Beogram 4004 DC Platter Motor: Sonicating the Rotor and the Brushes Made it Work

I recently received the DC platter motor of a Beogram 4004 located in California. It had the usual RPM stability issues that are a result of drying out Oilite bearings. This can be cured by infusing the bearings with fresh oil under vacuum. This shows the motor as received:

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I disassembled it to extract the bearings (the two small donuts on the black pad):

Then I immersed the bearings in motor oil and pulled a vacuum. Immediately strong bubbling started indicative of dry bearings:

The vacuum sucks out the air from the porous material, thereby making room for oil to penetrate. After about two days the bubbling stopped and I extracted the bearings and reassembled the motor. Then it was time for a 24 hrs RPM stability measurement with the BeoloverRPM device. It can log the RPM in 10s intervals for extended periods of time:

The blue curve in the graph below is what I measured:

Less than perfect! My first guess was this motor also had a bad spark snubber. They can short circuit, which disables one of the windings on the rotor. In the beginning this phenomenon is often intermittent, and that causes strongly erratic RPM behavior. I took the motor apart again. This shows the original snubbers soldered to the commutator poles:

I removed the original snubbers and prepared the modern replacements:

Then I soldered them in place:

After that I ran another test. Unfortunately, no cigar! The curve looked very similar like the above blue curve! So this was not it. Not many options left at this point. After pondering for a while, I remembered a case where the motor had been 'oiled' by a customer before sending it to me, and in that case I had to clean the oil from the interior of the motor before I was able to get it running again properly.

I opened the motor up again and inspected the brush carrier. And indeed, it seemed there was some sticky contamination on the plastic and the brushes. So I put the rotor and the brushes into the ultrasonic immersed in isopropyl alcohol and ran it for 30 min. After that I let everything dry over night and then ran another test. This time, the motor ran very nicely and gave me the red curve in the above graph!

Beolove can take a bit of patience sometimes!...;-). This motor seems ready for another tour of duty in a beautiful Beogram 4004!