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Wednesday, March 8, 2017

Beogram 4002 (5501): Repair of 'Knocking' AC Motor

I usually like to take my time for testing Beograms that I restored to make sure there are no intermittent issues. Sometimes also additional problems arise when one starts using them again on a routine day-to-day schedule. And so it happened with the AC motor Beogram 4002 (5501) that I completed recently. After playing a number of records I noticed a faint 'knocking' coming from under the hood. After removal of the aluminum plates I realized that the knocking was in sync with the rotation of the AC motor. First I tried to adjust the motor tilt, which brought some reduction of the noise, but after a while it always came back, and it also grew in intensity the more records I played until it was clearly audible whenever a quieter track was playing. Absolutely unacceptable for a turntable!

There was only one option: Take the motor apart and have a look! This shows the motor installed (with my 3D printed capacitor adapter in the background):
I removed it from the enclosure. Unfortunately, the AC motor is soldered in, i.e. one has to unsolder the leads to take it out:
This motor is held together by two rivets with M3 threads that also serve to hold the leveling screws. Sometimes these rivets have already failed at this age of the Beogram and it is easy to extract them, but not in this case. I had to remove them with force. One of the came out by applying a bit of pressure with adjustable pliers:
With the other one I was not that lucky and I had to drill it out on my work bench. After that was accomplished the motor opened up:
This shows the rotor and the parts that are on the shaft:
this shows one of the Oilite brass bearings:
They look very similar to the bearings that are in the later DC motors.
Unfortunately, one cannot easily extract them. Or at least I did not see an obvious way, and in the absence of a spare motor to replace this one, I elected to leave them in place of the oil infusion process. This meant I had to put in the entire motor housing into my mason jar and put it under vacuum:
The usual bubbles formed after the vacuum was pulled. After 24 hours I took the parts out and drained the oil onto an absorbent paper towel for another 8 hours. then I wiped everything dry of oil. This process worked fairly well. I put the motor back together. In absence of replacement rivets I designed 3D printed parts outfitted with M3 nuts to hold the adjustment screws. This shows the part on the side facing away fro the enclosure:
and on the enclosure side (there is less space, i.e. it had to be a bit more spartan):
This shows the motor installed with these parts:
When the motor is bolted in like this it is held together securely by the two mounting screws that are in the two other orifices in the motor housing.

Testing of the motor after this procedure demonstrated that the knocking was gone and that the RPM was stable. I guess the lesson learned here is that the 4002/4000 AC motors also needs restoration at this point in time to re-infuse the dried out bearings with oil. But it seems the issue may be a bit rarer than in the DC motors, where almost all of them seem to be afflicted by dry bearings. I am hypothesizing that this difference may be related to the lower RPM of AC motors, which should consequently result in less oil being removed from the bearings per number of played records. 

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