Beogram 6000 (5512): Final Adjustments and Test Drive with Isaac Hayes (Shaft!!)

After rebuilding the CD-4 preamplifier, it was time to do the final adjustments on this Beogram 6000 (5512), plug it into the Phono4 input of my Beomaster 6000 4-Channel and put on a nice record!

Once the platter, arms and chassis are aligned, the final adjustments are the tracking weight and the arm lowering limit. This shows the adjustment of the weight with a digital scale:

The arm lowering limit needs to be adjusted in a way that the needle does not hit the low sections of the black signature ribs on the platter. This is the final safeguard against needle loss should the control system fail and put the arm down without a record. The low sections are at the set-down points for 6, 10 and 12" records, i.e. let the needle pass unscathed if there is no record. This shows the adjustment for this Beogram:

And then it was finally time to give this Beogram 6000 a first spin! It is always a satisfying moment when a restoration comes together and the unit is performing perfectly. I put on the seminal Isaac Hayes soundtrack of the movie "Shaft", which I recently bought in very nice condition at End of an Ear in Austin, TX. Here is an impression:

I really like how the CD-4 pre-amplifier sounds...very clear and happy. It is a good idea, though, to turn the CD-4 decoder off with the switch on the right side of the enclosure for regular stereo records, since it can get confused if there is a high share of high frequencies in the music.

Ah, beautiful music! Let's sing along!...;-):

Who's the black private dick

That's a sex machine to all the chicks? (Shaft)

You're damn right!

Who is the man that would risk his neck for his brother, man? (Shaft)

Can ya dig it?

Who's the cat that won't cop out when there's danger all about? (Shaft)

Right on

You see this cat Shaft is a bad mother (Shut your mouth!)

But I'm talkin' about Shaft (Then we can dig it)

He's a complicated man but no one understands him but his woman (John Shaft)

I will give this Beogram 6000 some more play to ensure that there are no intermittent issues, and then it will be time to send it back to Norway!

Beogram 6000 (5512): Restoration and Characterization of CD-4 RIAA Preamplifier

The most interesting part of the restoration of the Beogram 6000 that is on my bench right now was  working on the 4-channel CD-4 preamplifier board. This board renders the 6000 capable of reproducing CD-4 quadraphonic vinyls from the 70s. While there are not so many CD-4 vinyls that were cut for this short-lived format, this preamplifier board is also a veritable stereo preamp, which is very convenient if the Beogram is to be used with a modern amplifier (who often do not have Phono input stages anymore). The CD-4 board allows connecting the Beogram to any standard high level input like a DVD or AUX input.

The first step was to replace all the electrolytic capacitors on this board. This shows the board in the turntable after taking the keypad out:

I removed the board

and then replaced the capacitors and the indicator light bulb:

The light bulb can be replaced with a standard red LED and a 1k resistor:

This is how the LED peeks out through the cutout in the keypad that permits its light into the CD-4 indicator:

This shows the CD-4 preamp in action playing my The Fisher CD-4 test record (my only CD-4 record):

The more interesting part here is however: How well does this preamp perform for listening to stereo records. The most important items here are how much noise is added and how faithfully does it deemphasize the RIAA curve. I connected the Beogram DIN5 to my QA400 audio analyzer and measured a noise spectrum:

These curves were measured for the left and right channels. They look quite identical. They were measured with a cartridge installed and the turntable running with the arm lowered next to the platter. 

This made sure that the measurement was performed with the motor running to see if there is any crosstalk through the power supply of the Beogram from the motor (there is not) and that the preamp was actually amplifying the signal from the cartridge. When the arm is up the signal in the preamp is grounded, i.e. one only sees the noise from the amplifier itself. This is a boring measurement, which yielded for this CD-4 preamp a flat line at about -130dBV. This number corresponds to the noise numbers that are often given for external phono preamps that are sold for considerable amounts of money. -130dBV is a big negative number and impressive. However, it is meaningless for any practical use of a phono preamp. The measurement shown above is much more crucial, since it gives the noise floor defined by the cartridge, which is what you hear when a record is played.

What we see from the graph is that the 1kHz noise is about -110dBV. Considering that a 0VU level  is at about -20dBV, we can say that with this amplifier and a cartridge we have about 90dBV maximum signal-to-noise. What does this mean in practical terms? 20dBV correspond to a 10x difference in the amplitude of the signal, i.e. 90dB means that the noise contributed by the cartridge is less than 1/1000th of the audio signal at the highest level. 

You may wonder why I stopped writing about the amplifier noise and just mention the cartridge. Well, the amplifier noise is -130dB, i.e. it is another factor 10 smaller than the cartridge noise, i.e. pretty irrelevant at this point. 

