Beogram 4002 (5513): Restoration of DC Motor: A New Tool and a New Spark Suppressor

I am collaborating with Sonavor on a Beogram 4002 restoration and took over the DC motor restoration. What was supposed to be a routine oil infusion of the porous brass 'Oilite' bearings turned into finding an exciting new failure mode for these motors. It seems there is just no end to the evolution of our restoration process for these DC motors.

First I set out as usual to infuse the bearings with fresh oil under vacuum. The motor ran rough when connecting it to my bench supply, so it was clear that it needed this procedure. This shows the motor as extracted:

When I took it apart I realized this motor had the (dreaded) old style brush carrier, which often is glued to the posts that align it and that hold the feedback coils in place:

It is immediately recognizable by the brown color. This design is a bit problematic when trying to get the bearing out that sits underneath this carrier. The carrier has to be lifted up, and I already broke the very fine leads to the coils a few times when I pried it out. When this happens the repair requires 'micro-soldering' to get the flimsy wires connected again. This time I thought 'no more of this' and I instead developed a special tool that fits exactly into the openings in the metal bottom:

Now one can simply put the assembly on the tool and while applying even pressure the carrier is pushed up together with the coils. This has the great advantage that the coils do not separate anymore from the carrier and the wires do not suffer stress. This shows the motor disassembled:

I put the bearings into the oil and pulled a vacuum:

Immediately bubbles emerged from the bearings indicating that air was being pulled from the pores in the bearing to make room for oil, which then diffuses into the opened up space in the brass material (nature abhors vacuum...;-). After 24 hrs the bubbling had stopped and I took the bearings out:

I assembled the motor using my tool for inserting the top bearing:

Then I tested the motor in one of my Beogram 4002s and measured the RPM performance with my BeoloverRPM device (this unit is available to other enthusiasts - just send me a message via the form on the right or send an email if you are interested). The blue curve in the graph below is what I measured:

Not very Beolovely, for sure! Still major RPM drops that would even make the most unmusical ear weep. 

After a bit of head scratching I remembered that I had to replace one of the spark suppressors in such a DC motor to get it to run again. So I hypothesized that these suppressors (or 'snubbers') could perhaps have intermittent issues. This shows the side of the rotor that has the spark suppressors installed together with the TVS diodes I used to replace it:

I removed the original suppressor 'ring':

and installed the new units:

They fit perfectly between the solder points of the rotor windings above the commutator. One needs to be careful that they sit as close to the coils as possible, otherwise they can scrape on the brushes when the motor is assembled.

After this repair I put the motor back together again and ran it from my bench supply. I immediately noticed that instead of 30 mA at 5 V it now only consumed about 20 mA. So something changed to the better reducing current draw.

I installed the motor again in my 4002 and ran another 24 hrs test. This yielded the red curve in the above graph, which appears to support my hypothesis of an intermittent spark suppressor. It seems this motor is back in business. Hopefully, this is the final chapter on DC motor restoration in Beogram 400x...;-)