Beogram 4002 (5501): Replacing the Sensor Arm Bulb with a SMD LED on a FlexPCB

***************************Sequel to this post see here******************************

I finally was able to design a LED based upgrade to replace the sensor arm incandescent light bulb in Beogram 4002 units. This was the most difficult bulb to replace due to the tight space available in the sensor arm and the requirement to put out an incandescent looking light spot, while being able to properly back-illuminate the lovely orange-red B&O logo at the end of the arm. 

I quickly realized that only a flexPCB would be able to do the trick. My first experiments involved high brightness amber LEDs combined with a red LED to provide some longer wavelength photons to give the B&O logo a more red appearance. Here is an impression of the design:

While this worked technically, the light spot on the record was way too orange, while the red LED could not compete with the amber unit:

Worse, the red LED could be seen reflected in the record surface separated from the amber spot due to the different projection of the red LED through the lens that focuses the light on the record surface. Anyway, this was not Beolovely, and so I did some research and finally found a latest-development warm white LED with a 95% color rendering index (CRI) value, which promised that this LED would be able to correctly illuminate the B&O logo while looking fairly natural when reflected from the record. This is how my board looked like with it on there:

And fired up:

It is a pretty warm white (2600K temperature equivalent). And this shows the board folded into the sensor arm cavity:

And in action:

Pretty authentic look, even though of course the light spot is not a projection of a bulb filament, i.e. it is a bit more homogeneous than the original bulb pattern. But I think without pointing this out to someone, it would be hard to tell that a white LED is at work here. So far so good...but the real test is of course whether this setup can make enough signal at the sensor output for a reliable detection of the absence of a record. For this I measured the sensor signal on the platter without a record present. This is the signal after being amplified by TR8:

Note that the dips go all the way to zero, which is what is required to properly disable arm lowering when there is no record. Bulbs rarely deliver a stronger signal, so this seems adequate!

Another critical factor is the current draw of the LED board. The Beogram 4002 is equipped with a detection circuit that disables arm lowering if the bulb filament breaks. This circuit requires about 50-60mA in the bulb circuit to enable arm lowering. So I added a resistor to the LED board to match 60mA (the LED on its own only draws about 40 mA...). This shows my bench power supply driving the board with 5.5V (same voltage as is applied to the bulb in the sensor arm):

It appears we have a functioning prototype in play! All that needs to be done now is to clean the board design up a bit to arrive at a final version. This is Beolove!