Beogram 4000: Restoring the Carriage Position and Solenoid Driven Switches

After rebuilding the keypad cluster of this Beogram 4000 it was time to see to the remaining mechanical switches that control the Beogram's performance. There are two clusters, one beneath the carriage where the switches determine the position of the tonearm, and the other next to the solenoid. The solenoid activated switches are responsible for the activating the tracking mechanism, opening the outputs when the needle is in the groove and to regulate the solenoid power down once the arm is lowered.

This shows the carriage position switches. For getting to them it is best to drive the carriage inwards and remove the red position indicator assembly. Then the board is fully accessible:

After unsoldering the two leads that power the carriage motor and removal of two screws the board can be pulled up and turned around:

Some oxidation is visible on the back of the terminal tabs. For some reason the carriage position switch terminals are usually less oxidized than the other switches under the keypad and next to the solenoid. I removed the terminals

and coated them with gold:

Then it was time to solder them back in and clean the plastic plungers that activate the switches from 40 year old hardened grease. Then I turned my attention to the switches next to the solenoid. The single switch assembly that controls the solenoid power can be directly accessed after removing the two screws that hold it in place:

I extracted the terminal and then removed the assembly that holds the output and tracking activation switches:

This assembly needs to be taken apart while not ripping the thin wires off that are connecting the switches to the control system:

Once this was done I extracted the three terminals. This shows all four terminals as extracted:

'Beautifully' oxidized, I'd say! I brushed the oxide off with 2000 grit paper and a fiber glass brush and then coated them with gold:

That looked much better! I put everything back together and gave the system a first test...Good news: so far everything seems to work...the carriage is now setting down the arm at the 12 inch point and the solenoid actuated properly. A significant milestone has been reached in this restoration!

Beogram 4000: Gold Coating of Solenoid Activated Switch Terminals

The third group of switches that control the Beogram 4000 is comprised of four switches that are activated by the solenoid when the tone arm is lowered. Like the other switches in keypad and under the carriage these usually suffer from oxidation. One of the switches is particularly important, since it controls the solenoid current during lowering activation. If it is oxidized, the arm will not lower anymore. Therefore, we strongly recommend to also get these switches gold plated like the others. This shows the switch assembly next to the solenoid (this photo shows already the gold plated terminals - I forgot to take a picture of the original condition):

The switches are installed on the three small vertical PCBs that are soldered to horizontal base-PCBs. The left most switch activates the tracking system once the arm is down. The two switches on the center PCB are the output switches that ground the cartridge signal lines while the arm is up, and the right most switch (under the wide yellow conduit) reduces the solenoid current to a lower value once the arm has been lowered.

These switches are a bit more difficult to get to than the others since they are connected to many flimsy wires. The solenoid current switch terminal is the only one that can be removed directly after unbolting its base-PCB:

The other two vertical PCBs need to be removed from their base-PCB to access the switch terminals. The first step is to unbolt the base-PCB and turn the board around that the solder points for the vertical boards can be accessed:

Unsoldering requires three hands, but with a de-solder pump and a bit of persistence it can be done with only two...;-):

After removing the oxidized and deformed terminals

I electroplated them:

And then it was time to put them back in:

Nice! On to the PCBs!

Beogram 4000: Gold Plating of Carriage Position Switch Terminals

After restoring the command center (aka 'keypad'...;-) of this Beogram 4000, it was time to restore the carriage position switches. Their functioning is crucial for the Beogram 4000's digital control center and the malfunction of just one of these switches will make it impossible to use the Beogram. Like the keypad switches, they are susceptible to oxidation and gold coating is a reliable remedy for potential contact troubles down the road. This shows the PCB that features the switches after removal of the carriage position indicator assembly:

After removal of the servo motor leads and the two screws that hold it in place, the board can be flipped up:

While the contact tabs look shiny and happy on first glance, their removal reveals oxidation in the contact areas (small black dots):

Electrochemical processes enabled by the humidity in the air acting as electrolyte the terminal metal oxidizes where it is in contact with the opposing contact terminal. Coating with a noble metal like gold is a good idea since it dramatically slows down this process. This shows the terminals after the electroplating process:

And after installation:

All good in the carriage position switches department! On to rebuilding the main PCB and the reservoir and motor capacitors!

Beogram 4000: Restoration of the Keypad Assembly - Gold Plated Switch Terminals and Light Bulbs Replaced with LEDs

After restoring the arm lowering and tracking mechanisms, it was time to rebuild the command center of this Beogram 4000. The keypad assembly contains an early digital control system, which responds to inputs by the keys on the keypad. The mechanical switches associated with the keys are one of the most often malfunctioning features of this design. This is why we recommend to gold plate the terminals in these switches to ensure trouble-free long term performance after the restoration process.

