Beogram 4002 (5501): Sensor Arm Light Bulb Replacement - Final Version

This is a follow up to my recent post about the replacement of incandescent sensor arm bulbs in Beogram 4002 turntables with a LED setup. Subsequent experiments revealed that the best position for the LED PCB is when it is pushed down by about 2.5 mm into the bulb cavity of the sensor compartment. This position yields the highest sensor response as measured at the collector of 1TR8 (5501; pre SSN 257556 notation), and I think this makes sense since the light bulbs have the filament always a bit lower than the upper rim due to the glass bulb.

I like reproducible solutions, and so I designed a small 3D printed wedge that can be epoxied on the outside of the flexPCB and that presses the LED down by the correct amount when the bulb compartment is closed:

This shows the LED powered up at 5.5V:

The board has a ballast resistor that adjusts the current draw to the prescribed ~60mA. This is necessary that the 'bulb malfunction detection circuit' is not triggered on the main PCB of the Beogram.

This shows the board installed and lit up:

This shows the TR8 signal:

Note how nicely the curves bottom out at 0V.

This shows the setup 'in action':

Note the nice red-orange glow of the B&O logo. This is a result of using a 2600K color temperature high intensity white light LED that has a considerable amount of red light in its emission spectrum which enables a 95% color rendering index (CRI). Also the light spot is nice and tight indicating that the light source is in the focal point of the exit collimator.

This pretty much concludes the development of the LED implant for the Beogram 400x sensor arm. Henceforth Beolover restorations can (if so desired by my customers) yield Beograms devoid of incandescent bulbs.