By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...
Thursday, May 7, 2015
Beogram 4000: Replacing the Light Bulb of the Tracking Sensor with a Drop-In LED Assembly
Whenever I restore a Beogram 400x I also replace the tracking sensor light bulb with a LED assembly. I recently developed a drop-in replacement for the light bulb assembly for Beogram 4002 and 4004 models. For the Beogram 4000 I modified this part to include the B&O prescribed intensity trimmer that allows to adjust the gain of the tracking feedback mechanism of the 4000.
Whenever the light source of the tracking system is replaced, the sensor position of the mechanism needs to be adjusted to yield the correct tracking sensitivity. I made a short video about the entire process. The video also discusses the tracking circuit and gives a detailed account of the adjustment procedure:
Here are a few pictures of the parts: This is the original bulb assembly, which is not available anymore due to the proprietary housing shape into which the bulb is glued:
This are the components of the replacement part:
The PCB holds a high brightness LED (Newark 15R2204) and a 3k current limiting resistor, suitable for the 6V power rail in the Beogram 4000. The blue part is a 3D printed replacement of the lower part of the original housing, which fits precisely onto the sensor base. Both parts are then glued together, which generates a drop-in ready replacement assembly, where the LED is very near the location of the filament in the light bulb:
This shows the top side. For the Beogram 4000 I added a 2k encapsulated cermet trimmer (Newark 70M7250), which is very helpful for the correct adjustment of the tracking mechanism:
Here is a picture of the implemented assembly:
The LED runs at a very small current of less than 1 mA in this set-up, which should make it last for a very long time. It is rated 20 mA at full output.