This shows the keypad cluster extracted from the Beogram and opened up:

One immediately sees that one of the light bulbs of the position indicator scale is missing. The other, still present one was not working, too. The chips that are visible are early TTL gates that comprise the digital control system that orchestrates the functions of the Beogram. The keypad switches are located in the lower PCB:

The black spots are oxidation, which often renders these switches permanently open circuit. A quick fix is to pull some 2000 grit sand paper through the terminals and their counter electrodes. But this is only temporary since the oxidation quickly grows back. It is better to remove the terminals, clean them and then electroplate them with a layer of gold. This shows the terminals removed in their original condition:

And after the plating process:

After soldering them back in the OFF button started to work again. After restoring the switches I replaced the light bulbs with LEDs. For the position scale illumination I use red/green LED-based assemblies that are tuned to emit an incandescent-looking spectrum which contains also red. This is important since the position indicator is a red piece of plastic, which would not appear red without the presence of red photons in the LED light. This is the reason I do not use amber LEDs for such tasks. The position indicator would look grayish in the light of amber LEDs. This shows the LED boards:

The RPM indicator bulbs can be replaced with standard red LEDs and current limiting resistors (the bulbs run on 12 V):

This shows the LEDs installed and in action:

And this is how the LED keypad illumination appears when looking at the keypad:

Beolovely! On to the carriage position indicator switches (you guessed it! they also need to be gold plated...;-)!

Beogram 6000 (5512): DC Motor Restauration and RPM Stability Test

After rebuilding the PCB and replacing the light bulbs in the RPM trimmer panel in the Beogram 6000 (5512) that I am restoring right now, it was time to restore the DC motor and do a RPM stability test of the platter drive system. RPM stability in DC motor Beograms is mainly affected by three components: Dry bearings in the motor, corroded relay and trimmers, and thermal issues caused by the incandescent light bulbs in the trimmer panel. 

This shows the extracted motor:

It needs to be fully disassembled to get the Oilite sleeve bearings out:

The bearings are the two small donuts on the black pad. Submerging them in motor oil and pulling a vacuum started the oil infusion process:

This is a beautiful example of dry bearings. The bubbles represent the escaping air drawn out by the vacuum, which is then replaced by oil. Once the bubbling stops after 12-24 hrs, the process is completed and the porous brass bearing material ("Oilite") is again full of oil, ready for another 30 years of duty. This shows the bearings after extracting them from the oil one day after. A bit like french fries...;-)

Then it was time to reassemble the motor and subject it to a 24 hrs RPM stability test. This shows the BeoloverRPM device in action that I developed a while ago:

While it is very convenient for precision adjustment of the RPM, its main function is to log the RPM over time. This allows the generation of graphs showing the stability of the RPM over time, such as the one measured with the above motor:

This curve shows that the RPM is very stable, and that the restoration of the platter drive system was successful.

Beogram 4002 (5513): DC Motor Restoration and Yet Another Exciting Aspect of Achieving RPM Stability

The restoration of the platter drive system of a DC motor Beogram 4002 (5513) requires the installation of modern multi-turn RPM trimmers, a new encapsulated RPM relay, replacing the light bulbs that back-illuminate the user accessible RPM trimmers with LEDs for better thermal stability, and rebuilding the motor with freshly oil infused oilite bearings. See here for RPM measurements taken after each of these steps to see how they individually improve the RPM stability.

The Beogram 4002 that is currently on my bench already received most of the above treatment except the motor restoration. And so here we go:

This shows the extracted DC motor:

To get to the bearings the motor needs to be fully taken apart:

The bearings are the two small donuts on the black pad.

Once the bearings are out, they need to be oil-infused. They are made from porous "Oilite" brass that is factory oil infused. After some run time the oil in the bearings is depleted and it needs to be replaced. This needs to be done under vacuum to draw out the air from the pores that over time replaced the oil. This shows the bearings in a mason jar under SAE30 oil after pulling the vacuum:

The air bubbles show that the oil infusion process has started. When the bubbling stops after 12 to 24 hrs, the process is finished.

Then it is time to put the bearings back into the motor housing.

This shows the bottom bearing re-inserted into the brush carrier plate (which also contains the white feedback coils for the tacho control of the motor):

This shows the process putting the top bearing in:

I use special 3D printed fixtures to press the tabbed ring back onto the bearing to hold it in place securely:

Once the motor is back together it is time to give it an initial test. Run it at ~5V and measure the current. It should be <30mA. If it is higher then the brush carrier plate needs to be loosened and retightened until the current is low enough. This ensures that the motor spins easily for good long-term stability.

