Beogram 4004 Type 5526: Securing the tangential drive motor and installing a new pulley

In the previous post of this project I identified the need to fix the tangential arm drive motor vibration damping. The old material had deteriorated and was starting to crumble into dust that can get into other components inside the Beogram 4004 cabinet. I also noticed that the tangential drive pulley had some cracks in it.

I removed the pulley and drive motor so I could clean the debris up and make the repair.

Ugly stuff but it is just old foam material and is easy to remove. You can see how fine the debris gets though. That stuff could end up on other moving parts of the Beogram so you really need to get it cleaned out.

There are a lot of options to replace the old damping material. These days remote control cars, helicopters, planes and drones are all the rage. Those all run on electric motors and there are a lot of products to dampen motor vibration. I am using some 1/16" self adhesive neoprene material.

The fit is perfect and the motor does not have any play inside the case anymore.

I added the new pulley and this part of the restoration is complete.

Beogram 4004 Type 5526: Preparing the Texas Beogram 4004 for restoration

To begin the restoration on this Beogram 4004 I need to clean up the loose debris from the deteriorated plastic bushings, The old plastic on these Beogram 400x units breaks into small pieces that show up all over the place inside the turntable cabinet.

I removed the components from the Beogram cabinet so I could access the three suspension mounting points where the deteriorated bushings are.

Front mount

Left mount

 Right mount

The only other place I saw a need for some mechanical restoration is on the tangential arm drive assembly. There are some of the usual hairline cracks on the original plastic pulley and the drive motor mount material is deteriorating.

Those problems are due to age and are pretty typical on the Beogram 400x units. I will replace the pulley with a remanufactured aluminum one. On the motor I will remove the old material and repack the motor in its housing with new material. When the old material deteriorates like this the motor will move around. You can push on the motor shaft and feel the play of the motor inside the housing. As you can see both problems are easy to access for the repair.

There are two circuit boards to work on in the Beogram. The main board and the output board. The restoration on those will be to replace the electrolytic and tantalum capacitors with new capacitors, replace the trimmers for the platter motor speed and replace the two relays (one on each board).

Main board

Output board ... note that the Beogram 4004 output board has additional circuitry to support the wired remote control feature of a Beomaster receiver (i.e. a Beomaster 2400).

Now to begin the restoration work.

Beogram 4004: DC Motor Restoration and RPM Stability Test

A Beogram 4004 DC motor arrived from France for restoration. This shows the motor as received:

I disassembled it to extract the Oilite bearings:

The bearings are the two small donuts on the black pad. This motor had a seized mounting screw on the inner enclosure, which I isolated in the small plastic bag for the time being.

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

Immediately, vigorous bubbling started, indicating the removal of air from the porous Oilite bearing material. Removal of the air from the pores enables diffusion of fresh oil into the bearing. Oilite bearings lubricate the shaft by diffusion of oil from the bearing. At some point the oil runs out and the bearings run dry. This vacuum infusion procedure restores the oil and the process can start anew.

After about 48 hours the bubbling stopped and that was the sign that these bearings are 'full' again. I extracted them from the oil

and assembled the motor. This shows the bearings re-installed:

At this point it is usually a good idea to check if the very fine wires to the feedback coils survived the surgery. This can be done by measuring the resistance across the two coils, which are connected in series:

If everything went well the resistance should come in at about 42-46 Ohms. If open circuit is found, some microsurgery is necessary to reconnect the coils.

I was lucky with this motor and the measurement checked out. I put the rotor back in and installed the motor in one of my Beogram 4002s for performing a 24 hrs RPM stability test with my BeoloverRPM device, which allows logging the RPM in 10s intervals for extended periods of time:

This is the curve I measured:

This is as good as it gets with the Beogram 4004 DC motor. This motor is ready for its journey home to France!

Beogram 4004 (5525): Replacement of Sensor Arm Bulb with LED Assembly and Calibration of Sensor Amplification Circuit

A Beogram 4004 that I restored a few years back returned to my bench for some TLC/adjustments. Along with the adjustments it was decided to also update the restoration to the latest Beolover standard, i.e. the sensor arm bulb needed replacement with an LED assembly, which was not yet available during this Beograms's first visit.

This shows the sensor arm bulb cabinet pulled out together with the flexPCB based LED replacement circuit:

After extracting the bulb I installed the LED assembly and fired it up:

The LED that is used on the board is a warm white LED, i.e. it has enough red emission to make the B&O logo light up nicely.

The next step of this procedure is to adjust the biasing of TR3 properly. TR3 is responsible for amplifying the weak signal of the photo diode that is in the sensor arm. By design the base of this transistor is biased with a 1MOhm resistor (R26) pulled up by the collector. Unfortunately, this biasing scheme depends on the transistor gain Hf, which can vary quite drastically across several transistors of the same production run.

