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Beolover SyncDrive: DC Platter Motor Replacement for Beogram 4002 and 4004 (Type 551x and 552x)

Late Beogram 4002 and the 4004 (Types 551x and 552x), which have DC platter motors instead of the earlier synchronous AC motors usually suff...

Thursday, October 3, 2019

Beogram 4004: DC Motor Bearings Oil Infusion and Spark Snubber Replacement

A Beogram 4004 DC Motor arrived from Oregon for restoration. A brief bench test revealed the usual shrieking noise coming from dry bearings. This shows the motor as received:
I took the motor apart to remove the bearings:
The bearings are the two small donuts on the black pad. I immersed them in motor oil and pulled a vacuum. Immediately vigorous bubbling started:
This indicated that the air was drawn from the pores of the Oilite bearing material to make room for oil diffusion. After about 3 days the bubbling stopped and the bearings were ready for reinstallation. I put the motor back together and installed it in one of my Beogram 4002s for a RPM stability test with my BeoloverRPM device:
The BeoloverRPM allows logging of the RPM for extended periods of time. The red curve in the graph below shows the curve that I measured:

Far from perfect! The sudden RPM drops indicated that a spark snubber was occasionally shorting out one of the rotor coils. They usually start going bad in this intermittent way. I took the motor apart again to extract the rotor for spark snubber replacement. This shows the rotor with the snubbers up (on the black pad you can see the replacement TVS devices that I had prepared for soldering onto the rotor):

I unsoldered the original snubbers ('ring' on the black pad)
and then installed the TVS units:
I put the motor back together for another RPM test. This second measurement is shown in blue in the above graph. The negative spikes are gone, i.e. this motor is ready for duty again!

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