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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...

Friday, December 26, 2014

Beomaster 8000: Signals at IR Remote Receiver Test Points

After the remote receiver came alive again in the Beomaster 8000 that I am currently restoring, I decided to take some measurements at the test points for future reference. I thought this memo might come in handy the next time there are some problems in this area.

Below is the relevant section of the circuit diagram. The IR sensitive diode 6D27 changes its impedance when IR light impinges. This drives the damped 41kHz resonator formed by L1/C2/R1. IC1 drives the signal into the input of IC2 (TP5), where it is amplified. The signal emerges from the amplifier at pin 3 where it has a 12V amplitude (TP7). The 41kHz modulation is removed in the filter formed by R16/C27 and a cleaned up digital signal containing only the bits of the remote codes is forwarded by TR25. TR26 finally changes the signal to a 5V compatible signal via a pull-up on the processor board (PCB#9)

It is interesting to note that this entire circuit is nowadays integrated into the receiver package (example IRM3638), which has pretty much the same form factor as the simple IR diode D27 that is used in the Beomaster 8000. Amazing progress in a couple decades!

This is the table of remote codes from the service manual:

It is evident that all codes start on a 1, which represents the start signal for the transmission and its evaluation in the microcontroller. It is interesting to note here that the signal of the remote is fed into pin 24 of the 6500/1 microcontroller, which is on its port B. Only Port A is interrupt capable on this processor (S. A. Money: Microprocessor Data Book) and it is completely used for the keyboard readout. This suggests that the remote function is fully software implemented. Anyway, after this trip down the memory lane (I am a proud Commodore 64 veteran...;-), here are the oscilloscope signals that I measured while pressing continuously 'Volume Up' (10100111) on the Terminal:

TP5 (input to amplifier) - pretty strong 700 mV signal, which is no surprise, since the Terminal was about 20 cm away from the IR diode (note the 20ms timebase...the other two pics below were measured at 10ms):

TP7: Output of Amplifier (signal is still modulated, but amplitude is now ~12V):

TP8 (after removing the modulation and translation to 5V a clean digital signal emerges):

Wednesday, December 24, 2014

Beomaster 8000: Rebuilding the Power Supply Board (PCB#6) and a Happy Remote Control Receiver

I always recommend to recap the power supply (PCB #6) board whenever I open up a Beomaster 8000. The golden reservoir cans are often prone to fail at this vintage. Another frequent candidate to interrupt the Beomaster bliss is 6C35 that is mounted directly next to 6IC4 at the voltage regulator heat sink, where it gets nicely warm (but not necessarily cosy for electrolytic capacitor standards...;-). The procedure is largely uncomplicated. PCB #6 can be left connected to the heatsink, and simply pulling it up vertically after removing the plugs from the board gives enough access to the capacitor solder points. The golden cans are usually glued to the board with some kind of silicone-type goo. It can be softened up with a rework heat gun (or a blow dryer) until the cans come loose. This shows the board before I started working on it (note the imposting yellow 'Sprague' capacitor in the top left corner of the board...this helped kill the remote received. See at the bottom of my last blog entry for a better picture of the 'modification' of this board - not sure what the nut who worked on this was up to...it all does not make much apparent sense):

 This shows the board after rebuild with quality 105C caps and removing the offending capacitor:

After this I woke the Beomaster 8000 with the Beolab Terminal by pressing '0' and it came on! All the other functions also appear to work remotely. It appears the remote receiver is back in the game! Beobliss!!

Out of interest, I did some measurements at the remote receiver test points TP5 (input of IR remote amplifier TDA 4050), TP7 (output of TDA 4050) and TP8 (cleaned up 5V digital signal) for further reference. I will post the oscilloscope shots in the next blog entry.

Tuesday, December 23, 2014

Beomaster 8000: Rebuilding and Testing of the Output Stages and First Power-Up

When I get a Beomaster 8000 of unknown provenance on my bench, the first thing is to check the main 10A fuse, and the color of the emitter resistors in the outputs (crispy brown indicates a burnout of the output transistors, the same can be said for an open circuit main fuse). The 8000 I am currently working on was negative on both counts, and so I proceeded to rebuild the output boards with new quality 105C electrolytic capacitors from name brand Japanese manufacturers (Nichicon, Rubycon et al.). I also put in 25-turn quality encapsulated Bourns trimmers to control the quiescent current of the outputs. This is probably the most important operation in any Beomaster 8000 restoration. Hot heat sinks during modest volume play, or, worst case, smoke coming from the back of the receiver are usually related to the failure of the often corroded original single turn 100 Ohm trimmers that were standard back then. These old trimmers also make it difficult to adjust the quiet current to its prescribed 18 mV equivalent voltage drop across the output resistors since they drift thermally and over time. 25-turn trimmers are much superior, and allow a precise adjustment that will stay put for a long time.

