutator contact changes (see here for some awesome animations and basic info). This suggests that the observed aberration in the pin 1 traces are related to one specific cycle of the motor rotation since the 'missing valley' phenomenon occurs every 6 wave cycles. So I took the motor apart again and throughly cleaned the commutator and the brushes and put it back together. This made the phenomenon much rarer but did not alleviate the issue completely. I may go ahead and put the motor of a parts unit in for now until I figure out how to fully fix this. The issue with intermittent problems is that they are intermittent. So it is difficult to make sure that a fix 'sticks'  100%. All this shows the principal issue with DC motors in turntables: They need a precisely working feedback mechanism to keep the RPM constant. All this is a non-issue with AC synchronous motors that simply 'obey' a sine wave that is fed into them.