Beogram 4002 (5523): Restoration of DC Motor and a Dead TR2 Transistor

After restoring the PCBs and the RPM trimmer scale backlighting of the Beogram 4002 (5523) that I have on my bench right now it was time to rejuvenate the DC motor. At this point in time practically all DC motor 4002/4004 need their Oilite brass bearings infused with fresh oil. Over time the oil in the porous bearing material is drawn out onto the motor shaft and after 40 years the oil reservoir is finally empty. This usually causes strong and intermittent RPM fluctuations. In order to do this procedure, the motor needs to be taken apart to extract the bearings. This shows the motor removed from the enclosure:

And in pieces:

The bearings are on the black pad. A closer inspection of the brushes revealed that this motor had been 'worked on' before:

With brushes in this bent state the motor probably had a hard time working properly. 

On we go: The next step is to immerse the bearings in motor oil (SAE30) and pull a vacuum:

The bubbles that immediately start to emerge from the bearings signal that the oil infusion process has started. The air is drawn out of the pores due to the vacuum, while the oil enters them due to molecular motion and the urge to lower the entropy of the system. After this brief excursion into physics it was time to wait until the bubbling stopped. In this case it took nearly 48 hours, which was about twice as long as it normally takes. This points to 'hungry' bearings that had a very low oil content before the immersion. 

After taking the bearings out of the oil, I put them on a paper towel to absorb the excess oil:

Then it was time to put them back into the motor enclosure. The upper bearing mount needs to be pressed into the motor housing to ensure that the bearing sits snugly against the upper end of the enclosure. For this I designed a 3D printed tool that makes this process simple:

This shows the bottom bearing inserted back into the brush carrier:

For this the bent brushes had to be straightened out:

After bolting the motor back together it was time to do a 24 hrs RPM stability test of this motor to ensure that nothing was missed in the restoration process. Sometimes the motors have bad spark snubbers, which often only reveals itself during a long term test.

After inserting the motor into the Beogram and starting up my Beolover RPM device to log the RPM over the 24 hrs period, the motor died after about 10 min. It turned out that the transistor that stabilizes the motor 9V power supply, TR2, had a problem. I extracted it

and replaced it with a new TIP31C. After this was done, I started the RPM test again:

This shows the Beolover RPM device in action. It is available to other enthusiasts. Just send me an email or use the contact form to the right if you are interested in getting gone. Aside from enabling a precise RPM adjustment, it can also log the RPM in 10s intervals over long periods of time. This takes out the guesswork and repeat listening to LP sections trying to discern whether there is a RPM fluctuation or whether it is the music...;-). This shows the 24hrs RPM graph that was recorded by the device:

This is as good as it gets with the Beogram 4002 DC motor. Very small fluctuations and an excellent stability over time. This motor is back in business!