Here are the PCBs after updating them (the original look is shown in this post):

Left channel:


Right channel

Below are the 25-turn trimmers before implantation in comparison with one of the original single turn trimmers. The 25ers need to be extended on the swiper lead to fit the old footprint. Also, it is important to orient them properly, so one gets clockwise increase of the quiescent current when turning the adjustment screw. If this is done reverse, then down the road another tech may get confused and accidentally fry the output transistors while adjusting the trimmers. Semiconductors behave exponentially, i.e. once the quiescent current threshold is crossed, the current will increase swiftly.
Once I had the new parts installed, I disconnected the power rails and the three plugs that are on the boards. Then I ran the PCBs with three external power supplies. While doing so, I adjusted the quiescent current to the correct 18 mV voltages at the test points:

Left channel:

Right Channel:

Both stages took in the prescribed ~100 mV via each of the main ±54V rails, so all is good now in Output Ville!
Now that the outputs were certifiably stable, the first power-up of the Beomaster could be attempted without worries about producing 'magic smoke'. Before turning it on, I reconnected the clipping light control line to its proper place (see heat shrink on red wire below the indicator light that is on - someone just cut this wire for whatever amateurish reason...gotta love it when dilettantes find a screwdriver (or wire cutter for that matter) and wonder how best put it to use...;-)

I connected my two bench-speakers and then the Beomaster came on with a health double relay click, indicating that the start-up sequence worked properly and that the microcontrollers were doing their job as they should. I tuned to a station, and I got some decent FM reception of a local station, which played nicely on the speakers. Unfortunately the displays of this Beomaster are in need of fixing. They show the frequently observed missing segments issue:

I tested all the functions of the Beomaster, and it seems that everything is working except the remote control. None of the buttons provoked any response by the Beomaster.

I poked around a bit and finally realized that the dilettante did not only cut off the clipping wire, but also did some strange experiments with the remote receiver. Here is a picture of a part of the power supply board which shows some 'additions', notably a Sprague capacitor soldered to D25 in the output of the TDA 4050 IR preamp, which has no obvious function. Furthermore, pin 1 on P48, which carries the remote signal is bent upwards, i.e. the signal is effectively disconnected from the microcontroller remote input. No surprise that the remote does not work...I will check this out when I rebuild the power supply. Let's hope that the remote just works when disconnecting the cap and reconnecting the output to the connector.

Monday, December 22, 2014

Beomaster 8000: First Peek Under the Hood - Some Remarkable Findings

Today I opened up the Beomaster 8000 that just arrived to do an initial assessment of the interior. Here are a few images of what I found:

Maybe most remarkable is PCB #9 (u-computer board), which looks uncharacteristically new. In difference to all the other 8000s that I opened up so far, this one is also a higher quality, thicker polymer material. Pristine green gloss!! It seems that this Beomaster received a replacement at some point, potentially in the 1990s.

Strange are the two white/red/blue striped cutoff jumpers that are soldered on to the board to wire connector for the volume encoder. Maybe they were used to check the encoder signals. A frequently found problem on these boards is that some of the vias malfunction. Some of them affect the volume encoder  functionality. Maybe the techs tried to figure it out, and then gave up and simply replaced the board.

The hypothesis that the Beomaster saw some care in the 1990s is supported by the electrolytic capacitors on the right output PCB, which have a typical 90's style light blue/silvery look:

Compare with the left channel, where the usual original 1980s models are apparent:
A good sign on both of these output boards is that the emitter resistors of the output transistors are not brown. This makes me hope that the transistors are still o.k.

Another thing I found is this cutoff wire (red) to P70 of the display PCB. It controls the clipping indicator. This probably explains why the seller mentioned an issue with the clipping light.

Aside from a missing screw for the control panel, this is the only major mechanical damage I found so far:

This is the left receptacle for the control panel cover hinge. Maybe I will find the missing piece of plastic under one of the PCBs and I can reattach it to give this hinge the support it needs. When I opened the Beomaster, the cover would not come up by itself, and it was stuck to the glass panel...this has probably to do with this loose hinge.

And finally a glance at the power supply board, which exhibits the usual golden Made in Germany reservoir cans that I have found on their last legs in most Beomasters that I opened up so far. Let's see if we have all the voltages when I turn this unit on for the first time (after I rebuild and test the output stages):

Sunday, December 21, 2014

Beomaster 8000: Double Boxed from Ebay! Amazing!

