Background

The Biotracer

In the golden era of vinyl, Sony developed a range of record players that employed an innovative method for control of the tone arm's resonances. They gave this technology the name of Biotracer. The Biotracer  turntables family included both models with normal pivoted arms (e.g. PS-X75, PS-B80, PS-X600) and models with linear-tracking arms like PS-X555 or PS-X800.  

In short, the Biotracer's principle of operation involves a pair of electric coils for either plane of the arms movement (vertical and horizontal). Either pair of coils is placed in magnetic field of their respective fixed magnet arrangement. One coil in either pair is the driver coil and the other serves as the velocity sensor whose output is fed back to the arm's control circuitry.  Using this arrangement of coils and magnets the control circuitry can achieve the following:

• drive the arm forwards and backwards to perform functions like automatic start, automatic stop/return or seek function

• exert static tracking force and static anti-skating force

• dynamically detect and compensate for resonances of the arm in vertical and horizontal planes 

Interestingly enough, the PS-X65 had an identical arrangement for the horizontal plane of the arm as the PS-X75 but had a classic statically balanced arrangement in its arm for the vertical plane. In other words, in the horizontal plane there was electronic drive, anti-skating and resonance compensation, and in the vertical plane good old counterweight to set the tracking force and no resonance compensation. For this reason it can be considered a semi-Biotracer, although Sony never used the Biotracer moniker in the context of this model.

The reason why the PS-X65 is mentioned in this project is that PS-X75 and PS-X65 share the same electronic design and PCB design with the only difference being that in the PS-X65 the part of the circuitry responsible for vertical drive of the arm is not populated on the PCB. 

The fragile micro

While all the Biotracer magic is performed by purely analog circuitry, the actual control of the record player, i.e. getting user commands through push buttons, actuating the arm, detecting the stylus drop-point, detecting the size of the record placed on the platter and many more is performed by a 4-bit microcontroller MB8841 by Fujitsu. It is one of the early microcontrollers of the era implemented in the N-MOS technology,  a technology which was notorious for its susceptibility to static electricity.  In fact the first two pages of the PS-X75's service manual provide precautions for handling of integrated circuits using MOS technology so as to avoid electrostatic discharge which could damage the chips. This fragility of the early microcontrollers is a significant factor for vintage turntable enthusiasts of today. Simply put, the microcontroller break-down, if it happens, spells the end of the story for the turntable as the replacement part MB8841 is extremely hard to find in the second hand market. The MB8841 was programmed at the Fujitsu chip factory with specific firmware from Sony and the only source of a replacement part is another PS-X75 / PS-X65. In other words, one may end up with a perfectly restored vintage turntable suddenly rendered useless due to the failure of the main controller. A failure quite common, judging by the discussions in various vintage vinyl frorums on the Internet.

While the Biotracer Sony models delivered very good sonic performance, many vinyl fans are hesitant to spend money on vintage PS-X75 / PS-X65 for fear of being affected by this problem sooner or later. 

The solution

Having successfully restored a few PS-X75's and PS-X65's as well as other Biotracer models the author has developed an in-depth technical understanding of both models. When a friend trusted the author with restoring his newly acquired PS-X75, it was supposed to be another routine restoration work - a walk in the park. Unfortunately this time, despite all the usual precautions taken, the machine's micro failed in the process. That inspired the author to start this project in order to create a solution to a problem that has been haunting vintage Sony turntable enthusiasts for years. 

The project started in January 2023 was successfully concluded in June 2023 and delivered a drop-in replacement for the original microcontroller in the same form factor as the original. All it takes is to unsolder the original micro and replace it with the PCB of the same size which restores all the original functions of the turntable and more. Thus the RCM (Replacement Controller Module) was born.

Features of the RCM module

• Same physical form factor and electrical characteristics  as the original micro (1:1 replacement)

• Restores all functions of the turntable

• Suitable for PS-X75 and PS-X65

• Additional functionality over the original device:

  • Easy drop point adjustment (manual and automatic)

  • Diagnostic mode to help diagnose other problems in the turntable circuitry with audible feedback through an on-board beeper

Installation procedure

Click here for detailed installation procedure.

Project development history