New Beolover Carriage Position PCB for DC-Motor Beogram 4002/4004s (Types 551x and 552x)

I always found the repair and adjustment of the carriage position sensor on the PCB underneath the carriage somewhat tedious. The board and circuit are designed in a way that make a precise alignment of the LED, sensor and plexiglass ruler across the entire travel of the carriage necessary to ensure proper functioning. At the same time photosensor failures and mechanical failures of the sensor housing or the two switches on this board were sometimes difficult to fix, while broken off wires occasionally caused operational issues.

So I decided designing an improved board that would provide a reliable fix for all these issues at least in DC-motor Beogram 4002/4004s (Types 551x and 552x), where this board can be easily replaced. This shows the final design of the board:

The board features new SO and ES switches, as well as a modern monolithic IR photo-interrupter whose signal is cleaned up and shaped by an opamp in combination with a Schmitt trigger. It connects with a modern harness using a 2.54mm 8pin adapter to match the classic Molex jack on the main PCB. This connection should last much better than the original soldered wires.

The part is now available at the Beolover Store. I also gave this board a built in sensor test feature. Flip the switch to the right and an integrated LED will light up every time a black band on the ruler is properly identified by the sensor:

This shows the board installed:

My tests yielded a proper carriage position detection, while the SO and ES switch were in the correct location and performed perfectly. It seems that the modern photo interrupter due to its fixed alignment between LED and phototransistor is much more forgiving regarding the distance between ruler and photosensor. This makes the alignment between ruler and sensor across the entire translation range of the carriage much easier.

A good test for the proper functioning of the position sensor is whether it can read out the run out groove detection pattern at the end of the ruler. I measured the signal at the collector of TR17 while this section passed through the sensor:

A nice clean trace! All good in the carriage position detection department!