Another Beomaster 8000 arrived at my door yesterday. It is from eBay and will receive some TLC before I send it on to its new owner in the UK. I was almost impressed by the packaging this unit received (at least for eBay standards). It came double boxed and fully cushioned by well-cut monolithic styrofoam panels.

I would have used industrial strength poly foam instead since it decelerates blows much better than styrofoam, which is pretty hard unless it reaches its yield strength and permanently deforms (I am thinking of some delicate plastic mounts inside the Beomaster, which can shear off in case of sudden acceleration). Also the unit was only wrapped into a too small piece of poly foam sheet inside the inner box which did not protect the sides, so there was potential for chafing of the veneer on the cardboard. I did not like, too, that the plug was positioned in between the heat sink tower and the glass panel.

But, as far as my eBay experiences go: Really Not Too Bad! Here are some pictures:

The Beomaster survived the first leg of its long journey fairly well (which is probably also a result of FedEx's capable ways of handling heavy packages - much better than the USPS where they let small women handle 70 lbs packages without giving them a hand truck - some sad stories there, that I could tell...)

Here are a few pictures of the cosmetic condition of this beomaster after extracting it from the box. It looks fairly pristine in a 20/20 way:

One of my main concerns with the exterior of the 8000 are the veneered corners of the cabinet...usually they take some hits during storage or shipping. This unit is not too bad, but there is a bit of damage. I will probably 'stabilize' this one with a bit of glue that the hanging off part does not fully break off:

The other corner is acceptable:

A point of concern is this slight scratch. I hope it will be reduced with a bit of Mr. Clean Magic Eraser work. There is also a slight dent in the panel at the front...not much one can do there.
 Some smudges around the program keys...hopefully just a cleaning issue:
And a pretty big mark on the back where the paint went missing...oh well, one usually does not spend much time looking at the back of Beomaster 8000s...;-)

Saturday, December 20, 2014

Beocord 5000 (4715/4716): Polishing of Plexiglass Panel

I started working on another Beocord 5000 (4715/4716) restoration. It appears that cassette tapes are making their comeback to catch up with vinyl...;-). What better way to spend a Saturday afternoon than with making a mixtape for a loved one! I miss this way of showing affection...these days, all we do is swap Spotify playlists...not much Labor of Love in that! 

In this spirit I started out with polishing the badly scratched panel of a spare Beocord 5000 as a practice run for the better-condition panel that will go with the Beocord once I am done with its internal restoration. I felt I needed a practice run, since I worried about the coating layer that is on the plexiglass panels of these B&O units. It appears they felt that this coating would improve the shininess of the surface, since it appears that it does not change the tint of the panel. After I polished it, the tint appeared very similar despite the missing coating.

Here is a picture of the heavy damage of this panel:

I essentially followed the polishing procedure that I applied to the panels of my Beolab 4500 speakers. See here and here for a detailed description of this process.

The main difference with the Beocord 5000 panel occurred during the first sanding step with 220 grit dry paper to get rid of the heavy scratches. When I started sanding, immediately the coating disintegrated into tiny shreds and I observed this wool-like appearance:

After I worked through this step everything was as usual and I progressed through the ever finer sandpapers of the Micromesh polishing kit. A pretty good workout! After I went through all the sanding steps I followed up with their polishing solution, and then I ended up with a pretty nice result:

There are still a few minor scratches that are visible to detailed scrutiny, but I think this panel would now look very nice in the usual setting in a living room etc...It is very difficult to get them absolutely perfect since the slightest oversight in one of the polishing steps requires to go back to this step and repeat the entire procedure from that point on.

Thursday, December 18, 2014

Beogram 4002/4004: Rebuilt Keypad Surface

A good day in beolover's life! I received a rebuilt Beogram 4002 keypad from a fellow 'beofriend' in Denmark. He is able to refinish the keypad surfaces to the original splendor. I was absolutely amazed when I received the package. The resurfaced switches looked 99% like the original. Very awesome! This will give the 4002 that I am restoring a pristine look. Well worth the money!

Here are a couple pictures. The first one shows the old pad with the circuit board removed for exchange to the rebuilt one. The usual damage to the keys from pressing on the surfaces with bare fingers is clearly apparent. Skin acids in combination with mechanical abrasion eats the surface finish over time, and a smudged appearance occurs. If you look closely, on the start key the finish is fully gone in a few spots. Absolutely un-beolovely!

Below is the rebuilt keypad. Absolutely pristine looking. If you hold it next to the damaged one, the pads have very much the same appearance. I am stunned! Like I time machined back to 1975 and picked one up at the B&O store! Excellent job! 

