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Beogram 4002: Restoration of DC Motor Video Published - Check It Out!

By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...

Monday, March 27, 2017

Beogram 4002 (5513): Final Adjustments and Test Drive with Freddy Hubbard's 1975 Album "Polar AC"

It is always a happy moment when a restoration concludes and another Beogram 4002 sits there in pristine Beoloving happiness playing a favorite album! However, before I was able to enjoy this moment with Freddie Hubbard, the final adjustments had to be done to this Beogram. The first step in the adjustment process is always to get the platter and arms perfectly aligned. The next step is to align the platter with the aluminum panels that surround it, while keeping the chassis floating freely suspended by the spring loaded arms that pull it up. This can be a difficult process since there is very little leeway to get all these parameters properly adjusted.

Once this is accomplished the arm lowering limit needs to be adjusted. This is a very important adjustment since it is the final safeguard against needle loss should the record detection mechanism malfunction. You may have wondered why the ribs on the platter have U-shaped recesses...their main purpose is to give the needle some space in case it is lowered accidentally due to a malfunctioning photodiode in the sensor arm or a confused control system. The lowering limit needs to be adjusted that the needle clears the ribs in those U-shaped troughs:
Once this is done, the tracking force can be adjusted. I usually replace the flimsy circlip that holds the  counter weight in place with a nut that can be tightened. This gives this mechanism much mor longterm stability and it stays calibrated during shipping. This shows the original setup:
and with the M3 nut tightened in place:
Then the weight can be adjusted. I always adjust everything that the adjustment scale is accurate around the 1.2g mark which is the recommended tracking weight for most B&O cartridges:
One more thing before the test drive: This Beogram had missing hood mounting springs. They often get lost when people unscrew the hood mounting bolts. Luckily one can replace them with the springs from Pilot G-7 pens which have a larger than usual diameter cartridge:
I cut them in half and widen their diameter a bit on one end. This allows to fit them to both of the hood mounting bolts:
And the it was time for the test drive. I selected Freddie Hubbard's "Polar AC" album, which he recorded in 1975 for the CTI label. It has some quite awesome tracks on it. My favorite is "People Make the World Go Round" (which has another great interpretation on CTI by Milt Jackson - I'll feature this one some other time on a different 4002!). Anyway, here is an impression:
This near-mint first release record was restored to its original glory with the CleanerVinyl Pro ultrasonic record cleaning system. In fact, we used this lovely restored Beogram to make a before/after video that demonstrates how impressively how well ultrasonic vinyl cleaning works by featuring the People Make the World Go Round track before and after cleaning in a video:

Beomaster 8000 & Beocord 9000: Completing the Restoration

The final step in this Beomaster 8000 and Beocord 9000 project is a system test. To do that I needed plenty of space as these units take up a lot of room. Especially when you add in a Beogram turntable to complete the system. It is also time to clean up and put parts away. My soldering bench is my largest desk so I used it to set up the system test.

Here is a picture of everything layed out for testing.  Quite lengthy! With all of the system connected together the dependent functions can be exercised. A really nice feature of this system is that turning on one component will notify and turn on any related component. For example, placing a record on the Beogram and pressing the Beogram play button will turn on the Beomaster and switch it to phono mode. Same thing for the Beocord. Conversely, selecting PH (phono) on the Beomaster will cause the Beogram to operate. Turning the Beomaster off turns all of the other components off. To make things even nicer, all of these features also operate from the wireless remote control. The well thought out design is what makes this Bang & Olufsen system my favorite. Of course B&O continued some very cool integrated systems after this one but this system was a few years before the digital age and is analog audio at its best.

I continued testing by doing some recording on the Beocord 9000 with the Beogram 8002 as the source. I used a new TDK MA90 cassette (metal type IV) and ran the Beocord tape calibration procedure. This is when you start really appreciating the complexity of the Beocord 9000. After calibration I selected Tape End so the Beocord would run through the tape once and figure out exactly how much recording time is available on the tape. The Beocord tape counter of course displays the amount of time used and remaining. Really handy if you want to fill up as much music you can on a side.

