Characterization of a MMC 6000 Cartridge Restored by Tonabnehmerservice.de in Comparison to Two Good Sounding Original Cartridges

I recently sold a fully restored Beogram 4002 (Type 5503 with AC platter motor) to a customer in California. The customer also wanted a newly rebuilt cartridge. He settled on getting a restored MMC 6000 from tonabnehmerservice.de. Franz Josef at tonabnehmerservice has provided a number of cartridges to some of my customers over the years, and they all seem pretty happy with his work. This particular cartridge is one of his top-of-the-line restorations with a beryllium cantilever and a nude Shibata diamond. 

In this case, my customer asked Franz Josef to send me the cartridge directly so I could check it out before sending it along with the restored Beogram. I received the cartridge last week. It arrived in a little box fitted with a proper plexiglass protector:

I carefully extracted it:

Looks very nice! This shows the beauty playing on the restored Beogram:

The test setup for cartridge characterization is simple. All I had to do was plug the Beogram DIN5 output plug into proper adapters to interface it with the BNC inputs of my new QuantAsylum QA403 audio analyzer. All measurements were done at 1.2g tracking weight (calibrated with a digital gauge).

A few years back I wrote a couple blog posts about this type of measurement. This one discusses the frequency response measurement and this one the 'trackability' measurement.

Let's focus on the frequency response first:

There are several ways to do it. In the old days, they used test tracks that actually played a frequency ramp from 20 to 20000 Hz. The cartridge was hooked up to an AC voltmeter, and a voltage vs. time plotter recorded the signal as the test track played. Add a proper scale to the time axis that translates the time it took to play the track into the proper frequency scale, and the frequency response becomes visible. 

These days we can play a noise track and use an audio analyzer to record a Fast Fourier Transform Spectrum (FFT). A FFT spectrum shows the volume of all individual frequency components of the measured audio signal on a frequency axis. A clean 1kHz test tone would, for example, show up as an isolated peak at 1kHz. A noise signal would look more like a line parallel to the frequency axis since noise is a composition of many frequencies.

I used Tracks 2 and 3 on Side 2 of the Analogue Productions "Ultimate Analogue Test LP" as noise tracks and played them directly into the QA403 analyzer. 

Tracks 2 and 3 are "Pink Noise" tracks: Unlike white noise, pink noise distributes its energy equally per octave rather than per individual frequency. Because of this, its Fourier transform (FFT) is not ideally a horizontal line along the frequency axis, but rather a downward-sloping line that drops at exactly -3 dB per octave (which translates to a -10 dB drop per decade). In plain English: pink noise emphasizes the lower frequencies and gradually reduces the volume towards the higher frequencies contained in the noise. White noise features all frequencies at the same level, i.e., would be a parallel line to the frequency axis.

Now let’s go from the theoretical construct of pink noise to its real embodiment on a test record. Records are usually cut with RIAA pre-emphasis, which superimposes an upward tilt of roughly +6 dB per octave (or about +20 dB per decade) to the signal. If you go to the link, you will see that the RIAA curve is not a line, but has some wiggles in it. That is a consequence of the fact that in real life electronic filters are used to produce the pre-emphasis. But for simplicity's sake, let's assume a +20dB/decade line.

Therefore, if a pink noise test track is played back from a record without using RIAA de-emphasis, it has the RIAA pre-emphasis superimposed on the original pink noise signal. In practical terms, this means that the -10 dB drop of the pink noise signal is overcompensated by the ~ +20 dB RIAA signal increase per decade. This results in a measured FFT spectrum that should roughly slope upward at about +10 dB/decade.

