Beogram 4002 DC Motor Restoration

This post reports about the restoration of the 2nd Beogram 4002 DC motor (1st: see here) that recently arrived from the UK. This shows the motor as received:

I took the motor apart to extract the bearings for oil infusion:

The bearings are the two small brass donuts on the black pad up front. I immersed them in oil and pulled a vacuum. Immediately, vigorous bubbling started:

This indicated that the air was drawn from the porous Oilite bearing material making room for fresh oil to diffuse into the bearing. After about 48 hrs the bubbling stopped and I extracted the bearings from the oil:

I reassembled the motor and installed it into one of my Beogram 4002s. I ran it for 24 hrs measuring the RPM in 10s intervals with my BeoloverRPM device. This is the curve that was measured:

This is a pretty good result, but this motor seems to have slightly more variation than the first one. It is very likely that it will 'calm down' after it runs for some more time. The root cause for this kind of behavior may be that the orientation of the bearings improves over time as the motor runs. See here for a documented case, where the improvement happened over three weeks of running the motor. The joys of analog audio!...;-)

Beogram 4002 DC Motor Restoration

Two Beogram 4002 DC platter motors arrived from the UK. This post reports about restoring the first one. This shows the motor as received:

I disassembled the motor to get the bearings out:

The bearings are the two small donuts on the black pad upfront. These bearings are made from a porous material ("Oilite"), which has a limited amount of Oil infused when new. Over time this oil leaches out lubricating the shaft. When the oil is gone the shaft runs dry, which seems to be the case with most Beogram 400x motors these days.

I immersed the bearings in motor oil and pulled a vacuum:

Immediately, vigorous bubbling started. This is a sign of the air drawn out from the pores by the vacuum. Once the air leaves, the volume is filled with oil that diffuses into the material to replace the air ("nature abhors a vacuum"...;-). After about three days the bubbling came to an end and I assembled the motor for testing. 

I installed it in one of my Beogram 4002s and ran it for 24 hrs while monitoring the RPM every 10s with my BeoloverRPM device.

This is the curve I measured:

That is as good as it gets. This motor is ready for duty again!

Beogram 4004 DC Motor Restoration: Yet Another Failure Mode - Strongly Oxidized Brushes

A Beogram 4004 motor arrived from Quebec for restoration. It ran a bit noisy, so the diagnosis was 'needs Oilite bearings re-infused with oil'. This shows the motor as received:

I took it apart to extract the bearings:

The bearings are the two small donuts on the black pad upfront. I started using a new 3D printed tool (grey plastic on the right that holds the bottom part of the motor housing) that allows me pressing out the plastic-type brush carriers of 1977 and later motor generations in their entirety. This reduces the risk of breaking the pickup coil leads or damaging the plastic carrier when extracting the bottom bearing.

I immersed the bearings in motor oil and pulled a vacuum. Immediately, vigorous bubbling occurred:

The bubbling indicates that the vacuum pulls out the air from the dry bearing pores. This makes room for oil to diffuse into the bearing, which then can lubricate the shaft of the rotor again.

After about three days the bubbling stopped and I extracted the bearings from the oil:

I re-assembled the motor and installed it in one of my Beogram 4002s for a 24 hrs RPM stability measurement with the BeoloverRPM device. It allows logging the RPM in 10s intervals over extended periods of time.

The blue curve in the graph below is the curve I measured:

Obviously, something was not right yet. Usually, if the oil infusion does not stop RPM drops entirely, one or more of the spark snubbers of the motor are gone. So I took the motor apart once more and installed unidirectional TVS diodes to replace the original spark snubbers. This shows the rotor with the original 'ring' of snubbers installed:

I unsoldered the connections of the devices:

One of the snubber devices was packed one with some glue. I had to gently heat up the area with my hot air blower to get the device off without destroying the rotor windings. Then I installed new unidirectional TVS diodes:

And tested the motor again. This yielded the green curve in the above graph. The drops were gone, as expected, but there were some new spikes to higher RPM. 

It turned out that the fix for this issue was to clean the brushes of the motor with a fiberglass brush. Upon closer inspection the brushes showed an unusual build up of carbonaceous residue in the area where they make contact to the commutator of the motor. My theory is that the motor was run for extended periods without proper spark suppression.

I put the motor back together again and measured another RPM stability curve (shown in red in the above graph). This curve is pretty much as good as it gets. In my experience the small variations that are still present will go away after playing the deck for a while (it seems the bearings and the shaft need some time to find perfect alignment after reassembling these motors). This motor is ready for duty again! 

Announcing Bang & Olufsen Audio Components For Sale On The Beolover Blog

Occasionally the situation comes up where we can sell one of our vintage Bang & Olufsen restorations. For some time we have wanted to do this and there is no time like the present.

Recently we restored two beautiful Beogram 4004 turntables belonging to the original owner who purchased them new in the late 70's.  The first turntable has rosewood trim and the second turntable has teak trim.

The owner has decided to keep the rosewood trimmed turntable and sell the teak trimmed one.

We are presenting that teak trimmed Beogram 4004 as our first audio component for sale here on the Beolover Blog and you can view it on our AUDIO FOR SALE page.  There is also a link to the for sale page on the right hand menu of the blog.