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...;-)

Beogram 4004: Replacement of Convertible DIN 7 Plug with a Female Socket and Adapter Cables

While enjoying the Beogram 4004 that I recently restored I was looking for an adequate modern replacement for its convertible DIN 7 plug. This plug is a special design that does not seem to be made anymore. Here is a picture of its business end:

The picture shows the plug in its DIN5 configuration, i.e. it can now connect with any standard DIN5 amplifier input. The two (threaded) orifices above the pins can hold two more pins, which can be bolted in if the Beogram is to be connected to a Beomaster 2400 to take advantage of its remote control, which can be used to start/stop the Beogram. In essence these two additional pins carry the remote control signals for the Beogram. But, of course, since this is a male plug, the pins have to be removable if the Beogram is to be used in a standard DIN 5 setting.

Sadly, these plugs are nowhere to be found these days, and so, if a new plug is desired, another approach needs to be found to preserve the above 'convertibility'. My solution was to replace the plug with a female DIN 7 socket. That way DIN5 (or DIN5-to-RCA) jumper cables can be used for non-B&O set-ups and DIN7 jumpers can be used to connect to B&O setups. In essence, this modification gives the Beogram 4004 the same connectivity design like the later Beogram 800x, which have a panel mount DIN7 socket in their back panel to receive a suitable jumper cable.

It was pretty difficult to find a decent female cable mount DIN7 socket, but finally Steve at soundsheavenly.com saved the day by providing me with a few DIN7 Deltron sockets for cable mounting.

Here are a few impressions: This shows the socket installed. The red and white leads are the signals and the yellow and blue leads carry the remote control.

It is important to note that the signal leads on a female sockets need to be soldered to the other side of the connectors since male and female plugs have mirrored pin assignments to properly maintain the signal chain through the connections. This shows the fully assembled socket:

This is how it looks with a DIN5 jumper cable (also provided by Steve) plugged in:

This Beogram 4004 is finally ready to be shipped back to its owner!

Beogram 4000: A New DIN 5 Plug and Installation of a Grounding Switch

The Beogram 4000 that I am restoring right now had a badly corroded and poorly installed DIN 5 plug. Obviously not the original plug. Here are a couple pictures of the sorry state of this plug:

I think the dangling black lead may have been an attempt to reduce some humming issues (no surprise, considering the way this plug was installed...;-). Besides this strange construction, one of the signal leads was also disconnected due to a sad soldering job. 

I cut off the plug and prepared the leads for the new plug:

This picture shows the advantage of DIN connections over RCA: The entire cable and connectors are double shielded. Each lead is shielded by the signal ground lead and then there is an outer shield that connects the enclosures of amplifier and turntable. That way the shielding is complete, which gives the best defense against EMI. RCA usually only have a single shield via the signal ground. That is why there is usually a separate wire that connects the amplifier and turntable system/enclosure grounds.

Then I soldered it on:

And this is how it looks with the shield on:

Beogolden I'd say!

The final step of refurbishing the signal path was to install a switch that allows connecting signal and system grounds. This is a convenient features if there is a hum when connecting to RCA inputs via an adapter.

Switch to the left (as shown) and the grounds are connected. Switch to the right and the grounds are disconnected as in the original configuration. 

Beogram 4000: Restoration and Adjustment of AC Platter Motor

After rebuilding the electronics of the Beogram 4000 that I am restoring right now it was time to look into the AC platter motor. This particular Beogram 4000 has the old style pulley that does not have belt guards integrated, i.e. they added guards on the posts that hold the motor:

These guards are not really needed since the pulley is slightly crested which securely holds the belt on the pulley. The vertical position of the belt can be adjusted via the tilt of the motor.

Similar to the DC motors of the later Beogram 4002 and 4004, the AC motors also develop dry bearings. This motor made this clear via a noticeable 'knocking' noise when driving the platter that could not be adjusted away via the motor voltage trimmer or motor tilt (without platter some 'cogging' can be normal, since the motor is 'bored' without platter, hence the phase lag for smooth operation between rotor position and AC voltage is too small) . So I took the motor out

and disassembled it:

Disassembly requires drilling out the rivets that hold this motor together:

The standoffs (left most parts in the above picture) need to be reused once the motor is reassembled, i.e drilling should be done carefully in small steps to limit damage to the parts. Since they also served as rivets, they needed to be drilled out with a 3 mm bit to allow 25 mm M3 screws all the way through. 

Anyway, the bearings cannot be extracted without doing damage, so I usually put the entire motor housing in the oil and under vacuum:

After 24 hrs I reassembled the motor using 3D printed 'nut arresters' that allow the replacement of the rivets with M3 bolts and nuts:

the purpose of these parts is to hold the nuts in place under the motor that the tilt screws can be adjusted when the motor is installed. This is how the parts fit on the motor:

After re-installing and adjusting the vertical position and tilt of the motor, I adjusted the motor voltage. Per manual this is to be done at 33 RPM that the sine wave measured at the connection point between motor coupling and phase capacitors is maximized without showing distortion:

This motor could be adjusted to 6.5V RMS as specified in the manual. At 45 RPM is still had 6.0V:

There is no way to adjust 45 separately, i.e. one has to live with whatever 45 shows when 33 is properly adjusted. But 6.0V is plenty to drive the platter vigorously. This motor seems to be back in business!

