Beogram 4000: Installation of a Grounding Switch That Allows to Connect System and Signal Grounds

The last Beogram 4000 that I restored had a strong hum while playing when I connected it to my Beomaster 6000 4-Channel phono input. This was fixed by simply connecting signal and system grounds. While I was not able to fully figure out why the hum occurred, I thought it wise to install a switch with which one can choose between the original grounding set-up with separate system and signal grounds (which typically results in a connection between them in the amplifier circuit) and a direct connection between the two right at the 4000 output. My original post about this procedure contains a detailed discussion of the electrical setup. I thought this to be a useful upgrade for any 4000 and so I decided to put a switch also into this one. Here are a couple pictures to document the installation of the switch on this Beogram 4000.

This picture shows the original setup. Signal and system grounds are connected separately to the shielded DIN cable. System ground is connected to the shield, while the signal ground is connected to the inner lead-shields:

Here is the set-up with the switch allowing the user to switch easily between the two setups. I also cleaned up the soldering mess of the as-found configuration:

A discussion of the grounding strategies of the later Beogram 4002 and 4004 models is included in this video (they actually went from the original separate grounds model to connecting them in the 4004, i.e. my switch simply implements both strategies in one set-up allowing a choice between them):

Beogram 4000: Adjusting the Gap at the Carriage Servo Pulley Bearing

When I tried out the carriage mechanism of the Beogram 4000 that I am restoring right now after repairing and adjusting the tracking mechanism, I found that the threaded rod that advances the carriage had a lot of longitudinal play. Here is a picture of the setup:

The play manifested itself in an unwanted 'clacking' every time the carriage changed its direction. This turned out to be an easy fix. All I had to do was to remove the servo pulley and remove the damaged locking ring and the worn out plastic washer:

This is the pulley. It is held in place by a setscrew whose threads are in the small brass plate that is inserted into the slot above the shaft receptacle. It should be noted that it usually sits very tightly on the shaft and that it took some careful 'convincing' with a flat head screwdriver to pry it off the shaft.

I replaced the locking ring and the washer by a thick 2-56 nylon washer which fit perfectly on the shaft. The locking ring is not needed since the pulley is locked in place with the set screw:

I also lubricated the shaft in its bearing and the nylon washer with a bit of silicone grease.

Then I put the pulley back on:

Now it has only a fraction of a mm play and the carriage moves very precisely and without strange noises. I added a bit of silicone grease to the threaded section of the shaft and that quieted the mechanism further.

Beogram 4000: Cleaning the Carriage Position Switches and Fixing a Bent Output Switch Contact Tab

Today was 'switch day' for the Beogram 4000 that I am restoring right now. It is usually a good idea to clean the contacts of the many open switches in this design. I usually clean the contact areas with a 2000 grit polishing paper and then give them a coat of DeoxIT D100, which prevents further oxidation very effectively and forms a protective conductive layer. Some in the field insist that the switches need to be unsoldered and completely cleaned, but I think the risk of damaging and bending the contact tabs is too great a risk for simple cleaning. One should keep in mind that all that matters for a switch to function well are the actual areas that come into contact. So it does not matter at all if the switches look a bit dark as long as the contact areas are not oxidized. In fact it is most important that the switch tabs retain their correct shape to give the switch its correct mechanical behavior (and right feel in the case of the control panel buttons). 

Here are a few pictures from cleaning the contacts of the switches on PCB7 under the carriage mechanism:

The pic below shows PCB 7 as it is installed. The switch trigger plate is already taken out for access. A while ago I made a video about this process. It is located here on my YouTube channel. The entire Beogram 4000 playlist can be accessed here.

This shows the PCB taken out:

The contact areas are beneath the 'bridges' that straddle the contact tabs. An exception is the center switch that is a break switch, and the contact pole is underneath the tab. For the functioning of the switches it is crucial that the contact tabs are straight and have enough 'spring' to make or break their contacts in rest position. The tabs I found here look perfect, but are oxidized. This shows how I clean them with the 2000 grit paper. I do not recommend to use smaller grit numbers since that runs the danger to scratch the surfaces. The oxide layer is only microns thick, i.e. light polishing can easily restore the surfaces to their original sheen. I fold the paper and then lightly press the switches to bring the contacts together, then I move the paper a couple times forth and back:

After this process I use a thin cardboard strip coated with DeoxIT and repeat the process. This is all it takes to achieve a reliably functioning switch.

