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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...

Friday, May 25, 2018

Beogram 4002: Restoration of a Very Oily DC Platter Motor

I recently received a Beogram 4002 DC platter motor from Berlin, Germany for restoration. I immediately noticed that the motor was quite oily on the outside, probably from an attempt to lubricate it to enhance RPM stability. This shows the motor as received:
I disassembled it to extract the bearings for oil infusion:
The bearings are on the black pad up front. The inside of the motor was also very oily, but of course this does not help the bearings, since they need to be infused with oil under vacuum. This is necessary to pull out the air in the pores of the Oilite brass material that the new oil can penetrate into the bearing. This shows the bearings submerged in motor oil after pulling a vacuum:
The bubbles rising from them is the air exiting the bearing. This process usually takes 24-72 hrs depending on the state of the bearings.
After the bubbling stopped I extracted the bearings from the oil and put them on a paper towel to wick the excess oil away:
Then I re-assembled the motor again and implanted it into one of my 4002s. Then I ran a 24 hrs RPM stability test with the BeoloverRPM device, which allows the logging of the RPM for extended periods of time:
The BeoloverRPM is available to other enthusiasts. Just send me an email or use the contact form on the right. The very unstable blue curve on the graph below is what I measured:
This indicated that I was not done yet with this motor. First I thought the motor probably needed new spark suppressors since that usually causes large downward spikes, but the overall pattern looked somewhat different. Finally, I remembered that I had a similar issue when I just started developing the restoration procedure for these motors a few years back. Initially, I also thought that simply putting a lot of oil in everything would probably help. But when I squirted some of the oil on the commutator of the motor (to 'further reduce friction'...;-), the motor performance dramatically worsened, like in this case. It seems that the oil creates an insulating layer on the rotor impeding the current flow though the rotor, which causes RPM reductions.
So I took the motor apart again and cleaned rotor and brush carrier (together with the pulley - it was also oily and I worried that I would not be able to glue it to the shaft once done) in my ultrasonic cleaner:
I put it in warm water with some dish washing detergent. Immediately after starting the cleaner the water became very cloudy.
This indicated that an emulsion was forming between the oil on the parts and the water. I replaced the water three times and ran the cleaner a couple minutes in each batch. Then the water finally stayed clear and I gave it a fifth run in pure ethanol to get the water out of the rotor windings etc...The I put everything out for drying:
After re-assembling, I tested the motor for another 24 hrs and I was able to measure the nice red curve in the above graph. So everything is fine now with this motor, and it is ready for return shipping to Berlin!

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