At this point we did not consider the influence of the electromechanical characteristics of the cutting and playback processes on the signal. The good news is that in a first-order approximation we do not have to! To understand why, consider the ‘constant velocity characteristic’ of the cutting and reproduction processes by the cutting machine and the cartridge. Both have cantilevers that end in iron pieces inside coils. On the cutting side, signal voltage changes fed into the coils translate into cantilever movements cutting the groove, while the cartridge does the exact opposite: It translates the 'wiggles’ in the groove back into a voltage signal at the coil leads. So in a first-order approximation, we can say that the influences on the signal by the cutting machine and the cartridge basically cancel out! Therefore, for understanding and discussing the measured FFT curves on the test track, we only need to consider the pink noise characteristic and the RIAA pre-emphasis (since we plug the cartridge signal directly into the QA403 without RIAA de-emphasis).

Let's have a look at what I measured:

This graph shows the frequency response of the restored MMC6000 cartridge measured on Track 2 (lateral pink noise):

I tried to 'graphically evaluate' the slope of the spectra and found maybe +9dB between 100 Hz and 10000 Hz, less than the above estimate of +10 dB per decade. This result probably has a bit of an error bar on it due to the superimposed 'RIAA pre-emphasis wiggle'. I think this probably means that the pink noise tracks do not precisely follow the theoretical -10 dB/decade energy distribution across the bandwidth. This is supported by the fact that I see the same slopes for the other cartridges I measured.

Nice is that both channels match very well. The two peaks at the bottom end of the spectrum at 60 Hz and 120 Hz are interference from the power grid. Not surprising considering the very small signals coming from the cartridge. The sharp drop-off after 20,000 Hz is probably caused by the pink noise signal itself. It probably only contains frequencies up to 20,000 Hz. Sadly, there is no detailed information about the Analogue test record available, and my emails have not been answered so far. I guess analog audio is only analog if there is a bit of woodoo in the signal path!...;-)

The next measurement I did with this cartridge was on Track 3 (horizontal pink noise):

This spectrum looks pretty similar, but there is an approximately 5 dB difference between the channels between 15,000 and 20,000 Hz. This difference means that the left channel will reproduce very high frequencies at a slightly lower volume. In the bigger scheme of things, 5 dB is not a very big difference. I certainly did not notice it when I listened to records with this cartridge. 

This is not surprising for several reasons. The most important one is probably that the 62-year-old Beolover likely has a significant roll-off at around 15,000 Hz and does not hear much of the frequencies under consideration here in the first place.

Furthermore, music has only very little information in this frequency range; only the sizzle of cymbals and similar noises are there. We also need to realize that the vertical signal contains the stereo localization (L-R), while the lateral (horizontal) signal contains the mono information (L+R). So a bit of difference in the vertical signal only affects the localization of instruments on the sound stage. Since the human brain does the locating mainly with mid-range frequencies, a small difference at high frequencies is not very significant for this process.

On to the next cartridge: A Beogram 4002 (5523) that I recently restored for a customer in Louisiana came with its original MMC4000 cartridge. I played it a bit with the restored unit, and I thought it still sounded very good. Here is Track 2 played by this cartridge:

and Track 3:

Very similar spectra compared to the MMC 6000. Left and right are in good agreement in both directions.

Next, I measured my favorite MMC20 EN that I use for daily listening in my workshop. These are the measurements:

First Track 2:

and Track 3:

These curves also look very similar to the previous curves, but here both directions have a small difference between the channels. Here, the left channel is a tad stronger than the left in both directions.

In my opinion, this cartridge also sounds very good.

On to the trackability measurements:

I use a Clearaudio "Trackability Test Record". This record has tracks of a 333 Hz test tone at different volumes. They give the 'volume' directly as lateral deflection, i.e. the physical dimension of the 'wiggles' superimposed on the groove. The tracks start at 50 um lateral (i.e. mono) deflection, and then they go to 100 um in 10 um steps. To put this into perspective: Loud dance music tracks have maximum amplitudes around 70 um. So if a cartridge can track 70-80 um without too much distortion, it is pretty good and can play most music ever pressed on vinyl.

