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 8002: Final Wrap up

Today is the final day here for this Beogram 8002 turntable. Tomorrow it will return to its owner to enjoy. I have spent two days now playing records on it while I work in my office. This Beogram is functioning perfectly.

Last night I started on the lid assembly for this turntable so I could attach it today. I discovered that the black, metal base in the tonearm compartment had a problem. It turns out someone previously attempted to repair the underside where Bang & Olufsen also originally used double-sided tape.

The original tape deteriorates by now and can no longer hold pieces together. Instead of cleaning all of the old tape residue off another piece of tape was installed. It has now come loose.

No big deal. I just pulled the current tape off and used some Goo Gone to help get rid of the original tape residue.

I like to use 3M double-sided tape for this repair. I cut a few strips to customize the fit and reattached to the pieces.

Now I can focus on the dust cover hinge. Here is the original hinge for this Beogram.

It is missing the black plastic trim piece that hides the metal hinge from view. There was a bunch of dry, pasty grease residue so I had to clean all of that up and re-lubricate the hinge pivot.

Once I get into this part of the restoration I tend to not think about things like taking pictures so I forgot to get some detailed shots of this Beogram's hinge assembly. I do have pictures from one of my previous restorations of this hinge so I will use those to show what the dust cover hinge attachment is like.

Here is what a Beogram 8002 hinge assembly typically looks like when first opened up. The hinge clip that mounts to the lid was originally attached with some double-sided tape that eventually gives out. On this Beogram project I re-attached it with some epoxy. These pictures also show the hinge trim cover piece that this Beogram is missing.

The lid hinge and lid clip slide together to form the completed dust cover hinge.

The metal part of the hinge assembly is mounted on the leaf spring and there is an allen screw to adjust the amount of spring force to keep the lid raised. Note that you actually attach the hinge assembly to the leaf spring bar prior to attaching it to the lid clip.

For this Beogram project I was able to find a spare hinge assembly trim cover so the dust cover assembly looks much better now.

I also re-installed the owner's SMMC3 cartridge and adjusted the tracking force to the recommended 1.2 grams.

This turntable is ready for one last test play.

I decided to pull out an old Renaissance "Song For All Seasons" LP from 1978 and give it a spin.

Very nice 70's progressive rock sound. I believe this turntable restoration is complete.

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 8002: Taking the restored turntable for a spin

The last bit of repair on this Beogram 8002 is to fix the problems with the dust cover lid assembly. Meanwhile I started playing records on the Beogram as a test drive (or burn-in test). This is always the best part of the restoration.

I set up the Beogram 8002 in my office so I could listen to it play for the next couple of days. One of my kids is a trumpet player so I find myself listening to a lot of music by trumpet players. This picture is the Beogram playing Freddie Hubbard's First Light  LP. My copy has a very slight warp in it but the Beogram had no problems tracking it.

Once the listening tests are over and the dust cover assembly is re-attached this Beogram will be returning home.

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 8002: Service Manual Adjustments

Going in to the Beogram 8002 service manual adjustments I already know that the fixed arm and tonearm horizontal parallelism needs adjusting. The tonearm appears higher than the fixed arm as you can see in this picture.

The tops of the two arms should be the same height and that height should be 19.5mm from the top of the Beogram platter. So the fixed arm needs to be adjusted up and the tonearm needs to be adjusted down.

To make all of the necessary adjustments I have to get to the underside of the tonearm carriage assembly. That means removing the rails and the tangential drive screw. I have to do this anyway to clean and re-apply some lubrication.

The red arrow in this picture shows the adjustment screw for the tops of the arms (the horizontal parallelism adjustment). The green arrow is for the vertical parallelism adjustment (the gap between the two arms). I am pointing that out as it turned out I needed to adjust it as well.

After adjusting the horizontal parallelism I got both arms to the 19.5mm height from the platter surface.

I installed a new tangential arm drive belt.

I also cleaned and checked the lubrication of the platter hub. The following pictures show the Beogram from platter hub removed to the sub-platter re-installed. I was able to find a spare tonearm assembly cover trim piece. The original one was missing from this turntable and the wires were exposed.

When I put the top platter back on and connected the main board the tonearm and platter worked okay but I noticed something was not right with the tonearm. When the Beogram 8002 tonearm is at rest you should be able to push the arm towards the fixed arm and have it spring right back to centered position when you let go. With this tonearm I was detecting an obstruction that was hindering the spring back action.

I was concerned at first because I couldn't see anything wrong. The vertical parallelism looked correct. I took a peek into the tangential arm tracking sensor assembly.

Note the spring clip in the picture below. That spring mechanism moves the tonearm back to a centered position when you move the tonearm away from the fixed arm.

Everything was fine in there. Nothing was scraping.

I finally realized the tonearm vertical parallelism adjustment screw was the problem. It had been adjusted incorrectly. While the gap between the fixed arm and tonearm looked fine, the tonearm raise/lowering bar was not operating correctly in the V-shaped channel of the brass adjustment screw.

The following picture of one of my own Beogram 8002 turntables shows the tonearm lift/lowering bar and the V-Channel adjustment screw. The lift bar has a spring mechanism that will attempt to keep the bar pressed down on the V-channel when the arm is in the raised position (not playing). If you manually move the tonearm towards the fixed arm the lift bar rides up one side of the V-channel. When you let go the tonearm should pivot back to a centered position as the spring loaded lift bar returns to the bottom of the V-channel.

On this Beogram 8002 project turntable the lift bar was getting slightly stuck at the top of the V-channel screw when the tonearm was moved towards the fixed arm. By re-adjusting the V-channel screw (which is the vertical parallelism adjustment screw) I was able to get the tonearm back to its normal, springy self.

The tonearm and fixed arms are also vertically parallel to each other.

I test played one complete side of a record and observed the linear tracking working correctly as the stylus remained always centered. I could see the tangential arm assembly motor adjusting the tonearm position pretty much ever half turn of the record. Very nice results after all of that disassembly/reassembly of the Beogram mechanics.

The last check I wanted to make was the voltage level of the light driven sensor in the control panel. This is for the Beogram forward/fast forward and reverse/fast reverse function. After the Beogram has been operating for at least five minutes the voltage of the forward/reverse sensor should be around 620mVDC in its normal state. This check is done my monitoring control panel connector P5-4 to P5-3 (ground) and P5-6 to P5-3 (ground). To adjust the sensor voltage levels there are two screws on the control panel that operate an aperture for the sensor lamp source as seen in the picture below.

The procedure requires checking the voltages at both test points a few times as each screw is adjusted. Each adjustment screw can affect the measurement at both test points. After a few iterations I got the sensor to a satisfactory level.

Tomorrow I will re-assembly the Beogram 8002 (except for the dust cover lids) and do some actual record playing testing.