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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...

Saturday, April 29, 2017

Beogram 4004 (5526): Installation of New Transport Lock Bushings

When taking out the main PCB for restoration it is a great moment to also repair the transport lock bushings in Beogram 4002/4004 turntables if they need replacing. The 4004 that I am currently restoring had the orange bushing type that usually is degraded at this point in time. This is the typical look when they start to go:
A closer look after removing the top plate of the lock:
It is a great idea to take the sub-chassis out to clean out all of the fragments. They can be lodged under the sub-chassis impeding its free motion during record play, which negates the vibration insulation afforded by the leaf spring mechanism. Luckily, it is fairly easy to take the entire chassis out in the 4004 since it is only connected by wire-to-board connectors. So after removal of the three transport lock top plates it can be lifted out:
A typical 'find' underneath it:
This shows the chassis solo:
Once it is out it is easy to remove the bushing fragments:
and insert the Beolover replacement bushings, which are available via the Beolover Shapeways store (two parts are needed per bushing since they are designed as two parts which insert from below and from the top):
first the bottom part is inserted:
and then completed with the top part:
After doing this for all three locks it was time to put the chassis back in:
This shows one of the bushings after reassembling the locks:
On to the electronics!



Friday, April 28, 2017

Beogram 4004 (5526): Restoration of Arm Lowering and a New Aluminum Pulley for the Carriage

The first step of any Beogram 400x restoration is usually rebuilding of the arm lowering mechanism and getting the carriage to move properly again. The owner of the Beogram 4004 (5526) that I am currently restoring already cleaned and re-lubricated the arm lowering mechanism, but the linkage between the damper and the arm itself was lacking the retaining washer and the small spring that holds the linkage in place. So I needed to remove the sensor arm to repair this issue:
I installed a 3 mm retaining washer and a nylon washer underneath to reduce friction:
After putting the sensor arm assembly back in I adjusted the arm parallelism:
One more thing was needed: A lovely new aluminum pulley provided by Nick (use the contact form or send me an email if you wanted one from him - I'd be happy to get you in touch). These pulleys are a great improvement relative to the often wobbly original plastic pulleys. This shows the original pulley:
and the shiny replacement:
On to repairing the transport lock bushings...





Beogram 4000: Repair of a Cracked Plexiglass Hood With Laser Cut Patches

The most fragile part of a plexiglass hood of any Beogram is the hinge region. That is where the highest torque is applied to the plexiglass, while it is structurally weakest due to the holes permitting the screws that hold the metal hinge to the hood. The Beogram 4000 that I recently restored had a hood with a large crack through the holes and a broken out piece:
Encouraged by my recent restoration effort on a Beogram 3000 hinge that was broken off I decided to develop a similar fix for the 4000/4002 hoods. Unfortunately, the space one can work with is much less in 400x hoods, since the metal hinge part is bolted to the inside of the hood, i.e. if the plexiglass gets too thick, the metal part does not fit anymore. So my approach here is to use two very thin 20 mil (~0.5 mm) patches that are precision matched to the bolt holes and the hood shape.
But the first step is to try to glue the cracked parts and re-connect the broken out part:
I use Weld-On #4 acrylic glue, which is so liquid that is is drawn into cracks by capillary forces. This is a great feature since one does not have to bend the plexiglass any further to apply it. All one does is squirt a bit on the crack and one can see how it is sucked in. One can then simply press the parts a bit together for ~5 min (play some nice music while you sit there holding the pieces together...;-) and then the next step can be done, which in this case was reattaching the broken out part:
Another 5 min of music listening and then I let it cure for a couple hours. The next step was to attach the laser cut patches. This shows the inner patch:
I used Weld-On #16 for these, since the surface was pretty rough due to the broken out part and previous 'repair' attempts. After curing the patch for ~2 hours (=50% strength of the bond) I did the same for the outer patch:
At this point I let everything cure for 24 hrs to achieve a close to final strength of the bond (~80%). Only after a few days such bonds are fully cured and 100% strength is achieved.
In order to have the metal hinge centered, I also applied an inner patch on the opposite side of the hood. This was a good idea anyway since closer inspection yielded that there already were some minor cracks under development around the screw holes.

Then it was time to bolt the metal hinge in with new stainless steel screws:
The next step was to reattach the aluminum trim to cover the screws. This requires removal of the old glue, which can be done with a razor blade and careful scraping. Take care that the aluminum trim is not bent too much or otherwise damaged. These side parts can break off easily if the bend is flexed too often. I usually use an industrial strength ultra-thin adhesive transfer tape (3M 300LSE) to glue them back on. This allows to generate a very thin and precise adhesive layer, which makes a strong bond. 'Thin' was especially important here to minimize the thickness increase due to the outer patch:
After removal of the tape backing, I pressed the trim on and cleaned some glue splotches from previous repair attempts on the metal surface with Mr. Clean Magic Eraser as good as possible:
This is how this side looks from the front:
It looks pretty good, albeit closer inspection of course will reveal that there is a thin patch underneath. The price for a stabilized hood hinge!

One more thing: Like most Beograms of this vintage, the rubber hood bumpers were broken off:
As usual, I drilled them out with a 2 mm drill bit:
and then glued in snippets of a 2 mm O-ring:
Using a 1 mm template, I cut them to size with a razor blade once the super glue cured:
And here is the end result:
The final step was to polish this hood since it had the usual scratches after about 40 hears in service:
I gave it my usual multi-step polishing procedure that started with 200 grit dry sand paper and then worked it back to a wet 3000 grit level through about 10 sanding steps. I had to be a bit more careful than usual due to the repaired crack. I definitely did not want to break it again...;-). This is how it looks now (still has some black protective tape on the trim):
Time to install it again on this lovely Beogram 4000!






Saturday, April 22, 2017

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!



Friday, April 21, 2017

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.