This past weekend, I began working on some of the checks and adjustments outlined in the Beogram 4002 service manual. The manual details both electrical and mechanical procedures.

On the electrical side, my primary focus is to ensure that the photo sensors generate strong signals when activated by their light sources. Maintaining correct voltages is crucial for the proper function of the Beogram 4002’s analog control system.

Mechanically, I’ll be aligning both the Beogram’s fixed arm and tonearm. This involves setting the platter height relative to the fixed arm and tonearm, adjusting the tonearm’s lowering limit, and calibrating the tonearm’s tracking force.

Finally, there’s an adjustment that bridges both electrical and mechanical aspects: the tangential tracking sensor. Following the service manual’s instructions, I’ll fine-tune this tracking mechanism to ensure the Beogram accurately advances the tonearm (equipped with the phono cartridge) during record playback.

These tasks are quite extensive, and many of the mechanical adjustments are interconnected. While the process can sometimes be straightforward, it often requires multiple iterations to achieve optimal results.

My initial adjustment involved the tangential drive spindle, responsible for moving the tangential arm assembly during playback. The tangential drive motor rotates the spindle, which in turn connects to a nylon nut on the tangential arm assembly, effectively pushing the assembly back and forth. The tangential drive spindle can be vertically adjusted relative to the floating chassis and horizontally adjusted for optimal distance between the spindle and front tie-rod.

The following three pictures illustrate the process of checking the vertical adjustment at various points along the spindle.

The horizontal positioning follows a similar method. I utilized calipers to verify that the distance between the front tie-rod and spindle remained consistent at both ends of the spindle.

With the spindle accurately driving the tangential arm assembly, I proceeded to examine the arm position sensor, specifically its voltage level.

The image below showcases the arm position sensor and its lamp. The tangential arm assembly incorporates an acrylic lens that passes between the lamp and sensor as the assembly moves across the platter. This lens has markings that influence the sensor voltage, which, in turn, informs the analog control system’s actions.

The arm position sensor is integrated into the following circuit:

According to the service manual, adjustment of this sensor involves positioning the Beogram tonearm so that the sensor lamp illuminates the sensor through a transparent section of the position lens. At this position, the collector of the sensor (4IC1) should register 5 VDC. If this reading is not obtained, trimmer 1R88 should be adjusted to achieve the desired 5 VDC. This adjustment should also result in a 0.7 VDC reading at the base of transistor 1TR17.

Given that the original position sensor lamp (4D1) was burnt out, replacement was necessary. However, the replacement LED differs from the original, leading me to anticipate an adjustment to 1R88.

As expected, the initial voltage at the sensor collector (4IC1) measured only 1.5 VDC.

Utilizing the trimmer on the main board (1R88), I successfully adjusted the voltage for the 4IC1 collector.

With that adjustment complete, I decided to take a preliminary look at the fixed arm sensor’s output signal. Beolover offers an informative video detailing the arm sensor, its functionality, and appropriate sensor signals: great video about the arm sensor.

For this Beogram 4002 project, I connected an oscilloscope probe to the 1TR3 collector to monitor the record detection signal.

The signal, with no record present on the platter, appears as follows:

The Beogram 4002’s record detection circuit is functional with this signal. However, it’s evident that the signal strength falls short of the service manual’s specifications. Ideally, the peak should approach 6V, and the dips in the signal should reach 0V.

This indicates the need to revisit this circuit and implement the adjustment demonstrated in Beolover’s video: Beolover shows in the video.

Next on my list of service manual adjustments were those related to the tonearm. While experimenting with the Beogram tonearm’s movements (lowering, raising, pivoting), I observed that it would drift towards the fixed arm when lowered, rather than moving straight up and down.

This type of pull often suggests friction between components at the tonearm’s pivot base, hindering their free movement. Additionally, I noticed that the spacing between the fixed arm and tonearm was less than the 7.7mm gap specified in the service manual. Clearly, some alignment issues needed addressing.

The tonearm and fixed arm can be aligned using a series of adjustment screws. However, these adjustments can influence one another, often necessitating several rounds of fine-tuning to meet the service manual’s specifications.

Rectifying the distance between the two arms involved adjusting both the fixed arm’s position and the height of the lowering/raising arm (the metal arm connecting the tonearm and fixed arm).

These adjustments successfully established parallelism between the two arms and achieved the desired 7.7mm gap.

During the lowering/raising arm adjustments, I also repositioned the tonearm counterweight. Its initial placement, too close to the back of the tonearm, caused the side nearest the fixed arm to make contact, resulting in the tonearm tilting towards the fixed arm when lowered. Readjusting the counterweight assembly corrected this issue.

Before calibrating the tracking force, arm lowering limit, and stylus position, I needed to verify the platter height relative to the fixed arm (and tonearm). The service manual recommends a distance of 23mm from the metal platter surface to the top of the fixed arm.

For this measurement, I use a tool that I keep set to 23mm. If the height is incorrect, the platter bearing needs adjustment. This particular Beogram required only a few adjustments to achieve the correct height.

Moving on to the tonearm lowering limit, the Beogram needs adjustment to ensure the tonearm lowers to a maximum position where the phono cartridge stylus rests approximately 0.5mm above the platter’s lower rib section.

To confirm correct phono cartridge positioning on the platter and re-check parallelism, I use a piece of thread. I tie one end to the center spindle and extend it to the right side of the Beogram. Then, I ensure that the stylus lands on the thread both at the platter’s edge and at the point corresponding to the end of a record.

With these steps done, the Beogram tonearm is now correctly configured for finalizing the tracking force adjustment. The tonearm counterweight is already fairly well-