As a consequence one rarely finds a Beogram 4002 or 4004 that has the proper manual-specified 4V at the collector. This issue can be corrected by adjusting R26 until 4V are present. This can be done by replacing R26 with a 2MOhm multi-turn trimmer, and adjusting for 4V. This shows the trimmer installed from the solder side of the board that it can be adjusted while the Beogram is running:

After adjusting the collector voltage to be 4V I unsoldered the trimmer, and installed it from the component side:

After this it was time to measure the sensor signal at the collector of TR3:

The amplitude of the signal is 6.3V, exceeding the manual specified 6V, which is good. It should not be smaller, but a bit larger is o.k. Important here is also that the signal drops (that correspond to the passing of a platter rib) go all the way down to 0V. 

This result indicated that the sensor circuit was working properly, i.e it was time to test with and without a record present. All went well. I will play it a bit more and then it will be time to send it back to its owner!

Beogram 4004 Type 5526: New Beogram project from Texas

It is time for a new Beogram 4004 project from Texas. This Beogram is actually one of two Beogram 4004 turntables I will be restoring from the original owner!  I love it when I get the opportunity to get a Beogram from an original owner back into working order. I always take a lot of pictures because most of the Beogram should be in place just as it was when it came out of the Bang & Olufsen factory.

That is definitely the case with this first unit. This has to be the most pristine Beogram 4004 I have encountered, if not the most pristine Beogram period. This turntable has been so well taken care of that I am amazed. 

Let's take a look.

Look at the DIN phono plug and the AC power plug. How are those still so clean? They are like they were brand new!

I always remove the cabinet parts that will not be involved in any restoration work so they can be stored away safely until the work on this turntable is complete.

I hate to sound redundant but this unit is remarkable.  Note: I was so tempted here to say "sound like a broken record" but that is too corny even for me.

I actually had opened this Beogram earlier and pulled the DC platter motor for restoration by Beolover. He recently posted that restoration on the blog here. 

This picture finally shows that there is some deterioration in this 1978 turntable....the usual pieces of the floating chassis locking bushings. The plastic bushings B&O used on these turntables almost always dries out and breaks apart over the decades. Beolover makes 3D replacements for them so no worries about that. It is just a matter of cleaning the old pieces out of the cabinet.

A nice improvement B&O made on these Beogram 4004 turntables was to use metal guide washers for the cabinet trim. The earlier 4002 Beogram units have plastic washers that are almost always broken.

The components for the tangential arm assembly all look great.

The small rubber belt for the tangential arm drive pulley is broken but that is expected.  I was going to change the belts anyway.

The black plastic tangential drive pulley looks to still be perfect. I will have to examine that closer once it comes time to start operating the Beogram. There could be some hairline cracks. We typically go ahead and change these pulleys to metal pulleys.

I can't wait to get this turntable back to playing records like it is used to.

Beogram 4004: DC Motor Restoration

This is the second Beogram 4004 DC motor from the batch that I recently received for restoration. The restoration report for the first of these motors was posted here.

To get to the Oilite brass bearings the motor needed to be disassembled:

The two small donuts on the black pad are the bearings. I immersed them in motor oil and pulled a vacuum:

Immediately bubbling started, indicating that the air trapped in the pores of the bearing was extracted into the vacuum. This makes room for oil diffusion into the bearing restoring their lubrication.

After about 48 hours the bubbling stopped and the bearings were ready for re-implantation:

I put the motor back together and installed it in one of my Beogram 4002s for the RPM stability test with my BeoloverRPM device, which logs the RPM in 10 sec intervals:

This is the curve that I measured after 24 hrs:

This is as good as it gets, and this motor is ready for duty again. It should be noted that this motor needs 10uF as C10 on the main board. With the often found 0.33uF or 0.47uF capacitors it ran less stable.

Beogram 4004: DC Motor Restoration

I recently received two Beogram 4004 DC motors for restoration. This is the report on the first one. This shows the motor as received:

I disassembled the motor to get the bearings out for oil infusion. These motors use Oilite porous brass bearings infused with oil. After decades of service these bearings run dry (oil gets slowly extracted on the shaft as the motor spins). Luckily, they can be replenished by immersing the bearings into motor oil under vacuum. This shows the disassembled motor:

The bearings are the two small rings on the black pad. I put them in the oil and pulled a vacuum:

Immediately, bubbling started. This indicated that air is drawn from the pores in the brass material, making room for oil to diffuse into the bearing.

After about 24 hrs the bearing stopped and I extracted them from the oil:

I reassembled the motor and installed it into one of my Beogram 4002s. I ran it for 24 hrs, while logging the RPM in 10s intervals with my BeoloverRPM device (available to other enthusiasts - just send an email/message using the contact form). This is the RPM stability curve I measured:

This is as good as it gets...this motor is ready to spin some records again!