After I did PCB 7, I went on to fix the damaged output switch that is operated by the tonearm solenoid. Here is a picture of the tab as I found it. The output switch is the double switch in the center. It grounds the outputs when the arm is not in its down position.

It shows the usual signs of dilettante-intervention when a non-working switch is 'adjusted' in a vain attempt to make it work again. Once such a tab is bent, the only way to fix it properly is by taking it out and straighten it. Luckily it was the upper switch that needed this treatment, and so I did not have to take the board out. I was able to simply push the wires out of the way and desolder the tab:

I straightened it out and gave it a slight downward slant relative to the press-fit installation clamp at its end. Then I soldered it back in. This gave it a solid spring force towards the contact. It looks now like the tab of the other channel below:

After this I cleaned the other contacts (left is the carriage servo enable and on the right under the yellow cable the solenoid current limiter). Beautiful! This is beolove!

Beogram 4000: Adjusting the AC Motor Voltage and Measuring the Voltage Differences Between the Phases

Today I adjusted the AC motor of the Beogram 4000 that I am currently restoring. The manual prescribes to adjust the main phase of the motor to 6V (RMS) and the frequency to 42.3 Hz for 33 RPM. This is easily done with an oscilloscope. To see both phases the probes can be simply connected to either side of the phase capacitor:

Adjustment procedure: First set the 33 RPM trimmer on the control panel to its null position (i.e. vertical). This ensures that the adjustment of the RPM offset potentiometer on the main PCB can now be adjusted to yield a 42.3 Hz motor signal when the control panel trimmer is nulled out. This ensures that the user can both trim the pitch for the same frequency deviations in + and - directions. Once the frequency is set the MOT potentiometer can be adjusted. It is important to do it in this order, since the frequency affects the amplitude of the signal. We are working with an analog oscillator here...;-).

Once everything is adjusted the oscilloscope should show about this:

The yellow trace is the main phase and the green trace is the 2nd phase. It is interesting to note that the 2nd phase has a smaller 5.3V (RMS) amplitude. This concludes the adjustment.

When switching the motor to 45 RPM one sees this:

Note that the 2nd phase is now much larger, about 8V (RMS), while the main phase dropped a bit to 5.6V (RMS). The manual also wants 6V (RMS) here, but there is nothing one can do about this since the MOT trimmer determined both frequencies, and 33 RPM is probably for most people the much more important speed. 

There is an interesting detail I noticed about the RPM trimmers on the control panel. The 45 RPM trimmer is nulled when the correct frequency of 57.3 Hz is adjusted with it:

When I took the control panel apart for converting it to LED illumination, I noticed that the red inserts that allow the adjustment of the trimmers with a screw driver are different for 33 RPM and 45 RPM: The driver slot is at an off-angle for the 45 potentiometer, while for the 33 trimmer it is symmetrical. Again, the designers gave the 33 speed priority and made a small compromise for 45: The consequence of this off-angle is that the 45 speed can only be adjusted by about a quarter turn to higher speeds, while one can go about a half turn to lower speeds. All this was probably dictated by the need for using standard resistors and trimmers in the Wien oscillator feedback circuits, which prevented them from designing the circuit in a way that the 45 trimmer would be exactly in its center position for an exact 57.3 Hz.

With modern resistor values one could easily fix this since nowadays one can buy resistors with almost any value. Apparently not so in 1973. Of course if one would now fix this by replacing the resistors, the trimer slot would be at an angle, which would not look good. What a conundrum! Will I be able to sleep tonight?? This is Beolove!..;-)

Beogram 4000: Replacing the Tracking Sensor Incandescent Bulb with a Drop-In SMD LED Assembly and a Peek at the Opto-Resistors

The fourth light bulb that was broken in this Beogram 4000 was the tracking sensor bulb. I would have replaced it anyway. My recently developed SMD LED based replacement assembly works very well now in several Beogram 400x, and why risk a bulb failure down the road if this can be done with a LED, that most likely will survive the next 50 years? I use high efficiency white LEDs, which I run at a very low power level. This will make them last very long. Also it reduces the power consumption of these units, which is a concern due to the main transformers occasionally giving out, since they are not very over dimensioned, i.e. run close to their rated power. If all LEDs are replaced this way this can easily reduce the power intake by a few Watt, corresponding to a 10-20% current reduction in the transformer.