For the recording test I like to use music with some distinct instruments and some that really get up there in range. That usually means some brass. So this time I selected an old Maynard Ferguson album from 1973, "M.F. Horn 3". It has that early 70's sound and of course Maynard screams on the trumpet. Quite fun if you like that era like I do. The recording testing all worked perfectly. I also tested pausing and restarting recording. The Beocord fast forward and reverse functions all work great (remember the belts and pulleys are all new). This is the payoff for the time and effort it took to get these beautiful machines fully restored.

With all of the Beosystem 8000 functionality working beautifully it is time for their owner to enjoy them so Beomaster 8000 and Beocord 9000 are ready to be packed up and shipped home.

For me it is time to prepare for the next project.

Sunday, March 26, 2017

Beogram 4004: DC Motor Restoration and RPM Stability Test

A Bang & Olufsen enthusiast from Texas sent me the DC motor from his Beogram 4004 turntable for restoration. The deck had developed the usual dramatic RPM fluctuations that go along with dried out Oilite shaft bearings. Most of the Beogram 400x motors from the 1970s have now dry bearings that need to be re-infused with oil under vacuum to give these motors another lease of life. 

The Beolover restoration process involves disassembling the motor, extracting the bearings, placing them in motor oil under vacuum, reassembly and then a 24 hrs RPM stability test to make sure that the motor performs again. Here are a few impressions from this particular motor. The motor is very easy to extract. Removal of four screws and unplugging is all it takes:

This shows the bottom part of the motor that houses the brushes and the lower bearing. It also supports the two small coils that are responsible for generating the tacho feedback that allows the control electronics in the Beogram to keep the RPM constant:

This shows the motor taken apart:
The two bearings are on the black pad to the right.
Then it was time to put the bearings into the oil and pull the vacuum:
This picture is a beautiful example of the process. Both bearings are very thirsty and a lot of air is being pulled out by the vacuum to make room for the oil to enter the porous brass material.
After about 24 hrs the bubbling ceased and the bearings were ready for reinstallation:
This shows the 3D printed tools that I use to install the top bearing back into the motor housing.
Once the motor was back together, I did a 24 hrs RPM stability test with my BeoloverRPM device that allows the monitoring of the RPM over extended periods of time. See here for more information - it is available to other enthusiasts for occasional RPM checks and motor diagnosis.
This shows the tool in action:
And this is the RPM graph that I measured on this motor:
The small fluctuations are well below the audible threshold and are a result of the control system interacting with the motor to keep the RPM constant...somewhat like driving along a straight road, one steers a bit left, a bit right...and so on. The initial slight drop of the RPM is normal and a result of the motor and the electronics warming up and then reaching a steady state. This motor is good for another few decades of service in its Beogram 4004.

Thursday, March 23, 2017

Beogram 6000 (5512): A New Arrival from Norway

Recently a Beogram 6000 (5512) arrived from Norway for some TLC. The Beogram 6000 are essentially 4002 Beograms that have the CD-4 preamplifier and decoder board installed. This renders this deck capable of playing 4-channel records from the 70s. Exciting!

Here are a few first impressions: The packaging was an old acquaintance that was used earlier to ship a Beogram 4000.
Double boxing with foam
is just the best way to ship a Beogram. And so this Beogram 6000 arrived in a happy condition without any shipping damage:
It has very nice aluminum surfaces and even an almost pristine keypad:
The veneer corners of the plinth are also pretty nice:
It is just great to see that some owners take really good care of their Beograms!
Here is a peek under the hood:
Everything seems original and signs of 'human creativity' are absent as far as I can tell. A great starting point for a full restoration. There are some 'usual issues', like cracked cabinet guiding washers
and disintegrating transport lock bushings as this orange fragment next to the motor suggests:
This shows part of the CD-4 board that is mounted underneath the keypad:
After this initial inspection, I set the voltage selector to 110V and plugged it in. Pressing start yielded the proper response and the arm moved to the set-down position and the solenoid was properly triggered. The DC motor ran but made some noise, an indicator that the oilite bearings are in need of an oil infusion. Overall, this Beogram is an excellent starting point for a restoration, and I am confident that it can be brought back to like-new performance and with great looks (the hood needs a bit of polishing, but nothing out of the ordinary). 