So I played all the tracks with each of the three cartridges and noted the total harmonic distortion (THD). THD is essentially the ratio between the fundamental peak and its harmonics. As an example, these L/R spectra were measured on the MMC6000 playing the 50 um track:   

You can see that the THD estimated from the ratio of the second harmonic to the fundamental peak is less than 3%. After measuring six sets of spectra like the above for the three cartridges for each of the test tracks, I ended up with this plot of THD vs. amplitude:

All three cartridges track fairly well up to 70 um. At larger amplitudes, the original cartridges begin to distort fairly strongly. The restored MMC 6000, on the other hand, is considerably better. It has the lowest distortion across the entire range and could even reproduce 100 um signals with tolerable distortion.

So the conclusion of these measurements is maybe that 'good-sounding' original cartridges are still good enough for everyday listening, but that for audiophile listening experiences a restored cartridge should be procured. My money is on tonabnehmer.de. I think Franz Josef does a very solid job, and you can send him your original cartridges for restoration without hesitation!

Beogram 6000: Oil Infusion of AC Platter Motor Shaft Bearings

I recently received the AC platter motor of a Beogram 6000 (Type 550x) from a customer in the Netherlands. The motor exhibited knocking noises that could not be adjusted away by reducing the motor voltage. This is usually a sign of dry motor bearings. My remedy for this issue is reinfusing the Oilite motor bearings with synthetic oil under vacuum.

This shows the motor as received:

After a trip to my garage for drilling out the rivets that hold the enclosure together, I disassembled the motor:

It is important to keep the parts on the shaft in their original order and orientation (the washers have an orientation, too!):

Unlike the later DC platter motors, there does not seem to be a straightforward method for removing the shaft bearings from the enclosure halves of the AC motors. Therefore, I simply put the entire enclosure into the oil. This is a bit messy, but it works well. This shows the initial bubbling coming from one of the submerged bearings while the pump labored to pull a vacuum:

A few minutes after the lowest pressure had been reached, the oil had foamed up:

These bearings were pretty thirsty! But I think some of the bubbling up air also comes from pores in the enclosure itself. The infusion process is completed when the bubbling stops. At that point, all the pores are again filled with oil. After two days, the oil became quiet, and I removed the parts from the vacuum chamber. This shows one of the bearings after I cleaned out the oil:

These bearings look pretty similar to the ones found in the later DC motors. They can also adjust their angle to minimize misalignment with the shaft.

I put the motor back together:

If you try this at home, make sure the stacked coils are re-inserted at the correct polarity (purple and green wires need to be on the same side as before, otherwise the motor will run backwards). Since opening these motors requires destroying the threaded rivets that hold the enclosure together, I developed 3D printed plastic parts (red) with integrated M3 nuts for the tilt adjustment screws. Note that without the rivets, the enclosure is mostly held together by the mounting screws after the motor is bolted back into its place in the Beogram. This is not an issue for the operation of the motor, but it needs to be kept in mind when the motor is reinstalled.

Beomaster 6000 (Type 2253): Runs Too Hot, Output Stages Rebuild, Volume Belt Replacement, and Repair of an Intermittent Power Supply

A few months ago I met a couple while riding my bike around in my neighborhood in Albuquerque. It turned out that they lived close by. And that they have close ties to Denmark. The plot thickens!...;-). Long story short, I was invited to look at a Beomaster 6000 (Type 2253) that they bought in the 1980s from a local store. It was running hot and the volume could not be adjusted anymore.

Recently they went on a trip for a month and it was a good moment to give it a bit of TLC. So I picked the unit up and put it on the bench:

It is in pretty good cosmetic condition. Only very minor damage to the veneered side panels:

I put the unit into service position (which was easy since the plastic enclosure screw fittings had broken out - see below for my repair of this issue) and had a look at the volume servo:

As expected the rubber belt was gone. Luckily, this is an easy repair if one knows that the pulley on the string mechanism can be pulled off for installation of a new belt:

So with a bit of care the new belt can be installed without needing to mess with the string that moves the volume indicator foil. I put a Beolover Volume Servo Belt for Beomaster 6000 on the shaft:

and after pulling it onto the motor pulley:

