Beogram 4002 (5503): New Monolithic Beolover Carriage Position Sensor for AC-Motor Beogram 4002 (Types 550x)

A little more than a year ago I designed a replacement part for the carriage position sensor PCB in DC platter motor Beogram 4002 and 4004 (Types 551x and 552x). This board takes the guesswork out when the carriage position sensor does not work properly. The original design is fairly sensitive to the alignment between the IR diode and the photoresistor, as well as the alignment of the ruler relative ot the sensor, and it can be tedious to get everything properly adjusted.

The Beolover Carriage Position PCB for Beogram 4002 and 4004 (Types 551x/552x) is based on a modern monolithic IR photo-interrupter and circuitry that generates a clean and precise digital output signal for driving 1TR17 on the main PCB.

Sadly, this solution only works in the later DC motor Beograms. The earlier Beogram 4002 with AC platter motor has a much more complicated hard-wired carriage position sensor PCB that is not very easily replaced. But when I recently restored an AC motor Type 5503 Beogram 4002, and it gave me grief with detecting the runout groove reliably, I thought it would be nice to have a solution for these models, too!

Since replacing the entire board is complicated and unnecessary, I designed a replacement for just the sensor part of the board. I designed a board that simply piggybacks onto the main PCB, replacing only the original sensor bulb and photoresistor. This board adapts the same reliable circuit that I used on the DC motor PCB. 

This shows the final version of the Beolover Carriage Position Sensor for Beogram 4002 (Types 550x):

Let's see how it is installed:

This shows the original setup. The photoresistor is in the black housing in front of the 'plexiglass ruler' bolted to the carriage assembly:

After removing the two screws that hold the ruler assembly in place, it can be removed. This reveals the black bulb housing under the ruler.

Here is a view from a different angle (I already had the orange capacitor and white solenoid resistor replaced when I took this photo):

After removal of the bulb cover, the bulb is visible:

The first step for the installation of the new Beolover Carriage Position Sensor for Beogram 4002 (Types 550x) is removing both the photoresistor and the bulb and cleaning the five solder pads indicated below of all solder:

This is best done with a desolder gun since the pads need to be clean and flat so that the Beolover board can be placed onto the original board surface. The board needs to be aligned as shown in the picture below:

This shows it aligned and soldered in place:

When aligned properly, the three round connection vias on the Beolover board are aligned with the respective solder pads where the board connects to power and the base of 1TR17. All that needs to be done at this point is to put a bit of solder inside the vias.

It is a good idea to solder one via first and then adjust the board precisely, while making sure it is fully flat on the original board surface. If it is not flat or misaligned, it may be difficult or impossible to adjust the plastic ruler for proper function of the position sensor.

This shows the plexiglass ruler assembly bolted back into place. 

Adjust the ruler that runs at a constant ~1mm distance from the front-facing part of the sensor during the entire travel of the carriage.

Flipping the switch in front of the sensor activates the on-board LED:

The LED makes it easy to test the proper functioning of the sensor. It should light up whenever a black bar passes between the legs of the sensor.

After installation of the sensor, the Beogram reliably recognized the runout grooves of all records I played. Beautifully!