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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...

Sunday, November 30, 2014

Beogram 4002/4004: New Belts and RPM Adjustment

A few months ago I received new Beogram 4002/4004 belts from 'Dillen' (Beoworld.org). Now was finally the time to install them into the Beogram 4002 that I am currently restoring. It seemed to need a new drive belt, since I had to considerably increase the speed trimmer to achieve 33 RPM. The Servo Belt also showed cracks. It seems it may have still been original. It had the prescribed triangular crossection, matching the groove in the pulley that drives the carriage spindle.

As in the case of the Beocord 5000, I thought it useful to take some measurements of the old and new belts for future reference (like when diving archeologists bring up this turntable in the year 2358 from the ocean floor where Florida used to be before global warming...;-):

Belt
Old ID
New ID
Old Crossection
New Crossection
Drive Belt (flat belt)
17” length when pressed flat
17” length when pressed flat
Width: 5 mm
/0.2”
Thickn.: 0.5 mm
0.020”
Width: 5 mm
/0.2”
Thickn.: 0.85mm/
0.033”
Servo Belt
31 mm
/1.2”
31.5 mm
/1.23”
Triangular
Height: 1.5 mm /0.059”
Base: 2 mm /0.078”
Square
0.75 mm/0.030”

I really like the much tauter feel of the new, thicker, flat belt...and indeed, after I put the new drive belt in, the speed was considerably higher, indicating slippage with the old belt. Now the speed trimmer was much closer to the '0' mark for a precise 33 RPM, i.e. the slippage was much less. I decided to make this perfect, and adjusted the primary speed trimmers on the circuit board (R14 for 45 RMP and R15 for 33 RPM) for a correct RPM setting when the user accessible trimmers are set to '0'. I did this with a small all-metal watch maker screwdriver for 33 RPM while holding down the '33' key (as one does for sweeping the platter before starting to play a record). There I noticed a remarkable behavior: The speed of the platter would immediately increase, seemingly beyond 45 RPM, whenever I stuck the screwdriver into the slot of the trimmer. After extracting it the speed would normalize...it appears that my body's resistance path to GND via the key pad affected the time constant formed with C3, which then throws off the difference amplifier in IC3, resulting in a change of speed of the motor.

Lesson learned: Finally get rid of the all-metal screwdrivers...they got me in trouble before when I fried one of the outputs of my Beomaster 6000 4-Channel by a screwdriver-caused short when adjusting the quiescent current...;-).

Saturday, November 29, 2014

Beogram 4004/4002: Tracking Force Adjustment and Zero Force Balancing

I noticed that the Beogram 4002 that I am currently fixing up needed a tracking force setting of 1.5g to produce an acceptable undistorted signal with its MMC20EN cartridge. At 1g the signal would be completely distorted. A clear sign that something was wrong with the tracking force adjustment (the cartridge works well on my Beogram 4004...).

It turned out that the arm needed a zero force rebalancing. The counter weight in the back of the tone arm was way too far back. I made a video about the procedure. The video shows how to do the zero force adjustment, and then how to use the tracking force gauge that comes with B&O cartridges to get a precise adjustment of the tracking weight:




It is interesting to note that the scale on the adjustment bushing at the back of the cartridge appears to be only a rough estimate of the weight acting on the needle. In my opinion, this is not surprising considering the design of the mechanism using a variable torsion spring to press the arm down.

There is another issue with this mechanism: The adjustment screw is only held in place at the back plate with a retaining clip, which gives the counter weight 0.5-1 mm play along the arm axis. This, of course, can throw off the zero balance when the turntable is moved after an adjustment. I fixed this in Beogram 4004 with a M3 counter nut and a washer. Since the nut needs to be tightened after the adjustment the weight position changes slightly once the nut is tightened. This requires multiple attempts to get it right, but I think this is worth the effort, since this will lock the adjustment in place for long-term stability. Here is a photo of the installed nut and washer:






Friday, November 28, 2014

Beogram 4004: Characterizing a MMC20EN Cartridge with an Analogue Productions Test LP

I recently bought 'The Ultimate Analogue Test LP' from Analogue Productions. I felt I needed to be able to characterize the output of my cartridges and I read about this LP. It is a heavy duty 180g high-quality vinyl that has all the relevant test tracks for basic measurements.

I connected the oscilloscope to the DIN5 connector via a DIN-to-RCA breakout and two RCA-BNC adapters. This allowed me to form a shielded signal path from the output of the Beogram 4004 to the oscilloscope. However, the measurement was still pretty noisy, so I resorted to using a low pass coupling for the trigger and averaging. This gave me usable traces. Here is a shot of the oscilloscope screen on the 1 kHz test track:






























It is obvious that the MMC20EN cartridge that came with my Beogram 4002 puts out a well balanced signal on both channels. The peak to peak amplitude of the 1 kHz signal is about 15 mV. This also seems to be a pretty good way to calibrate the RPM of the platter...