Beomaster 8000 & Beocord 9000: Re-Installing the Lid Dampers

In the last post on the Beomaster 8000 lid damper restoration I left off with the first damper cleaned out and fitted with new damping grease. I had to experiment with the re-assembly of the dampers to get them sealed and working properly. Out of the six damper assemblies I had available I have restored four so far. Here is a lid damper completely disassembled.

Here is another picture of the damper with new damping grease ready for joining the two main pieces.

Next is the joining of the damper and Beolover replacement arm to the deck lid. It should be done with all of the components removed from the Beomaster. Snap the new arm to the deck lid first. After that the other end of the arm attaches to the damper by sliding over the keyed mounting post on the lever. 

Now it is ready for re-installation in the Beomaster 8000.

The Beomaster 8000 deck lid now opens slowly and smoothly as controlled by the damper device. This is the result I was looking for.

Also part of this lid damper restoration is the damper for the Beocord 9000 that goes with this Beomaster. Again, installation of the reworked damper is done with the lid removed from the unit.
The Beocord still has its arm intact. Note that while the damper unit is identical to the Beomaster, the arm for the lid is not the same. The Beocord arm is shorter in length.

Here is the reworked damper installed back on the Beocord deck lid. Note the position of the damper lever and arm. As was the Beomaster, attaching the arm to the damper is done by sliding the arm on from the side. To install this on the Beocord deck lid the result will be the arm on the lower side of the damper. After the arm is attached to the damper you must rotate the damper 360° so the arm is on top (as shown in the picture below). Otherwise the damper won't be able to do its job.

Now the deck lid assembly is back in place and this Beocord 9000 has a properly working deck lid again.

This concludes the lid damper rework for these two units. The new damping grease should not liquify and leak like the old grease did. At least not for another thirty plus years.

Wednesday, March 22, 2017

Beogram 4002 (5513): Replacement of the Sensor Arm Bulb with an LED and Improvement of the Record Detection Circuit

On the way to an all-LED Beogram 4002 the replacement of the sensor arm LED is usually my final step. And so, after restoring the DC motor, it was time to implant my recently developed flex-PCB based LED assembly to replace the incandescent light bulb. This shows the sensor arm with pulled out detector assembly:
If you click on the picture to get the full resolution and magnify it a bit on your screen, you will see that there is already a small black spot on the glass bulb, indicating that significant tungsten evaporation has taken place from the filament. This points to a likely near term demise of this bulb, which would incapacitate the Beogram. The installation of a LED promises a much better long term stability, and this is the main reason for LED upgrades outside the scarcity of fitting small light bulbs - they are hard to find these days.
The challenge with this particular LED upgrade is of course the small space and the need to replicate the power draw of the light bulb to not confuse the record detection circuit (see below).

My LED replacement therefore contains an additional resistor that compensates for the much lower power draw of the LED (which is a high brightness low color temperature model running at a very low emission level). This shows the assembly:
It is built based on a flex-PCB that can fold into the bulb compartment:
The glued on 3D printed red wedge on top assures that the LED sits in a place comparable to the bulb filament.
This shows the assembly in action:
Note the accurate color of the B&O logo, which is a result of the use of a low color temperature LED which has a high red component.

Any light source replacement in the sensor arm should be accompanied by measuring the sensor response when the arm is over a spinning empty platter to verify that the signal is strong enough to precisely and reliably trigger the protection circuit that prevents arm lowering in absence of a record. This is the trace I measured on the collector of TR3 after installing the LED:
On first glance it looks textbook, the dips going all the way until they bottom out at 0V. However, closer examination yields that the amplitude of the signal is only about 3.7V, while the circuit diagram prescribes something closer to 6V. This amplitude is too small to reliably trigger the protection circuit.

