Beogram 4002: Failed Restoration of DC Platter Motor and Substitution with Beolover SyncDrive

I recently received the DC motor and the RPM panel of a Beogram 4002 for restoration. The unit displayed the usual RPM variations associated with dry running motor bearings.

In the end I was not able to restore this DC platter motor with a satisfying result. The reason may be that the motor seemed to have been opened before. The pulley was loose and the screws only lightly tightened. As shown below the motor still has RPM jumps around 1%, even after oil infusing the bearings and replacing the spark snubbers. My hypothesis is that the reason when I get such jumps is an mis-orientation of the top bearing after the oil infusion. When I open a motor I usually mark the bearing position and then put it back in exact the same orientation. This seems to increase the chances for a successful result.

My theory for the occurrence of the observed RPM variations is that over time the bearing gets polished in a spot facing the center of the platter due to the pull of the platter belt. If the bearing is re-installed in a different orientation the polishing process needs to start anew and this causes the speed variations. So when motors come in that were worked on previously it is possible that the bearing is already not in the original position anymore when I extract it. But I guess we will never know exactly with this motor.

Luckily, there is the Beolover SyncDrive DC motor replacement. It upgrades any DC motor Beogram to AC motor RPM stability with a simple plug-and-play installation (no soldering required):

I usually offer the SyncDrive at a discount whenever I am not able to restore a DC platter motor.

This summarizes the work I performed for this project: 

The motor as received:

I took it apart to remove the bearings:

The bearings are the two small donuts on the black pad upfront. I inserted the bearings into synthetic oil and pulled a vacuum. These bearings were very thirsty and the air bubbles drawn from them by the vacuum were so many that they caused the oil to foam before I could take a nice picture of the process starting:

After a couple days the bubbling stopped and I extracted the re-filled bearings from the vacuum:

Then I reassembled the motor and hooked it up to my bench supply. I measured a fluctuating current around 0.7-0.9 Amps at a voltage of about 1.4 V:

Way too high current at such a low voltage! The motor itself made strange pulsating noises and did anything but run smoothly. Sounded a bit like a one-cylinder farm tractor from 1938....;-). This was a clear indication of one or more spark snubbers having gone bad. They seem to occasionally short-circuit and with that one of the coils on the rotor is dead making a short circuit. Hence the high current and erratic behavior.

I set up to replace the spark snubbers with modern uni-directional TVS diodes. This shows the original spark snubbers still in place. They are the three silvery devices soldered to the poles of the rotor:

Next to the rotor on the black pad are the TVS diodes prepared for soldering in. I removed the spark snubbers:

And then soldered the TVS devices in-between the poles:

If you try this at home, the TVS packages cannot protrude into the plane occupied by the commuter terminals. Otherwise they can interfere with the brushes and/or the iron pieces of the rotor. Then I put the motor back together and tested it again:

Now I got the proper 0.2-0.3 Amps at 5 V. So I implanted the motor in one of my 4002s and measured the RPM stability over 24 hrs. This shows the BeoloverRPM in action:

In its slow mode it logs the RPM in 10s intervals, perfect for detecting RPM inconsistencies. The blue curve is the graph I measured after 24 hrs:

Sadly, the motor did not run stable enough to declare victory. The blue curve shows RPM jumps of the order of 1%, about 20x larger than what is tolerable according to specifications (<0.05%). These fluctuations may still be too small to be detected by the average human, but when listening to piano music or similar it may well be noticeable. 

So my customer decided to let me replace the motor with a Beolover SyncDrive. I added a typical SyncDrive RPM graph measured on the same Beogram (red) for comparison. This curve is as good as the curves I usually measure on AC platter motor Beogram 4002 and 4000. This will fix my customers RPM issues!

On to the RPM panel. This shows it as received:

And flipped around, revealing the bulb covers:

I removed the covers:

And replaced the bulbs with Beolover RPM Panel LED Backlights for Beogram 4002 and 4004 (Types 551x/552x). They solder directly to the solder pads used for the bulbs, i.e. are drop in replacements:

The LED boards do not interfere with the bulb covers, which can be installed the same way as before:

Then I tested the RPM panel in my 4002:

The scales are backlit in a nice warm sheen, exactly like an old fashioned incandescent bulb would do it!

All good again with this RPM panel!