Beogram 4002 (5513): Replacement of Sensor Arm Bulb with an LED Assembly

After rebuilding the DC motor of the Beogram 4002 that I am restoring right now it was time to replace the light bulb in the sensor arm. This bulb is critical for the record detection circuit, and replacing it with an LED makes sense since LEDs last much longer than light bulbs. This shows the bulb compartment extracted from the arm together with the flexPCB based LED assembly:

The LED assembly 'mimics' the light bulb in terms of drawn current to be compatible with the light bulb detection circuit. This circuit monitors the current through the bulb, and disables the record detection circuit in case the bulb dies. LEDs draw much less current than the bulbs, and therefore an additional current path needs to be provided on the LED board in parallel to the LED itself.

The LED board folds into the compartment:

After soldering it in, I started the Beogram to see if everything works:

The warm-white LED has enough red emission to render the B&O logo properly red as if it were illuminated by the original incandescent bulb.

The final step of any light source replacement in the sensor arm is to check if the ribs on the platter are properly detected. TR3 amplifies the sensor signal and its collector needs to be at 4V DC when there is no signal (record present). Unfortunately, the circuit design of the 4002 biases the base of TR3 via a single resistor connected to the collector. This makes the working point of TR3 dependent on its current amplification factor, which can vary widely between transistors of the same type. Hence, many Beograms have a too small voltage at the collector of TR3, reducing the signal generated by the ribs on the platter as they go by underneath the sensor. This can be rectified by replacing R26 (1 MOhm) with a 2 MOhm multi-turn trimmer:

I usually install the trimmer first on the solder side to be able to adjust the collector voltage  with the board installed and the turntable on. Once the 4V are set, the trimmer can be installed from the component side:

The final step is to measure the sensor signal at the collector with the platter spinning underneath:

This Beogram passed the test with flying colors: The signal is very close to the prescribed 6Vpp. If the signal is too small, then TR3 is not biased correctly, and if the signal does not go to zero when a rib goes by, then the LED or the bulb is not in the correct location within the bulb compartment of the sensor arm.