Wednesday, December 17, 2014

Beogram 4002/4004: 3D Printed Cabinet Guiding Washers

I am getting closer to finishing up my Beogram 4002. One of the still remaining annoying issues was the loose cabinet, which would not remain in its locked position due to the absence of the guiding washers that hold it in place. In the 4002 these washers were made from plexiglass, which easily cracks during the rigors of transport or just due to old age.
I opened up my Beogram 4004 to see how things look inside there, and it turns out that the 4004 has metal parts instead of the plexiglass ones in the 4002. I took measurements of one of those washers, and printed replacements on a Formlabs Form 1 stereolithography printer.

Here is a picture of the results: 

I also made a short video to demonstrate the functionality of the guiding washers:

The video shows that the new washers perform well. They are a bit elastic since the stereolithography printer plastic appears to be that way, so one needs to be a bit careful to not over tighten the screws when installing them. Otherwise, I was quite happy with the result. The cabinet slides snugly in its tracks.

Wednesday, December 10, 2014

Beogram 4002/4004: Replacement of the Transport Lock Bushings With 3D Printed Parts

I recently got into 3D printing with stereo lithography. This proved a valuable skill for replacing the decayed rubber bushings in the transport locks of my Beogram 4002. Here is a picture of how I found these bushings upon opening the Beogram up:

Clearly the rubber had seen better times. So I removed the fragments and designed a two-part replacement solution with Autodesk Inventor that could be conveniently installed without having to extract the floating turntable chassis. I printed the parts with a Form 1 stereo lithography (SLA) 3D printer, which produces relatively soft and pliable parts, ideal for the task of dampening impact during transport of a precious Beogram! Here is the result of my labor:

The 10-15 micron resolution of these SLA printers is much superior to filament based printers (~100 microns), i.e. it is a much better technology for small parts that need to have an exact fit. In this case, the fit needed to be pretty good to be able to generate a press-fit, that would hold both halves of the bushing in the opening in the chassis. This worked very nicely as you can see in this video, where I demonstrate the installation. Enjoy!:

Thursday, December 4, 2014

Beogram 4002/4004: Speed Selector Circuit and Replacement of Its Incandescent Light Bulbs with LEDs

The last incandescent light bulbs are now gone in my Beogram 4002. I decided to completely replace all light bulbs since I like the longterm stability of LEDs. Furthermore, with red-green LEDs one can achieve a very authentic incandescent feel (much better than with single-color yellow or amber LEDs). I made a video about the process. The video also explores the speed selector circuit and it shows the relevant circuit diagram sections. Here it is:

Here are a few pictures of the LEDs after installation, and the look of the scales with LED illumination (for comparison - the insert in the video title shows an amber LED, which did not produce a satisfactory outcome - more in the video):

Wednesday, December 3, 2014

Beogram 4004 Plays Cannonball Adderley's Somethin' Else (Blue Note 1595)

A happy moment in my recently restarted life as vinyl enthusiast: I recently ebayed a new 200g pressing by Classic Records of the seminal Blue Note recording (1595) "Somethin' Else" with Cannonball Adderley, Miles Davis, Hank Jones, Sam Jones and Art Blakey. Today it arrived. One of my all time favorites! Right up there with Kind of Blue. And now playing on my lovely Beogram 4004 via the Beomaster 6000 4-Channel. A great harvest from all that restoration effort! Here is a picture of my bliss:

I am quite impressed by this Classic Records edition. Hardly any cracklings or other audible issues, and a just very transparent and natural sound. I got the stereo edition. I read that the Mono pressings of Classic Records are something special (interesting discussion here), so I am hoping to find one soon for a reasonable price to be able to compare (the 45 RPM issue from Analogue Productions would probably also make for an interesting comparison). I also like the quality of the sleeve a lot. Very heavy duty cardboard, and no funny added labels or anything...just looks like the original would have in 1958. Even the stereo sticker looks like an authentic reproduction. The record is also exceptionally flat. The only issue I could find is that it is slightly eccentric. But it is really difficult to find a record that is fully centric. 

I recently bought a Blue Note reissue of Herbie Hancock's "Maiden Voyage", and I have to say that the quality of sound of this Classic Records album blows the Blue Note reissue clean out of the water. Much more crackles and other issues on the Blue Note in-house reissue. Furthermore, the Classic Records package has a much better feel. The Blue Note reissue has added labeling like bluenote.com etc...also the inner sleeve is a crappy advertising paper sleeve, while the Classic Records issue has a three-layer plastic sleeve with an integrated paper to stabilize it. Much nicer! Well worth the additional expense. One immediately recognizes true love...;-). Like Beolove! 

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...;-):

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
Thickn.: 0.5 mm
Width: 5 mm
Thickn.: 0.85mm/
Servo Belt
31 mm
31.5 mm
Height: 1.5 mm /0.059”
Base: 2 mm /0.078”
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!