An interesting question is: Why does the cartridge make most of the noise? (actually not-the vinyl surface is even more noisy than anything 'electronic' I am discussing here...see my discussion of the BeoloverRIAA internal amplifier for the Beogram 4002) 

After all it is a passive component that has no active (powered) electronic components, except one coil per channel that picks up the signal from the moving magnet connected to the needle. Here is where physics comes in: All conductors generate Johnson-Nyquist noise, which is generated by thermal movement of electrons in the conductor. This movement is random in direction, i.e. the electrons move forth and back at high speed through the wires of the cartridge coils and the connecting leads, which generates a small fluctuating current that is permanently fed into the amplifier input, hence one can hear a bit of hiss (white noise) even if the needle does not touch the platter.

One more interesting question: Why is the noise in the above spectrum higher at low frequencies than at higher frequencies? The answer is: This is a direct consequence of the RIAA deemphasis of the amplifier. Records are recorded in a way that low frequencies are engraved at a lower amplitude than higher frequencies. The reason is that lower frequencies need larger 'wiggles' in the groove to generate the same acceleration of the magnets (=mV output signal from the cartridge) like higher frequencies at the same audio volume. This trick allows to squeeze the grooves closer together and more music can be put on a side of a record. This shows the theoretical RIAA curve (from wikipedia):

The red curve is the playback curve. So if we have a flat noise spectrum (white noise) coming from the cartridge, the spectrum should drop by about -40dBV across the 20Hz-to-20kHz range. And that is what we see in the above measurement. The drop is about -36dBV, i.e. the RIAA deemphasis of the CD-4 board is slightly off from the theoretical curve from wikipedia. This difference, however, is pretty irrelevant in practical terms since a) -4 dBV it is hardly discernible when listening to music, and b) the RIAA emphasis (blue curve) of records from different labels are all somewhat different, too, i.e. they use different RIAA curves to begin with.

One last point: I just assumed that the noise coming from the cartridge is 'white', i.e. flat. Is this really the case? Yes, it appears so. Thermal noise is constant per frequency up to the GHz range:

This graph was taken from this interesting webpage, where thermal noise is discussed in some detail. This means that, in absence of a white noise generator, using a cartridge connected to the pre-amp seems to be a pretty decent way for measuring the quality of the RIAA deemphasis of an amplifier. 

Another interesting point here is that the thermal noise level depends on the resistance of the conductor that produces it. Hence, shorting the input of an amplifier connects essentially 0 Ohms, which kills most of the noise at the input, and one then measures only the noise generated within the amplifier.

Allright...after this little excursion into the land of physics, it is time to put this Beogram back together, do some final adjustments and then finally enjoy some lovely vinyl! This time through the line-level Phono4 input of my Beomaster 6000. Exciting prospects!

Beogram 6000 (5512): Replacement of a Corroded DIN5 Plug with a New Gold Plated Unit

Practically all Beogram 4002/4 and 6000 need their DIN5 plugs replaced due to various levels of  corrosion on the contacts, which were not gold plated back in the 1970s. This Beogram 6000 (5512) is a 'beautiful' example:

and with the shield removed:

I cut the rusty plug off and installed a new gold plated all metal plug:

and with the shield on:

Beolovely! The CD-4 preamp board is next!

Beogram 6000 (5512): Repair of a Wavy RPM Trimmer Scale Background

After replacing the background illumination bulbs in the RPM trimmer panel with LEDs I realized that the white  reflective background that homogenizes the illumination was wavy causing a 'zebra pattern' in the illumination:

This seems to be an issue that only affects a subset of Beogram 400x. I only had to do this once before so far. To fix it one has to take the panel assembly apart. This shows the panel from the back:

The white part needs to be separated from the aluminum panel. This can only be done by cutting the locking washers loose with a wire cutter:

Then the parts can be separated:

This shows one of the wavy white stickers close-up:

The wavy pattern causes a variation in the reflection of the light piped in from the previously installed LEDs. The original white stickers can simply be pulled up with pliers (one needs to be careful to not break the red dial hand off). Once removed it can be replaced by white electrical tape cut to size:

The panel can be reassembled by using new 3 mm circlips and nylon washers:

After this procedure the background illumination was smooth again:

Beotiful! On to replacing the badly corroded DIN5 plug...

Beogram 6000 (5512): Replacement of the Sensor Arm Bulb with a FlexPCB Based LED Assembly

The final light bulb to replace with an LED in the Beogram 6000 that I am restoring right now was the bulb in the sensor arm. It is arguably the most difficult bulb to replace due to the space constraints, as well as the design of the control system, which demands that a current comparable to a light bulb flows though the light source in the sensor arm. More detail about the specifics can be found in this recent post where I implanted it into a Beogram 4002.  Here are a few impressions of the process for this particular Beogram 6000:

This shows the original light bulb:

This is the replacement assembly based on a Kapton flex PCB that folds into the bulb compartment and places the LED in the right spot:

And this shows the LED in action:

Like all Beolover parts featured in this blog, this assembly is available to other enthusiasts. Just send me an email or use the contact form on the right.

