Beogram 4002 DC Motor Restoration - Oil Infusion, New Spark Snubbers and an Ultrasonic Cleaning

A Beogram 4002 DC platter motor recently arrived from Ohio.

The motor barely ran when I gave it a first test on the bench. I opened it up to extract the bearings for oil infusion:

The bearings are the two small donuts on the black pad. They are Oilite types, which means they are porous and, when new filled with oil. Over time the oil leaks out and keeps the shaft lubricated, until not anymore. That is the point when the motors get noisy and intermittent RPM fluctuations start cropping up. The restoration of these motors requires the re-infusion of oil into the bearings under vacuum.

I immersed the bearings in motor oil and pulled a vacuum. Immediately vigorous bubbling started, indicating that the vacuum was pulling the air from the bearings to make room for oil diffusion:

After about 3 days the bubbling stopped, indicating that the bearings were replenished with oil, and I  extracted the bearings from the oil:

Then I reassembled the motor. Unfortunately, it still responded very weakly to the applied voltage. This usually means that one or more of the spark snubbers have short circuits which disables one or more of the rotor windings.

I opened the motor up again and extracted the rotor to exchange the snubbers. This shows the rotor with the snubbers 'ring' removed:

Behind the ring the new 6.5V unipolar transient voltage suppressor (TVS) devices are shown, prepared for soldering across the three motor coils by bending the tabs of their DO14AC packages a bit outwards. This shows the TVS devices installed:

The DO14AC packages fit perfectly between the solder tabs for the coils. After I put the motor back together, it finally ran smoothly, or so it appeared on the bench at least. Now it was time for a for a RPM stability test with the BeoloverRPM device, which allows logging of the RPM over long periods of time. The red curve is what I measured after the above procedures:

Obviously this was not good enough. At this point I let myself be reminded that my customer had included a note with the shipment indicating that the motor had been opened and 'oiled'. At that point I remembered another oiled motor that I needed to clean in the ultrasonic bath before I could get it to run properly (see here). It seems these motors do not run well if the commutator and the brushes are oil covered.

So I took this motor apart again, and cleaned the rotor and the brushes first in water with detergent, and then in isopropanol (to get the water out of the rotor windings). After reassembly and another 24 hrs test, this motor now produced the nice blue curve, which is pretty much as good as it gets with the Beogram DC motors. So it appears this motor is finally ready to be returned back to duty!