Then it is time to put the motor back in and do a multi-hour RPM characterization. I usually run them for 12-24 hours. This usually shows if there are any problems left to tackle or if the motor is good to go. This shows the measurement process with my Beolover RPM device:

It clamps on to the Beogram frame and delivers a stream of RPM measurements to a computer serial port, which allows graphing the RPM stability over time. It is available to other enthusiasts. Just send me an email or use the contact form on the right. The blue curve is what I measured after the above procedure:

And that was a pretty disappointing result! Pretty big RPM spikes towards higher RPMs. Absolutely Beounlovely!

After a bit of head scratching I decided to put the motor into my own Beogram 4002 5513 and run it there. It performed flawlessly as expected after the oil infusion procedure. Then I ran the motor again with my main PCB installed in this Beogram. Again, flawless performance! This pretty much narrowed the problem down to an issue with the main board of this Beogram. I compared the two boards and I realized that C10 on my board had a 10 uF capacitor installed, while the board in this Beogram had a 0.47 uF capacitor installed. I had noticed in the past that some Beograms have 10 uF, while others have 0.47 uF or even 0.33 uF installed as C10.

I replaced the 0.47 uF capacitor with 10 uF and voila! I was able to measure the red curve in the above graph. So we can conclude that it is a good idea to replace C10 with a 10 uF capacitor if there still are RPM issues after doing all the other tasks outlined above to make the system stable.

Of course I wondered why a smaller C10 causes these RPM fluctuations. This shows the control circuit of the DC motor on the PCB:

C10 is the smaller capacitor to the left of the C1003 motor control IC. This shows the circuit that controls the motor:

Essentially, the motor induced feedback ("tacho") signal from the small coils in the brush carrier plate shown above is fed into an amplifier at pin 1. The overdriven amplifier changes the sinusoidal feedback signal into a square wave, which is compared with a time constant network formed by the RPM trimmers and C3 in the Schmitt trigger. This results in a square wave with a duty cycle that now depends on the motor RPM. If the motor RPM is too low the duty cycle increases. If the motor is too fast the duty cycle becomes smaller. That way C8 is charged to different voltages, which changes the DC voltage at pin 5 that controls the amplifiers that control the current that goes into the motor.

The sensitivity of this process is controlled by the feedback fed from pin 4 into pin 5 via the C10/C9/R20 voltage divider. Depending on C10 this feedback gets stronger or weaker. Since the feedback voltage divider is build from capacitors it is essentially only active when the motor voltage changes at pin 4. The impedance of a capacitor is 1/wC (w being the frequency). This means that the impedance of C10 is smaller for 10 uF than for 0.47 uF. This means that the voltage divider tries to reduce voltage spikes across the motor more resolutely, tamping down more resolutely on sudden changes. 

In other words the system becomes slower and less severe in its reaction to RPM changes, which we clearly see in the red curve. It appears to me that 0.47 uF leads to an overreaction of the system and even the establishments of a number of unstable oscillatory states. This can be inferred from the tendency of the blue curve to always jump to similar RPMs. Essentially, it flip-flops between several RPM states in a random way.

The fact that some Beogram 4002 were already factory-fitted with 10 uF (while the above diagram from the service manual shows 0.47 uF) indicates that B&O at some point realized this problem and took corrective action. I'd love to get my hands on this service bulletin! It was probably not so simple in the 70s to quantify long term RPM aberrations due to the absence of digital tools like my Beolover RPM device.

Beogram 4000: Restoration of the Control Panel - LEDs and Gold Plating of Switch Terminals

After gold plating the carriage position switches, it was time to rebuild the control panel of the Beogram 4000 that I am restoring right now. I usually replace the incandescent light bulbs with LED assemblies, and I gold-plate the switch terminals to ensure trouble-free operation down the road.

This shows the original keypad switch terminals, which were quite oxidized:

I removed the terminals:

And gold coated them after removing the oxidation with a fiber glass pen:

And like with the last Beogram 4000 that I did, I forgot to remove one terminal in my first approach...not funny! Here we go:

and after coating:

Pretty, aren't they?? Then I soldered them back into the PCB:

The next step was to replace the light bulbs with LEDs. This shows them in action:

The position scale illumination bulbs were replaced with my custom designed dual red/green LED assemblies:

These boards are available to other enthusiasts. Just send me an email if you want to upgrade your Beogram 4000!

The bulbs that illuminate the RPM trimmers from the back were replaced with through hole high brightness LEDs that are dimmed with 3.3k resistors:

Using strongly dimmed high brightness LEDs saves more current further reducing the strain on the transformer of the Beogram (which gets pretty warm during play) along with the other LEDs that I implanted.

This is an impression of the LEDs in action:

The red-green LEDs create a very authentic background illumination for the position indicator while the presence of red photons in the LED light ensures that the indicator shows up red like when illuminated with incandescent bulbs.