Beogram 4004: Replacement of Convertible DIN 7 Plug with a Female Socket and Adapter Cables

While enjoying the Beogram 4004 that I recently restored I was looking for an adequate modern replacement for its convertible DIN 7 plug. This plug is a special design that does not seem to be made anymore. Here is a picture of its business end:

The picture shows the plug in its DIN5 configuration, i.e. it can now connect with any standard DIN5 amplifier input. The two (threaded) orifices above the pins can hold two more pins, which can be bolted in if the Beogram is to be connected to a Beomaster 2400 to take advantage of its remote control, which can be used to start/stop the Beogram. In essence these two additional pins carry the remote control signals for the Beogram. But, of course, since this is a male plug, the pins have to be removable if the Beogram is to be used in a standard DIN 5 setting.

Sadly, these plugs are nowhere to be found these days, and so, if a new plug is desired, another approach needs to be found to preserve the above 'convertibility'. My solution was to replace the plug with a female DIN 7 socket. That way DIN5 (or DIN5-to-RCA) jumper cables can be used for non-B&O set-ups and DIN7 jumpers can be used to connect to B&O setups. In essence, this modification gives the Beogram 4004 the same connectivity design like the later Beogram 800x, which have a panel mount DIN7 socket in their back panel to receive a suitable jumper cable.

It was pretty difficult to find a decent female cable mount DIN7 socket, but finally Steve at soundsheavenly.com saved the day by providing me with a few DIN7 Deltron sockets for cable mounting.

Here are a few impressions: This shows the socket installed. The red and white leads are the signals and the yellow and blue leads carry the remote control.

It is important to note that the signal leads on a female sockets need to be soldered to the other side of the connectors since male and female plugs have mirrored pin assignments to properly maintain the signal chain through the connections. This shows the fully assembled socket:

This is how it looks with a DIN5 jumper cable (also provided by Steve) plugged in:

This Beogram 4004 is finally ready to be shipped back to its owner!

Beogram 4004 (5526): Final Adjustments and a Test Drive with Herbie Mann!

The restoration of this Beogram 4004 (5526) is finally coming to a close. After fixing the bent alignment pin for the aluminum panel around the platter it was time to adjust the floating sub-chassis and then the platter height. Once this is done, the arm lowering limit can be adjusted so that the needle misses the ribs on the platter should the record detection circuit malfunction at some point:

Once the limit is established, the arm counterweight can be adjusted and then the tracking weight. I usually install a M3 nut at the counterweight screw that one can fix it in place once the adjustment is done. This is a prerequisite when the turntable needs to be shipped and the weight calibration is to survive the trip. This shows the counterweight assembly in its original condition with the retaining washer that holds the adjustment screw in place:

And this shows the nut installed and tightened after calibrating the weight scale:

After this was done, I put on a cartridge and adjusted it to the prescribed 1.2g tracking weight with a digital gauge:

These low-priced gauges are very accurate these days and I highly recommend to get one for the occasional check of the tracking weight.

And then it was finally time to play this Beogram! I selected a recently acquired record by Herbie Mann, which is a pretty unusual and highly interesting album: Stone Flute, which he recorded for his own Embryo Records label in 1970. One can easily say that this is probably Herbie Mann's least commercial album. It is a highly addictive lush soundscape and quite different from his other outputs. In my opinion a seminal 70's jazz album that belongs in any serious jazz collection. Anyway, here is an impression of the Beogram and Herbie after cleaning the record with a CleanerVinyl ultrasonic record cleaner:

A Beolovely combination! Wouldn't you agree?? 

