The tracking sensor bulb in my Beogram 4000 recently burned out. This was the fourth bulb to fail. I decided to use this opportunity to install my new SMD LED replacement assembly. This assembly uses efficient white LEDs at low power for longevity, likely exceeding 50 years. It also reduces the unit’s energy consumption by a few watts, lessening the load on the aging transformer. Details about the assembly and adjustments can be found in this video.
This image shows the original bulb assembly:
This image shows the updated design. The blue component replicates the original bulb housing’s lower section to shield the sensor’s opto-resistors from ambient light. The PCB houses the LED, a current-limiting resistor, and a trimmer potentiometer for fine-tuning the LED intensity. This upgrade allows for precise calibration of the tracking sensor to B&O standards (see video).
Here’s the assembled light source viewed from below. The left ‘window’ houses the LED, positioned near the original filament location.
This view from the top shows only one contact lead, as the second contact is grounded through the mounting screw, simplifying installation.
Here is the installed unit:
Testing confirmed the LED’s functionality. However, this Beogram 4000 surprised me by only tracking in reverse after installation (the video demonstrates this). Unlike later models, the 4000 can play records in reverse.
To troubleshoot, I examined the tracking sensor’s lower section, which houses the two sensors controlling the carriage motor. I was concerned one of the opto-resistors had failed. Here are some images of the assembly:
This image shows the top with slots housing the sensors. Carriage direction depends on which sensor receives more light.
This image shows the bottom:
These classic semiconductors represent a pivotal time in circuit design, marked by rapid advancements.
Testing revealed both sensors were functional, showing megaohms of resistance in darkness and around 1 kilohm under a bright LED lamp. This indicated the issue wasn’t the sensors.
Ultimately, I discovered a cracked trace on PCB #6, disrupting the connection between one sensor and the circuit. The pad connecting the 24V rail to the sensor lead had broken. I repaired this by soldering a small extension to the affected lead, visible in the above image.
After this fix, I calibrated the tracking sensor, following the steps in the video. I mechanically adjusted the sensor using the excenter and then fine-tuned it with the trimmer potentiometer until it consistently achieved two rotations. The potentiometer simplifies the process significantly, making it quick and painless.
Here is the Beogram 4000 playing Cannonball Adderley’s “Somethin’ Else” on Blue Note:
