Beogram 4002 (5513): Restoration of DC Platter Motor and Main PCB

I recently received another main PCB and DC platter motor of a Beogram 4002. As usual I started with the motor restoration since it can take a few days until the bearings are fully infused with oil. This shows the motor:

I took it apart to get to the bearings (the two small donuts on the black pad up front):

This motor was quite oily on the inside, an indication of 'previous human interaction'. This was confirmed by sorry state of one of the commutator brushes, which was bent out of shape:

I immersed the bearings in motor oil and pulled a vacuum. Immediately vigorous bubbling started:

This is an indication that the vacuum draws the air from the bearing pores, making room for the inter diffusion of oil.

While the bearings were infusing, I focused on the main PCB. This PCB also had already seen some work, as was obvious from the two trimmers that were soldered to the copper side of the board (upper right corner):

This shows the component side in original condition:

Here a detail shot of the sensor arm amplifier (TR3):

I replaced all the electrolytic capacitors and the RPM relay and RPM trimmers with modern encapsulated parts, and I installed a new transistor for TR3 since these have a tendency to go out of spec, and then the record detection mechanism stops working or becomes unreliable:

This shows the rebuilt RPM section in detail:

After the board was completed, I installed it in one of my Beogram 4002s and I adjusted the bias of the new TR3 to the specified 4V at the collector:

Then I moved the adjusted bias trimmer to the component side:

Now it was time for a measurement of the sensor response. This shows the trace that I measured with the empty platter spinning beneath the sensor:

Each dip corresponds to a passing by black 'rib' on the platter. The amplitude is larger than 5V, which is a good signal.

After about 72 hrs the bubbling of the bearings had stopped and I removed them from the oil:

Then I straightened the brush as good as I could 

and cleaned the motor components in an ultrasonic bath to get rid of the excess oil from the previous repair attempt. These motors run very unstable when the commutator or the brushes are oily. Then I put the motor back together for a RPM stability test with my BeoloverRPM device, which allows logging the RPM in 10s intervals over extended periods of time:

This is the curve I measured:

This is as good as it gets with the 4002 DC platter motor. This motor and PCB are ready for duty again!