I recently had two Beogram turntables (a BG4000 and a BG4002) in my workshop with malfunctioning photocells in the detector arm. Usually, these photocells have a long lifespan, but the light bulbs can experience broken wires. While a pre-existing LED replacement for these bulbs exists (see sensor arm light bulb replacement), in this instance, the photocells themselves had broken wires and even a fractured cell, rendering repair impossible. Finding replacements proved futile as well.

The damaged photocell is shown below:

The Beogram 4000 series utilizes a BP100 photovoltaic cell, which generates DC voltage upon contact with light, similar to a miniature solar panel. Unable to locate a photovoltaic cell of the same size, I explored alternative solutions.

After testing various options like photodiodes and photoresistors, I settled on a phototransistor due to its superior sensitivity and reliability for this purpose. While integrating it into the detector arm presented a challenge, an “Osram opto sensor” proved to be a perfect fit, requiring no modifications.

The sensor measures 4.6 mm x 5.8 mm with a thickness of 1.7 mm and is an OSRAM LPT 80A.

To install, remove the light bulb, bend and insulate the sensor wires, and then carefully snap it into the black detector housing. Ensure the sensor makes contact with the brown PCB, aligning the sensor’s lens with the detector unit’s larger lens.

Insulate the wires using heat shrink tubing.

Bend the wires for seamless insertion into the housing and PCB holes.

The new sensor is now in position and soldered.

The light bulb is back in place.

The updated schematic with the additional resistors is shown below.

Here’s a breakdown:

The OSRAM phototransistor is an NPN type with an open base, meaning it has only two terminals (collector and emitter) and relies on incoming light to trigger conductivity. To establish a functional circuit, a power source is necessary. Through trial and error, a 22K resistor (R1) connected the 6V DC power supply to the collector.

Due to the phototransistor’s function as an on/off switch, the output pulse measures around 5V peak-to-peak, significantly higher than the 20-30mV delivered by the original BP100 photocell. To address this discrepancy, a voltage divider (R2 & R3) was incorporated to ensure the pulses after transistor 1TR14 remained around 2.5 VDC, consistent with the service manual’s specifications.

Note: As this sensor is an NPN transistor, confirm the correct wiring from the sensor to the PCB: the phototransistor’s longer lead corresponds to the collector, while the shorter one is the emitter.

An oscilloscope reveals well-defined pulses generated by the phototransistor at the collector:

…and those measured at the collector of 1TR14, closely resembling the service manual’s values.

Given the need for only three additional resistors, I directly soldered them onto the PCB and the incoming detector arm wire connectors. Surprisingly, a spare, unconnected connector was present, its purpose unclear. However, it proved quite useful in this situation - as if B&O engineers anticipated this very modification.

These photos are from the Beogram 4000. While a Beogram 4002/4004/6000 would require different wiring, the fundamental principle remains consistent.