Beogram 4004 (5526): DC Platter Motor Restoration - Patience Pays Off!

The Beogram 4004 (5526) that I am restoring right now (previous post) seemed to be a 'standard project' until I restored the DC platter motor. The process started out as usual. I extracted the motor,

and disassembled it to get the bearings out for oil infusion:

The bearings are the small donuts on the black pad. I immersed them in motor oil and pulled a vacuum. Immediately bubbling started:

This bubbling indicates that air is being extracted from the pores of the Oilite bearing material, making room for oil to diffuse inside. After a couple days this bubbling stopped, indicating that the infusion process had completed. I extracted the bearings:

Then I reassembled the motor for testing. The top bearing needs a special 'tool' that the tabs for the mounting ring of the bearing can be pressed down properly:

Then it was time to give this motor its 24 hrs RPM stability test. I installed it in the 4004 and measured for 24 hrs with my BeoloverRPM device:

It measures the RPM in 10 sec intervals and logs the data via the serial port of a computer. The top curve in the graph below is what I measured...not very Beolovely! The variations are a bit too large for a restored motor (but probably still too small to be audible):

I took the motor apart again and exchanged the spark snubbers, since they are often the culprit when the motors still run rough after the oil infusion. This shows the rotor of the motor with its original snubbers in place:

I removed them. This shows the original part with the modern (TVS diode) replacements:

I soldered the TVS diodes in:

One needs to make sure that the TVS diodes do not protrude into the commutator plane, since they can interfere with the brushes when the rotor is turning...not good for the life expectancy of the brushes...;-).

After this I put the motor back together and ran it again and measured...and measured...and measured and the results did not get much better than the first curve. The above graph shows a curve that I measured after about a week of run time. You can see that the stability got better, but that there were still some irregularities. That it got better encouraged me to simply run it for some longer. And indeed after about 3 weeks the motor showed a stability that is similar to many other motors that I rebuilt (bottom curve, red).

So in the end this motor is ready again for playing vinyl smoothly. But Beolove has a lot to do with Beopatience, and every 4004 or 4002 has some new story to tell, for sure...

With regard to a root-cause for the phenomenon described here, well, the only thing that I can imagine at this point is that the bearings had perhaps some larger caked on residues on their running surfaces that needed to smoothen out or something of this nature...I think it is unlikely that there is an electrical issue with the rotor etc...since the aberrations became smaller over time. I guess, we will see if this happens again in the future. I do think I experienced this on shorter time scales before, that initial measurements of motors were a bit more erratic than follow ups, but I never systematically looked into this...some more Beoexcitement for future late-night explorations!...;-)