This shows the pertinent part of the circuit diagram:

This is how it works: The base of TR3 is biased by the voltage divider formed by R26 and the BE-diode in the transistor. The signal from the photocell in the sensor arm OPH1 is coupled in via C12. When there is no signal (like over a record), the voltage at the base is such that the collector of TR3 is at 4V. This means that the transistor is slightly ON creating a voltage divider with R27 (and R30). When OPH1 delivers its AC signal of about 20 mV amplitude, the voltage at the TR3 collector starts oscillating between 6 V and 0 V, i.e. TR3 operates as an amplifier. This 6Vpp signal is then fed into the base of TR4 via C16, which takes out the DC component. Since the base of TR4 is pulled up to 21V via R32 and protected against negative voltage (relative to its emitter) by D15, the resulting signal at the TR4 base is an oscillation between 21.6V and 20.4V. While at 20.4V the transistor is on, and C18 charges. This in turn biases it towards 21V. This pulls up the base of TR6 via R34 as current limiter and the collector of TR6 is pulled to ground. This signal then prevents the arm lowering circuit to work and the arm stays up.

There is a second way to pull up the base of TR6, which is via the D18/R37 link. This connects to the collector of TR5 via the Zener diode D17. This is the circuitry that can invoke the 'no record present' signal at the collector of TR6 if the light bulb is broken. If the light bulb is open circuit, no current flows in R36, causing the voltage at the collector of TR5 to go up. This pulls up the base of TR6 and the collector of TR6 goes down and the arm cannot be lowered anymore under any circumstance. This is the reason that the LED replacement needs to be designed in a way that the current through TR5 is similar to the current caused by the original light bulb (~50-60mA).

Back to the signal on the collector of TR3, which measured to be less than 6Vpp. I long ago noticed that some Beograms have issues with the detector circuit and that sometimes they do not recognize that there is no record and happily lower the arm on the platter even though everything seems to be in working condition. If that happens the stylus' life depends on the proper adjustment of the arm lowering limit keeping it from hitting the platter ribs.
I addressed this problem in the past by soldering in a 10M resistor between the base of TR4 and GND. This biased TR4 slightly on and even a weaker signal from TR3 would be enough to stop the arm lowering circuit. At that point I did not understand the root-cause of the problem, but rather discovered the fix by accident: The circuit tended to work whenever I connected my oscilloscope to the base of TR4 and I realized that the internal resistance of the scope fixed the problem...a bit of trial and error yielded that a 10M resistor to GND tugged enough on the base of TR4 to keep things working after disconnecting the oscilloscope.

But as I know now, there is a much better solution: Double R26 to 2M. This causes a reduction of the current through the BE diode of TR3, and so the transistor is less on, and the voltage at the collector goes up to the prescribed 4V. This fixed the problem in this Beogram:
The amplitude of the oscillation is now close to 6V and can trigger TR4 reliably.

An interesting question is: Why did B&O let Beograms leave the factory with 1M resistors in place and a too low TR3 collector voltage?

One of the issues with the design around TR3 is that the collector voltage depends on the DC current gain of the particular transistor used as TR3. The current gain is the most variable parameter of any transistor series, i.e. even within one production run it can vary considerably by up to a factor two or three. It would have been better to bias the base with a stiff enough voltage divider to GND, and to implement a feedback for setting the gain of the amplification. With the design as it is, every Beogram has in essence a different TR3 working point depending on the particular BC182 that is soldered in. With a new light bulb in place in the sensor arm this variation may not have been a problem due to the high initial light intensity making enough signal even with a non-spec working point of TR3. But as the bulbs aged the intensity gradually went down due to deposited Tungsten on the bulb glass. And after a while the signal became weaker and at some point the mechanism stopped working. So they may not have noticed back then that there is a problem when they manufactured these Beograms.

Interesting stuff (at least to the Beolover...;-)!

Tuesday, March 21, 2017

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.