Anyway, here are a few impressions of this procedure. Details and instruction for the implementation of the drop-in ready assembly and the necessary adjustment of the tracking mechanism can be found in this video.

This shows the original light bulb assembly:

This shows the replacement design. The blue part matches the lower half of the original bulb housing shape to shield the opto-resistors in the sensor from ambient light. The PCB carries the LED and current limiting resistor, as well as a trimmer potentiometer to adjust the intensity of the LED. This is a great upgrade for fine tuning the tracking sensor performance to B&O spec (see video).

This shows the light source assembled from the bottom. The left 'window' contains the LED, which is located close to the original position of the bulb filament.

This shows the unit from the top. There is only one contact lead since the other contact is made to GND via the mounting screw. This simplifies the implementation of the assembly:

Here it is installed:

A test revealed that the LED performed as designed. 

Before I got it to work, however, this particular Beogram 4000 surprised me after the installation with a tracking mechanism that would only work in reverse direction (check the video for a demo - the 4000 can actually track a record in reverse direction, which makes it different from the later 4002 and 4004 models). I removed the bottom part of the tracking sensor unit, which contains the two sensors that control the carriage motor movement to check the two opto-resistors. My fear was that one of them had died. Here are a few pictures of how this assembly looks like.

This shows the top. You see the two slots under which each one sensor is placed. Depending on which one is brighter the carriage moves left or right. 

This shows the bottom:

Lovely old semiconductor devices. This is one of the reasons why I like this design so much. It is from a point in time where circuit design started taking quantum leaps due to the advent of integrated and other more advanced semiconductor devices.

Anyway, a test of the sensors showed that they worked. Here is what I measured: Several MOhms in darkness, and about 1 kOhm when blasting them with a quite bright white LED desk lamp. Both showed identical values indicating that I was searching at the wrong location for the root cause of the tracking issue. 

Of course, what I found was a cracked trace on PCB #6 that connects the sensor to the rest of the circuit. One of the pads that connect the 24V rail to the upper end of the sensors broke from the connecting trace. I fixed this by soldering a short extension to the sensor lead in question. You can see it in the above picture if you look closely.

Now the deck was ready for the tracking sensor adjustment procedure. I did pretty much exactly what I show in the video. First, I adjusted the sensor position mechanically using the excenter and then after being close to the prescribed two-record rotations, I used the trim pot to get a precise two rounds. I think I will implement this trim pot also in future Beogram 4002/4004 restorations. It just takes all the pain out of the adjustment process and makes it a quick affair.

Here you see it in action playing Somethin' Else by Cannonball Adderley on Blue Note:

Beogram 4000: Replacing the Reservoir and Motor Phase Capacitors

I completed the recap of the Beogram 4000 on my bench by replacing the reservoir and the motor phase capacitor. I recently developed a 3D printed adapter for that, which secures the new smaller capacitors in the location of the old ones using the original mounting strap. This post gives more detail about the replacement process and a detailed 'wire map' of all the connections that go to these capacitors. It can get a bit confusing when doing this for the first time. Here are a few impressions from this particular Beogram 4000:

This shows the original cans:

This is the replacement assembly with the new capacitors placed into the 3D printed adapter. This shows how to prepare the capacitors to accept the wiring. Note that the two capacitors on the left are the replacement for the unipolar original 150 uF motor capacitor. I use two 300uF caps back to back:

And here is a shot of the installed assembly:

Like all the parts that I develop, this is assembly is available as a kit to other enthusiasts. Just send me an email.

Beogram 4000: Replacing the Electrolytic Capacitors on the PCBs

While I worked on the breakout board needed to replace the RPM relay with a modern encapsulated signal relay, I also recapped the PCBs. I think it is worthwhile to replace these vintage electrolytic capacitors since the tantalum types can actually catch fire when they go, while the orange aluminum caps are often out of spec at this age. I usually use quality 105C grade capacitors from major Japanese manufacturers, good for another 40 years...;-)

Here are a few impressions:

This is the main board in original condition:

And here after replacing the capacitors (the RPM relay is still in there):

The power supply PCB also has two electrolytic capacitors. This shows the original board:

And here after the replacement:

All good now in the capacitor department! Here is a picture of the lovely old color coded tantalum capacitors. I wish they would make them still that colorful!

Ain't they pretty?? This is Beolove...;-). On to the reservoir caps.