Beogram 4004 (5526): Adjusting a Bent Aluminum Panel Alignment Pin

When I initially received this Beogram 4004 (5526), removing the aluminum panels took me a while. The main panel around the platter was completely jammed in a 'too far forward' position, which prevented its release by sliding the plinth forward (basically the plinth could not be moved far enough to release the panel). Once the panel was finally off (convinced with a bit of 'gentle violence') it became clear what the issue was:

The alignment pin on the left side was bent forward, which forced the panel to seat too far forward, too. These pins go into holes drilled into the bottom of the panels. This shows the corresponding hole:

The dab of white goo at 5 o'clock is a bit of Alene's glue transferred from the pin, which I used to get me an approximate position of the bent pin relative to the hole when the panel was aligned. After bending the pin a few times iteratively and watching where the glue moved I was finally able to get it properly aligned holding the panel in the correct spot:

On to the final adjustments (which require the aluminum panels to be installed properly)...

Beogram 4004 (5526): Replacement of the Sensor Arm Bulb with an LED Assembly

The sensor arm light bulb is the most difficult to replace with an LED in the Beogram 4000 series. I recently developed a flex PCB based assembly based on a warm-white LED, which fits into the small bulb compartment of the sensor arm. This picture shows the original light bulb that was installed in the Beogram 4004 (5526) that I am restoring right now:

This shows the light bulb in comparison with the flex PCB based replacement:

The LED assembly folds precisely into the bulb compartment and is held in the right position by the 3D printed red wedge that is glued on top:

This shows the assembly in action:

The warm-white LED ensures that the B&O logo is lit up properly in red...amber LEDs do not contain any red photons, and so the logo would look grayish-amberish.

Once the LED is installed, the detection circuit needs to be checked and adjusted for maximum sensitivity. I wrote a detailed explanation in the above link, which explains how the circuit works. One of the issues of the circuit is the amplification of the sensor signal, which itself depends on the position and shape of the light source in the bulb compartment, as well as the biasing of TR3, which takes care of the initial amplification of the fairly weak sensor signal. The biasing depends on the current gain of the particular transistor installed as TR3. Current gains notoriously vary throughout a transistor production batch, which suggests that feedback-based amplification schemes are much more stable across a circuit production run. The circuit used here, however, is not feedback based and so the amplification can vary. The key is to adjust the working point of TR3 that 4V are at the collector when there is no sensor signal (record present). This involves adjusting R26 until 4V are measured. In the above post I simply implanted a 2M resistor and hoped for the best (it worked) but in the spirit of Kaizen, this time I used a 2M trimmer that I could adjust to achieve a precise 4V collector setting. To do the adjustment it is a good idea to first install the trimmer on the solder side of the board that it can be accessed while the board is installed and the turntable running:

After adjusting the trimmer to get 4V at the collector

I flipped the board up and installed the trimmer on the component side:

It fits snugly into the place of the original 1M resistor.

After this installation I measured the sensor signal at the collector while the platter was spinning (if you do this yourself, you would need to install a jumper cable on the board feeding out the signal that the platter can spin while doing this measurement):

The achieved 6.62V amplitude is more than sufficient to inform the record detection circuitry about the absence of a record, i.e. this installation was successful.

Beogram 4004 (5526): Restoration of the DC Platter Motor and RPM Stability Test

After replacing the RPM trimmers, the RPM relay and the light bulbs in the RPM trimmer panel with LEDs, it was finally time to restore the DC platter motor of this Beogram 4004 (5526). This sequence is important, since one can only test the RPM stability performance of the motor properly after the entire RPM control circuit has been rebuilt. This shows the extracted motor:

The motor needs to be completely disassembled to get to the dried out Oilite bearings:

The bearings are the two small donuts on the black pad. I infused them with motor oil under vacuum:

The raising bubbles indicate that air is drawn from the porous brass bearing material. This creates space for oil to enter the bearing. After about 12-24 hours the process usually stops, at which point the bearings are again full of oil.

This shows the bearings after the infusion process:

After reassembling the motor I put it back into the deck and adjusted the RPM with the BeoloverRPM device:

Then I used the device to measure a RPM stability curve over 18 hours:

This motor showed an exceptionally stable RPM performance and can be considered ready for duty.

