Beogram 4004 (5526): Replacement of Sensor Arm Bulb with a LED Fixture

The final incandescent bulb to be replaced with a LED in the Beogram 4004 (5526) that I am restoring right now was the one in the sensor arm. It is the most difficult one to replace due to the tight space and the requirement that it is positioned perfectly to yield a good sensor signal. I removed the original bulb

and then implanted the LED assembly. This shows the part, which I recently developed in comparison with the original bulb:

The flexPCB based LED fixture folds precisely into the bulb compartment. This shows how I soldered it in:

The red wedge holds the LED in the place where the filament of the bulb was located. This is important that the optics in the arm focus the light into a spot on the platter where the light sensor can 'see' it.

Once the LED assembly was installed, I upgraded the detection circuit with a 2M 25-turn trimmer that allowed me to precisely calibrate the working point of TR3 (which amplifies the sensor signal). TR3 needs to be adjusted that the collector is at 4V DC when no signal (record present) comes from the photosensor. In order to adjust the trimmer with the board powered up, I temporarily installed it on the solder side of the board so I was able to adjust it:

After the adjustment, I removed the trimmer and inserted it from the component side and soldered it in place permanently:

It perfectly fits in the space of R26 that it now replaces. After this was completed, I measured the sensor signal at the collector of TR3 to make sure everything was up to spec:

The Vpp signal should be between 5-6V in order to ensure reliable operation of the record detection mechanism. A test with no record and spinning platter revealed that the the arm would not drop, i.e. the mechanism was working.