Beogram 4004 (5526): Restoration of the 'Drive Train'

I recently received a Beogram 4004 exhibiting RPM stability trouble. I rebuilt the 'drive train' components of the unit to restore RPM consistency. Most Beogram 400x have this issue at this point in time, and the most likely trouble spots are dry brass bearings in the DC motor, corroded contacts in the RPM relay and/or thermal influence of the light bulbs in the RPM trimmer panel on the trimmers themselves. I started with the motor. This shows the motor removed from the Beogram:

This shows it disassembled to extract the bearings (they are the two small 'donuts' on the black pad):

I submerged the bearings in motor oil and pulled a vacuum:

The typical bubbling started immediately, indicating air was leaving the bearing, making room for oil to diffuse into the bearing to refill its pores. This was a particularly thirsty set of bearings and the bubbling took about 72 hrs to subside (which indicates that the bearings are full of oil again).

While the bearings were infusing, I updated the main board with a new RPM relay and new 25 turn 

RPM trimmers. This shows the RPM section on the main board in its original condition:

and with new components installed:

Then I focused on replacing the incandescent bulbs in the RPM trimmer panel. This shows the panel extracted from the Beogram:

I removed the bulb covers

and unsoldered the bulbs:

Then it was time to implant the LED assemblies replacing the bulbs. This shows the two LED boards before installation:

Like all parts featured on this blog they are available to other enthusiasts. Just send an email or use the contact form on the right. This shows the boards implanted:

They are 'drop-in' replacements, i.e. perform in the circuit identically to the bulbs. The covers fit back on like nothing happened:

Once the bubbling stopped I extracted the bearings from the oil:

And installed them back in their places in the motor housing. This shows the tools I am using for placing the upper bearing:

After reassembly of the motor it was time to test the RPM stability with the BeoloverRPM device, which allows logging the RPM over long periods of time. A very valuable tool to identify intermittent (most are) RPM stability issues:

It is of course also great for quickly checking the RPM and adjust it if necessary. The BeoloverRPM is also available to other B&O enthusiasts.

I ran the Beogram for ~24 hrs and the blue curve in the graph below is what I measured:

Unfortunately, that was not a good result indicating that the motor also needed new spark snubbers. In some motors they seem to develop some significant intermittent conductivity at voltages they should not, which partially short circuits one or more coils on the rotor. This typically results in RPM drops. I opened the motor up again and extracted the rotor:

The two (visible, there are three, one is behind the commutator) yellowish devices around the commutator are the snubbers. They can be removed by unsoldering the three points where they connect to the coils. This shows the original snubbers with their modern replacements ready for implantation:

This shows the TVS diodes soldered in place:

If you try this at home, make sure that the TVS diodes are located as close as possible to the coils that they not interfere with he brushes when the rotor spins.

After putting the motor together again, I measured the red curve in the graph above, and we can conclude that the patient was finally cured! I listened to a few albums on they Beogram, and it seems to be working very nicely now. Time to send it back home!