Beogram 6000 (5512): Replacement of Transport Lock Bushings and Cabinet Guidance Washers with 3D Printed Parts

The Beogram 6000 (5512) that I am restoring right now is slowly coming together and it was time to replace the deteriorated transport lock bushings and the cabinet guidance washers. Replacement parts are available via the Beolover Shapeways store. Six of the bushings parts (two per bushing) and five of the guidance washers (there is one underneath the keypad) are needed to replace all original parts.

It is a good idea to get these items into proper condition before doing all the adjustments to line up the arms with the platter and the rest of the chassis. There are two types of transport lock bushings that were used in this type of Beogram, grey and orange plastic. The grey plastic seems to survive much longer, while the orange type usually is about to break up into pieces. So if you find telltale orange plastic fragments in your Beogram enclosure like this one found next to the motor 

then it is time to have a look at the bushings. This is what I found in this Beogram:

and with he lock components removed:

Before replacing the bushings it is a good idea to take the entire floating chassis out to remove plastic fragments that may possibly be underneath the chassis. If there are, it is possible that they impede the free movement of the chassis negating its isolating properties against vibrations. DC motor Beograms like this unit definitely have an advantage here, since the chassis can be fully removed once the transport locks have been removed and the PCB connectors are unplugged:

After cleaning out the enclosure and removal of the bushings remnants with needle nose pliers, the new bushings could be installed:

This is how they look once the locks are reassembled:

On to the cabinet guidance washers. This is what I found:

They usually crack into two halves. They can be replaced with my identical shape replicas:

It is a good idea to order one of them in black since the one at the front can be seen a bit between the wood plinth and the aluminum plate on top of the unit, and if it is black it is more or less invisible, similar to the translucent originals. Here you see one of them (under the keyboard with the CD-4 board in the background) installed:

Perfect fit! Having these renewed makes sure that the plinth seats properly and symmetrically around the top aluminum plates. Beolovely!

Beogram 6000 (5512): Dead IR Diode in Carriage Position Sensor

After restoring the PCBs and the platter motor of Beogram 6000 (5512) that I am restoring right now it was not working properly. The platter rotated, so I was able to do the RPM stability measurement, but  the carriage did not go to the 30 cm set down point, but rather traveled across the entire platter without intervention. It also immediately switched to 45 RPM after start. This suggested that the culprit was not the record detection system, but rather an issue with the carriage position sensor. This sensor uses an IR diode as emitter and a photo diode as detector. The plastic 'ruler' that is attached to the carriage has black markings on it, which allow the control system to detect what action is needed depending on the carriage position and speed of travel.

That it switched immediately to 45 RPM after traveling for about 1 cm after pressing start suggested that either the IR diode or the photodiode were dead. A strong light beam from a LED flashlight elicited a response from the photodiode. That left the IR diode. I measured the voltage across it and the result was 0.95V instead of the prescribed 1.1V. This was remarkable, since these diodes usually die by going open circuit, i.e. one usually measures a much higher voltage across it. First I tried adjusting R88, which controls the brightness of the diode, but it was maxed out already...so someone tried to adjust it previously...I took the carriage PCB out, which is an easy task in DC motor Beograms since it is just plugged in (one does have to unsolder the two leads to the carriage motor) and exchanged the IR diode with a low intensity amber LED and put the board back in. This is how it looks in action:

One can pretty much use any standard LED to fix a broken IR diode on this board. The photodiode is responsive to visible light. The issue is that modern IR diodes are designed for high output and high current, and so it is better to use an easy to find low output standard LED. The sensor does not need much light intensity. A crucial point of this fix is to adjust the working point of TR17 properly via the LED brightness trimmer R88. This shows my multimeter hooked up to the base of TR17 where 0.7V are specced in the circuit diagram. Just adjust R88 until 0.7V appear while the sensor is under illumination (i.e. none of the black markings can be between the LED and the sensor for this adjustment).

After this procedure the deck operated normally.

Beogram 6000 (5512): Replacing the Light Bulbs in the RPM Trimmer Panel with LED Assemblies

After rebuilding the main PCB the next step for this Beogram 6000 (5512) was to replace the light bulbs that back-illuminate the RPM trimmer scales in the control panel. Aside from their much better longevity, LEDs also emit much less heat than incandescent light bulbs and that reduces thermal gradients within the RPM potentiometers. This is beneficial for the long term RPM stability of these units. 

This shows the RPM panel extracted from the Beogram:

and with covers removed:

The bulbs need to be unsoldered and then pulled out:

Then the LED assemblies can be soldered in. This shows the units before installation:

and here inserted:

and with closed covers:

The LED assemblies are drop in replacement parts for the original bulbs and do not need any further circuit modification. They are available to other B&O enthusiasts. Just send me an email or use the contact form on the blog. Here is an impression of the 33 RPM LED replacement in action.

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.