Beogram 4004 (5526): Installation of LED Light Sources into the RPM Trimmer Panel

After rebuilding the PCBs, the final step of the restoration of the electronics of a Beogram 4004 (5526) is replacing the incandescent light bulbs in the RPM trimmer panel with LEDs. This reduces the heat load on the RPM trimmers, which improves their stability over time. This is recommended since thermal changes can affect the RPM stability of the platter motor. Here is a photo of the panel taken out:

After removal of the two covers the bulbs can be accessed:

This shows the bulbs replaced with the LED based assemblies:

They are direct drop in replacements with the proper resistors installed to make the LEDs compatible to the 24V that are used to drive the bulbs:

The covers install exactly like before:

And this shows the 33 RPM LEDs in action:

They achieve a warm incandescent glow due to mixing red and green LED light in the right proportions to yield a color temperature similar to an incandescent bulb, while ensuring that red light is present to properly illuminate the red indicator.

Beogram 4004 (5526): Restoration of PCBs and Reservoir Capacitor

After restoring the arm lowering and tracking systems of the Beogram 4004 (5526) that I am restoring right now it was time to rebuild the PCBs and install a new reservoir capacitor. This shows the original main PCB of the deck:

And after replacing all electrolytic capacitors as well as the RPM trimmers and the RPM relay:

This shows the rebuilt RPM section in detail:

I usually replace the original single turn trimmers with modern encapsulated 25 turn units, which greatly enhances the accuracy with which the RPM can be adjusted. The replacement relay is a Beolover design that uses a modern encapsulated SMD relay to replace the original National relay. This shows the original and new relay in comparison:

On to the output PCB with the output relay circuit and (in the case of the 4004) additional circuitry to enable remote control via the Beomaster 2400. This shows the board in original condition:

A detail shot of the relay section:

I usually install a new relay and a switch that allows connecting signal and system grounds together, which usually is a reliable method to quench any hum that may occur when connecting to non-B&O amplifiers that use RCA jacks:

This shows the entire board in restored condition:

The remaining part of the PCB restoration is to replace the original reservoir capacitor with a new one. This shows the original capacitor:

And this shows the replacement, which uses a 3D printed adapter to fit a modern (smaller) capacitor under the original clamp and into the compartment in the metal chassis:

The next step will be to install LED lights into the RPM trimmer scales.

Beogram 4004 (5526): Restoration of Arm Lowering and Tracking Systems

After a first look 'under the hood' and a discussion with my customer about what should be done I set  out to rebuild the arm lowering mechanism and the tracking system. This shows the arm lowering mechanism in place:

I took it apart and cleaned and lubricated the components:

After reassembly, I realized that the solenoid was not bolted in the correct position. This can be seen by the alignment of the solenoid lever that did not align in parallel with the threaded rod that drives the carriage when fully extended:

I shifted the solenoid until parallelism was achieved:

Then it was time to look after the linkage that connects the damper with the tonearm. This requires the removal of the sensor arm. This shows the arms from the back:

and after extraction of the sensor arm and removal of the linkage (be careful not to loose the small spring that is under the linkage - it easily jumps to an unknown location in the space-time continuum...;-):

After cleaning and lubricating I reinstalled the arm and adjusted the arm parallelism:

The next step was to replace the incandescent light source of the tracking sensor with a Beolover LED based replacement unit. The original set-up is shown here:

This shows the original and the replacement in comparison:

The LED is approximately in the same position like the filament of the bulb. This picture shows the replacement installed:

The final step is cleaning/re-lubricating the threaded rod that moves the carriage and the installation of a new pulley (provided by Nick - let me know if you would like to obtain such a pulley and I will be happy to get you in touch). This shows the original plastic pulley:

They are cracked in most Beograms, i.e. I usually replace them even if they are still in decent condition such as this one. Beolover restorations aim to achieve long-term reliability and so we usually exchange all parts that have an established failure history. This shows the new aluminum pulley installed:

Beolovely! On to restoring the PCBs and replacement of the reservoir capacitor.