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Saturday, May 28, 2016

Beogram 4004 (5525): Restoration of DC Motor with Re-Infused Oilite Bearings

The final step of the restoration of the platter drive system in a DC motor Beogram (551x/552x models) is an overhaul of the motor itself. These motors came with Oilite brass sleeve bearings, which contain oil inside their porous brass material. After 30-40 years of operation this oil supply runs dry and needs to be resupplied. Dry brass bearings are the foremost cause of strong RPM drops in the DC motor Beograms. RPM increases are usually associated with oxidized RPM relay contacts or defective RPM trimmers. See here for a summary.
In the following I will show a few impressions of the process as I performed it on the Beogram 4004 (5525) that I am currently restoring. If you are interested to do this on your own, I recently made a video that demonstrates this process in detail. 

This shows the motor in place:
Three screws later the motor was on the bench:
To get at the bearings one needs to completely take it apart:
The bearings are the two small 'donuts' at the bottom of the photo. I immersed them in SAE 30 oil in a mason jar and evacuated the jar with a FoodSaver pump. Immediately the characteristic bubbles formed that indicate that the oil immersion process had begun:
After 24 hrs the process had stopped and it was time to reassemble the motor. It can be a bit of a challenge to get the bearings back in without damaging the motor. The concern with the bottom bearing are the very thin wires that connect the windings of the feedback coils. They easily break off and then some micro-solder-surgery becomes necessary to get things right again. The top bearing's challenge is to reinsert it in a way that it sits tight in its place. I designed two 3D printed fixtures to simplify this process. They allow me to apply a high but very controlled force on the tabs that hold the ring in place that presses the bearing into its spring:
After the bearing is in place I usually test it with a toothpick
It needs to move but not wiggle or be loose in any way. 'Tight fit' is the keyword here.
After the motor is reassembled it is time to test it before going further:
My benchmark is a current smaller than 30mA at an applied voltage of 5V and that the motor rotates in clockwise direction. If this is not the case, then the brush carrier plate needs to be realigned until the correct values are achieved.
After passing this test the motor can be reinserted into its noise reduction housing and implanted back into the Beogram. I usually give a restored platter drive system a 24hrs test with my BeoloverRPM device to make sure everything is o.k.. This shows the result before and after for this Beogram 4004:
The black curve is the initial test. Strong RPM spikes can be seen after a few hours of operation. They point towards issues with the RPM relay or RPM trimmers. Since there were no significant RPM drops I am speculating that the motor still would have had a few hours left on its bearings. Not surprising since the keypad looks fairly pristine on this deck and the usual finger patterns are absent, suggesting that the deck was not used much in its earlier life. But I think it is a good idea to do a full restoration of the drive system once the unit is on the bench. This will ensure trouble-free operation down the road.
The red curve was measured after the motor rebuild and exchanging relay and trimmers. The curve is very good and well within the ±0.0025% wow and flutter spec stated in the service manual.










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