I turned the unit on to test the volume drive. I noticed that one of the two light bulbs that illuminate the volume scale was dead:

I replaced it with a red LED and an appropriate resistor:

Once I had figured out what resistor to use for a good intensity I also replaced the still working bulb:

This is how it looks like with the scale bezel installed:

Beolovely! On to the other major issue, the hot heat sink. I removed the metal shroud from the heat sinks, which revealed the output amplifier board. It is bolted to the heat sink, which cools its output transistors:

Removal of two screws from below releases the entire assembly and it can be pulled up:

I unplugged it and put it on the bench:

After removal of the screws that hold the board to the heat sink, the board can be flipped up so one can access the solder side:

I usually replace all the electrolytic capacitors and the quiescent current trimmers. This Beomaster was one of the initially produced units, which have two non-polar electrolytic 47uF capacitors installed. They are evident by the absence of polarity markings:

In later production runs of this design they were replaced with back to back standard polar units and a slightly modified board to accommodate them. Of course I did not have any non-polar 47uF electrolytic capacitors since they are a special item. But I have a wide selection of surface mount components for my development work. So I made two small adapters for standard SMD multilayer ceramic capacitors in a 1210 package using small Dremeled pieces from a development board and male board headers:

This shows one of them installed:

Next came the quiescent current trimmers. They are a bit of an Achilles heel of this amplifier design since they can lead to overheating output transistors when they oxidize as they age. Oxidization can increase the resistance across them and higher resistance means more quiescent current through the transistors, even if no music is playing. In the extreme, these trimmers can go open circuit and then smoke will come from the heat sinks, followed by a shutdown of the receiver due to blown fuses. I removed the two trimmers for the left and right channels and measured them. Unsurprisingly, one of them yielded a fairly high resistance of around 80 Ohms:

Normal is maybe 60 when they are adjusted properly. I replaced them with modern encapsulated 25-turn trimmers, which allow a much more precise and long-term stable adjustment of the current. Before the installation I made sure they were set to a resistance lower than 50 Ohms (usually, trimmers come set to center from the factory, i.e. in this case of 100 Ohm trimmers they should be set to around 50 Ohms, but it is a good idea to check):

Then I soldered them in. This shows the restored board together with the evicted original parts:

I plugged everything back togehter and turned the unit on. Then I adjusted the trimmers to get 22 Ohm (cold receiver) between the emitter resistors of the output:

this concluded my work on the output stages. In this vintage B&O it is a good idea to also replace the electrolytic caps in the power supply. They also get pretty warm. They are often out of spec due to leakage or drying out. The power supply is to the right of the output stages using the remaining heat sink real estate in the 'chimney' in the back of the enclosure. Removal of another screw from below the chimney liberates this board with its attached heat sink. This shows it in original condition. Two of the reservoir capacitors are mounted directly on the board. They are the two big cans in the center:

I replaced them with a couple new replacements:

The next step was replacing the two main reservoir cans (that stabilize the output stages) sitting in their own separate compartment next to the power supply board:

I installed two Beolover Reservoir Capacitors for Beomaster 6000 and 8000, which use 3D printed adapters to give modern capacitors the same form factor like the original ones: 

Assembled:

This makes transferring the wiring a snap. I removed the original capacitors with their ground connection still intact:

Then I transferred the ground wire to the new assemblies keeping the orientation of the terminals about the same:

Then I put the connected cans into their compartment and soldered the power rail wiring back on:

This concluded my work on the power supply (or at least so I thought at this point!...;-) and I focused on repairing the broken out enclosure screws. This shows the 'as-received' situation on the right side

and on the left side of the enclosure:

(I put the screws and nuts where they need to be when in their unlocked position).

