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Monday, April 20, 2020

Beogram 4002 DC Motor Restoration: Oil Infusion of Bearings, Replacement of Spark Snubbers, and Repair of a Disconnected Pickup Coil

A Beogram 4002 DC Motor arrived from Michigan. This shows the motor as received:
A bench test quickly revealed that the rotor was mechanically impeded, i.e. the motor would not spin at all when connected to a bench supply.
I opened it up and found that someone already had given it a try:
The brushes were bent out of shape and the pickup coil pin broken out. I took everything apart to get to the bearings:
The bearings are the two small donuts on the black pad. When I pressed out the brush carrier, the coil next to the missing pin turned out to be loose and disconnected.
I immersed the bearings into motor oil and pulled a vacuum:
Immediately, vigorous bubbling started indicating that air was drawn from the porous Oilite material, making room for oil diffusion into the bearing.
After about 72 hours the bubbling stopped and I extracted the bearings from the oil:
Before reassembling the motor I cleaned everything in the ultrasonic bath to remove the oil from a previous lubrication attempt, and then I reinstalled the ripped out pickup coil. Since the very fine leads of the coil were ripped short, I needed to extend them with some thin wire to be able making contact with the contacts on the brush carrier:
I put the lower bearing in place and pressed the brush carrier back in. Then I installed the straightened brushes, put in the coil and soldered one of the leads to the contact on the carrier.
I pulled the other lead through the hole left behind by the broken out pin and then glued the lead into the hole with some superglue gel to fix it in place:
Then I tested the resistance across both pickup coils:
This measurement yielded 34 Ohm, which is a normal value (it typically ranges between 32 and 45 Ohm depending on the vintage of the motor). So far so good!
I put the motor back together and installed in one of my 4002s for a RPM stability test with the BeoloverRPM device:

After about 24 hrs I was able to plot the blue curve in the graph below. Not very Beolovely at all!
This measurement result suggested that one or more of the spark snubbers was shorting out occasionally with an increasing tendency. I took the motor apart again to replace the spark snubbers with modern transient voltage suppressor (TVS) devices. This shows the rotor with the original snubbers installed:
I removed the snubber ring. This shows it together with the new TVS devices prepared for soldering across the rotor windings:
I soldered the TVSs between the winding poles:
Now it was time to reassemble the motor and run another 24 hrs test. The red curve in the graph above shows the result of this measurement. This is as good as it gets with the 4002 DC motor. This motor is ready for duty again!

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