Here is a shot of the 100 Hz track (timebase is 10 ms/sq):





























As expected the amplitude is considerably lower (maybe 4 mV without the noise) than for the 1 kHz signal due to the RIAA pre-emphasis. This is necessary at low frequencies to keep the groove width reasonable (Faraday's law prescribes that the induced EMF is proportional to the change of the magnetic field, which in turn is proportional to the velocity of the moving magnet, iron or coil in a cartridge). I found an excellent summary that explains the background behind the RIAA curve and the historic path towards it. The article is by Gary A. Galo and can be found here.

In short, the RIAA curve lowers the amplitude progressively towards lower frequencies to reduce the groove width (otherwise long playing records would not be possible). At high frequencies the amplitude is increased to stay ahead of the noise, which, due to its high frequency content is an effective EMF generator. Hence the signal amplitude needs to be a few magnitudes larger than the noise amplitude, that in the amplifier signal + noise can be reduced together, thereby reducing the noise relative to the original signal level. This is a similar approach as is used in Dolby noise reduction systems. 

The 10 kHz track on the Analogue Productions LP produces a standardized -20 dB amplitude (i.e. 10x smaller than the other two tracks, which are at 0 dB. 0 dB apparently corresponds to a 7 cm/s stylus velocity, as suggested on the LP sleeve. Here is the (pretty noisy, disregard the Freq measurement indicator) measurement (timebase is 100 us/sq):





























Without the noise the amplitude might be about 10 mV or so, i.e. it were at about 100 mV at 0 dB. That would give us about a factor 7 relative to the 1 kHz signal. 20*log(7)=16.9 dB, which, considering the noisy measurement, is reasonably close to the RIAA curve (from stereophile.com, the blue curve is the curve used on records, the red one is an idealized 6 dB/octave curve, which would be best, but cannot be implemented due to practical reasons), which suggests a pre-emphasis of about 13 dB at 10 kHz:


























The 100 Hz measured amplitude above is also reasonably close to the curve: 20*log(4/15)= -11.5 dB (compared to about -13 on the curve....

All in all another interesting evening at Beolover's lab...;-)





Tuesday, November 25, 2014

Beogram 4002: Signal Path between DIN5 Plug and Cartridge, Plug Replacement

I finally wanted to try out the Beogram 4002 that I am getting ready for the 21st century. While everything seems to work now, the DIN 5 plug looked really bad. Rusty and bent...so I decided to replace it with a modern quality plug (Neutrik AG Rean NYS 322G) with gold plated pins to ensure lasting contact at my Beomaster 6000 Aux input. Here is a pic of the old plug...pretty ugly! Not sure what the previous owner was up to...:























Before I soldered the new plug on, I noted the signal path down to ensure that I would make the right connections after cutting off the old plug. Here is a schematic of my measurements (note that the cartridge connectors are shown on a mirror that I held below the arm):

















It is interesting to note that the cartridge is essentially directly connected to the amplifier with the recommended 'star arrangement' ground (all ground connections come together at one central point and loops are avoided). The Beogram has its ground connected to the chassis, which could cause a ground loop if it were connected to the phono preamp ground and there were a conductive connection between the Beogram chassis and earth (usually the Beogram chassis floats due to the isolating feet and the absence of a earth connector at the standard two-prong power entry). The shield of the cable, however, is connected to the chassis, and will make a direct connection to the amplifier chassis, i.e. in the end the Beogram ground is connected to the amplifier ground. But this should be o.k due to the star-arrangement, even if there is a potential difference between the two units. And indeed, I was not able to discern any hum from the speakers after hooking the Beogram up.
Another interesting detail is that the back channel connections are fed to the plug, even in a 2-channel Beogram 4002 or 4004. I guess, when these decks were new, the CD4 upgrade was available for an easy plug-in at the Bang & Olufsen dealerships. Sometimes, I wish I had a time machine!...;-)

Here are a few pics from the plug replacement procedure. This was actually not trivial due to the heavy duty double shielded cable, which required a few sweat producing manipulations of the 22 gauge leads that carry the signal, while soldering on the heavy duty shield and signal GND connection:























And here the final result: 























Just hooked the Beogram to my Beomaster 6000, and it sounds beautiful! Andrew Hill: Dance with Death on an original 1970s Blue Note pressing. Fantastic!

Thursday, November 20, 2014

Beogram 4002: Replacing the Detector Arm Light Bulb with a LED

The Beogram 4002 that I am currently rebuilding also needed a new light bulb in the detector arm. The previous owner had done a poorly executed 'repair' attempt, and ripped out two wires from the detector arm lamp assembly, while also bending the photo cell backwards - unbelievable what some 'experts' are coming up with. Here is a picture of the assembly as I saw it the first time:






















For comparison I pulled the compartment out of the detector arm of my Beogram 4004, which is in a pretty good condition:








These light bulbs are hard to come by and I also believe in LEDs as a way to get these units stable for another 30 years. So I decided to replace the light bulb with a small 3 mm LED.