Beogram 8000: Replacing the Platter Tach Disc and Cleaning the Tangential Arm Assembly

Tonight I completed the tangential arm assembly cleaning and lubrication as well as changing the platter tach disc to a new, metal replacement.

The Beogram 8000 turntables started out with a plastic tach disc containing transparent and black markings that a photo-sensor uses to send pulses to the Beogram microcomputer. The microcomputer dials in the 33 1/3 RPM and 45 RPM speeds with these pulse signals.

Unfortunately, over time these plastic discs did not hold up so Bang & Olufsen replaced them with metal discs. As I mentioned at the beginning of this project, it is quite surprising that this Beogram 8000 actually has an original, functioning tach disc. Feeling that it is on borrowed time we decided to go ahead and give it a more permanent tach disc that Martin Olsen supplies. 

Here is the Beogram 8000 center hub with the tach disc next to the replacement disc I got from Martin Olsen.

...and here is the replacement installed.

Before testing the new tach disc I took the opportunity to clean the floating chassis and take care of some maintenance on the tangential arm assembly. I always clean off old oil and grease from the tangential arm drive so I can inspect the parts and apply fresh lubricants.

I was surprised to discover this Beogram's tangential arm drive screw bushing has a badly worn edge. It appears to come from the stress of the arm return travel.

This picture shows the worn bushing and the floating chassis before cleaning.

Here is a better look at the worn bushing after cleaning the parts.

For now I decided to turn the bushing around. That gives two solid edges pressing against the black, plastic transport bracket during the return travel of the tonearm. The stop travel puts the most stress on the bushing. During play the forward travel is in very small increments so there is very little stress on the bushing.

I reassembled the tangential arm assembly to give the new tach disc a test run. 

The Beogram speed testing with the new tach disc went great. The two speeds immediately lock in. I did some quick tests of the tonearm navigation and so far it runs very smoothly. Later, when I get to the record tracking tests I will re-evaluate how solid the tangential arm drive is working.

Next up I will re-install these components back in the main Beogram chassis and start on the mechanical service manual checks. It is getting close to actually playing some vinyl on this baby.

Beogram 4000: Restoration of the Main and Power Supply PCBs

After replacing the reservoir and motor capacitors of the Beogram 4000 that I have on the bench right now, it was time to rebuild the main and power supply PCBs. It is a good idea to replace the RPM relay and the RPM trimmers along with the electrolytic capacitors since they are often corroded. This shows the main PCB in its original condition:

This is a detail show of the old Siemens relay and the RPM trimmers:

And here an impression after replacing the parts:

The Beolover replacement relay is a drop-in assembly that has the same footprint as the original non-standard Siemens design:

It uses a modern encapsulated relay that promises upward of 2 Min switch cycles before failure...that should last for a few extended vinyl listening sessions!...;-).

The remaining task here was to replace the two Ta capacitors on the power supply board. this shows the original:

The two brown-white units on the right side are the 1uF caps that need replacing:

And this concludes the restoration of the electronics. The next step is the rebuild of the motor.

Beogram 8000: Electrical Work Checkout and Forward/Reverse Voltage Adjustment

After testing some basic Beogram 8000 control functions I am satisfied that the electrical work is done. It is time to bundle up the microcomputer board box so I can move on to the service manual adjustments.

The microcomputer box lid contains a heatsink for the processor device. I had cleaned off the old thermal compound and applied new compound.

The lid attached securely and I can peak in the end and see the thermal compound is sandwiched nicely between the heatsink and processor.

While the main Beogram components are still out of the chassis this is a good time to do some of the service manual adjustments. There is still one that is related to possible electrical tasks and that is the adjustment of the forward and reverse voltages for the tangential arm transport.

There is a common lamp with two photo resistors on either side in the Beogram control panel. One photo resistor (R9) is for forward movement and the other (R10) is for reverse movement. The voltages on those two resistors should be 650mV when the Beogram 8000 has been on for at least five minutes and the Play button is activated (with the tangential arm assembly stationary).

I initially measured around 600mV for the reverse photo resistor (P5-6, R10) and 300mV for the forward photo resistor (P5-4, R9). Here are the measurements after the adjustment.

Here is what the related connector, P5 looks like on the Beogram 8000 control panel.

This picture shows the final position of the forward/reverse adjustment screws.

The relative positions are a little unbalanced to say the least. I will recheck those forward/reverse arm motor voltages again before wrapping up this project to verify the settings are stable.