Rabco SL-8E Vs Top Quality Pivoted Tonearms

I ran one of these for years. But before I could do it, I had to fix the servo. Once that is done, not only is it reliable, but the motor noise is reduced and the arm keeps up with the LP properly.

This is done by adding a timing constant to the servo that allows the motor to pick the right speed so its always on target.

The bigger problems are these:

* lack of precision in the track, some of which can be corrected by replacing the wheels with something more substantial.
* Mechanical resonance in the track, solved by damping compounds
* really poor bearings in the arm! They are cup and point, but not hard enough to hold up right.
* poor arm tube rigidity. I replaced mine with a carbon fibre wand...
* bearings above the plane of the LP (a common engineering bug to this day). This is solvable but would take some tinkering...

^^ Thanks for that!

I used a simple opamp (FET input) to solve the servo issues. As long as the contacts made a connection of less than 1 Meg ohm, it worked fine. The reduced current through the contacts got rid of contact noise as well. Later I sorted out that if the opamp had a cap in the feedback network, by being careful of the value of the cap the motor would come on slowly and turn off slowly- in effect it would find a speed of the groove cut and settle in on it, thus reducing the tracking error even more, and also eliminating the motor noise.

I ran the opamp off of a separate supply (a 12Volt wall-wart). The opamp drove a transistor that actually turned the motor on and off. Once it was all set up correctly, it was quite reliable and low noise.

I've often thought about revisiting the concept using modern materials- in particular there are motion tracks now that have zero slop in the bearings. Such parts are costly compared to the original arm but nothing compared to modern arms.