This effort turned into an interesting experience, since on first try, I was not able to get the record detection mechanism working after I put in the LED with a 220 Ohm resistor. The turn table simply ignored any record that I would put on its platter.

Study of the circuit revealed that the original designers of this gorgeous piece of analog control electronics proofed the circuit against failing light bulbs (a failed light bulb would result in always detecting a record whether there is one or not, with potentially fatal consequences for the pickup). The much lower current in the LED resulted in a partial activation of this fail safe mechanism, which prevented the Beogram from auto-lowering its arm. This was fixed by replacing R36 with a 560 Ohm resistor, which resorted the original balance in the circuit.

I made a video about the entire process. The video discusses the detector arm circuit and explains how the LED exchange was performed:





Here is a photo of the final result:







Thursday, November 13, 2014

Beogram 4002: Conversion of the Tracking Sensor Light Bulb to an LED and Adjustment of the Sensor

I started working on a Beogram 4002 that I obtained locally from Craigslist about a year ago. I thought it would be a nice practice run before I give the cherished Beogram 6000 4-Channel a go that I recently found.

I plugged it in after opening it up in the hope to see the location of the smoke should any develop...;-). No smoke, but no tracking either. Pretty much nothing worked, no tone arm lowering etc...also some traces of amateur repairs.

I determined that the light bulb in the tracking sensor was out, and I decided to replace it with an LED since the bulb cannot easily be replaced since it is glued into the sensor housing, i.e. cannot be separated. I also am a fan of LEDs since they have a much better long-term stability, especially should the unit be shipped at some point.

I ended up using a 3D printed part to hold the LED and the current limiting resistor. The part essentially replaced the light bulb fixture on the sensor housing. Here is a picture of the end result:























I made a video that explains the whole procedure including the adjustment of the tracking mechanism for the right tone arm angle relative to the record grooves. The video also discusses how the tracking mechanism works. Enjoy!:





On to the dead sensor arm!

Monday, November 3, 2014

Beocord 5000 (4715/4716): Fader Fixed and Putting it Back Together

I looked into the Fader of the Beocord 5000 (4715) that is (was, hopefully) on my bench. It turned out that the contacts of the Fade In and Fade Out control buttons were strongly corroded. They simply did not make contact. I pulled a strip of paper coated with DeoxIT D100 through the contact pads, while pressing the buttons, and this cleared things up. The paper had nice black streaks of them, indicating a fair amount of oxide removal. I did the same with the other buttons, giving this deck an as-new user feel. Swift response whenever a button is lightly pressed...very nice!

This encouraged me to put the deck back together. But first I needed to re-glue the veneer on one of the corners of the wood panels. I used some wood glue that I carefully inserted in between the veneer and the particle board using a thin needle. This allowed me to avoid bending the veneer to much. Here are a couple of pics of the procedure. I used snippets of transparency foils to prevent accidentally glueing on the cardboard pads that I used to protect the wood from damage by the carpenter clamps.




























Once the corner was stabilized I put the wood panels back on the BC5000, and connected it to the Beomaster 6000 for a first tryout with the Beogram 4004. I taped Water Babies by Miles Davis, one of my all-time favorites of my vinyl collection. This went very well. There is almost no difference between the recording and the original. Also, the 'infamous' double capstan mechanism does not seem to have a problem...no eaten tapes so far, and the recordings seem to be very consistent. Here are a couple impressions of the kit in action:
























And a somewhat shaky GIF of the entire set-up while taping (it is pretty long and a linear sweep is pretty difficult with a cell phone...;-):








Saturday, November 1, 2014

Beocord 5000 (4715/4716): First Recordings!

Today I finally hooked up the still open Beocord 5000 to my Beomaster 6000 4-Channel for some basic testing. I made a recording on a new TDK SA 90 via my Beogram 4004 (Miles Davis: Porgy and Bess, a nice pressing from the 70's; CBS 32188). It worked beautifully! The Beocord worked and responded like new (well, almost...see below). I pressed the record button, and adjusted the level sliders to yield a signal just below the red indicators, then re-set the tone arm to the beginning of the track ("The Buzzard Song"), and pressed Start. The 5000 started recording. After the song was over, I stopped and rewound, and played back. It was perfect. I am quite amazed of the quality of this 1977 deck's reproduction. Almost no difference, just a bit more hiss (I did use Dolby NR, though). Here is a first impression of the deck in action with and without the plexiglass cover:


What a beautiful machine! I just love the internal layout...wonderful analog pre-microcontroller design work. A slight downer of this blissful moment: Unfortunately, further playing around revealed that the Fade function is not (yet) working...well, tomorrow is another day! I am sure I will get this figured out, too!