A typical issue of this generation of B&O designs from the early 1980s when plastic was king (remember the 1981 Plymouth Reliant? What breathtaking progress in the automotive sector!!...;-). Luckily, 3D printers were invented since, making repairs of such design sins a bit easier. I designed parts that fit on the remaining stumps. This shows the right side component including the original bolt and square nut:

This one needed to be epoxied to the remains of the original fixture for lack of space. I used two toothpicks against the circuit board to hold the part in place while the glue cured:

This shows the corresponding part for the left side:

On the left there is some space below the fixture that can be used for putting in a bolt. I drilled a hole through the bolt orifice in the part for precise fit:

Note that this photo shows an earlier version of the part, which had to be modified later since the left side has much less space available due to a metal bracket that is bolted to the mating area on the bottom part of the enclosure. This shows the final part in place. I also used epoxy to fuse the part as good as possible to the stump, while the bolt gives it additional structural stability:

A shot from a different angle:

At this point I thought I was done and I set the Beomaster up with a Beogram 4002 that I just restored to listen to a record.

Sadly, I was not able to turn the Beomaster on anymore. Something had happened between my bench and the sideboard where the Beogram was! Frustrating! The symptoms were these: I plugged the unit in and the lights under the volume indicator came on, and I also heard the click from the relays. But otherwise it was completely dead: Nothing on the displays, no power dot, no response to the keypad. Nice! Exactly what one wants to happen when fixing something!...;-).

I opened it up and had a look around. My earlier experience with units from this B&O vintage suggested a cracked circuit board, a cold solder joint or a broken board wire (they often disconnect right at the soldering tab within the insulation, and therefore one cannot see it when it happens). I started 'wiggling things' and after a while the power dot in the display came back on and I was able to turn the unit on again by selecting an input source button on the keypad.

Some more wiggling things and I was pretty sure that the power supply board (16) was the culprit. Not surprising, since its voltage regulating transistors are bolted to the heatsink, while also soldered to the PCB. This is generally a solid recipe for getting faults like this since the solder points can easily get damaged due to thermal expansion strain or just vibrations of the heat sink against the board when moving the unit, like I did.

Indeed it turned out that the fault could be reliably provoked by bending the board a bit against the heatsink. So my first response was to re-solder all the sink mounted transistors. But this did not fix it. So I thought it is probably a cracked trace close to the transistor solder points. I soldered short magnet wire (which has a heat removable polyurethane coating) pieces to the transistor solder points and made direct connections to the next circuit node, thereby circumventing the traces in close proximity to the points. Sadly, this also did not affect a change. At this point I started bending the board a bit more 'selectively', i.e. just on one end and then the other trying to get some insight into which area of the board might be affected. It seemed to me after a while that the left side of the board was a bit more sensitive. So I soldered some more wire bits between connected nodes and this finally cured the problem! I was now able to bend the board in any direction I wanted and the unit stayed on! This shows the final result of my efforts:

It was finally time to enjoy a record through this Beomaster! While I listened to it, I noted that I was not able to lower the volume below maybe 10% on the volume indicator. Even the '0' button of the volume presets did not mute the sound. In theory one should be able to lower the volume to zero with the volume up and down buttons. So I opened it up again and had a look. The problem became clear quickly: the volume indicator foil had an issue to go all the way to the quiet end, and due to this blockage, the motor was not able to drive the volume potentiometer all the way to its end point. 

This shows the volume indicator foil as it appeared to me at this point:

If you look closely at the right end of the foil mechanism, where the white string connects, it is apparent that the white 'plate' that connects to the string hangs down a bit before the plastic bracket that keeps the ribbon on track. And that prevented the foil from being pulled all the way to the end. I figured the white piece could use something like a ramp to guide it up a bit, so it could move all the way up to the plastic bracket. I 3D printed a few shapes that did not obstruct access to the solder points underneath the ribbon but were able to act as a ramp. This is the shape I ended up using:

It seems to work well and now the white pice makes it to the end of its travel every time zero volume is dialed in:

Here a photo from a different perspective:

Ok! Back to some more testing of this Beomaster 6000! I will play it for a while to make sure there are no more intermittent issues!