The Kenwood KD-500 and FG Servo controlled motors.
Updated: Jul 11, 2022
What an impressive turntable - both in looks and performance. The cabinet is formed from compression molded resin concrete called ARCB (Anti-Resonance Compression Base) This is a mixture of limestone particles, unsaturated polyester resin and glass powders, resulting in a marble like finish.
This composition provides superior damping qualities for frequencies under 1Khz, but Kenwood did not stop there. A sub base of 18mm thick lauan plywood is incorporated to reduce resonance at +2Khz, with the tone arm mounted on the same plywood at a thickness of 21mm. Combined with rubber matt on the platter, a universal stable for reducing high frequency vibration, and rubber feet for dampening isolation. All these design features combine to provide ultra low sensitivity to resonance feedback over the entire audio spectrum.
This one came to the workshop with an audio issue, the result of not replacing tone arm wiring, just soldering a jumper onto the broken one and then onto the cartridge pin. Please dont try this yourself, ever (it was the previous owner in this case) This level of fine pitch soldering is taught at an industry workshop, from experienced technicians - never a secondary college of TAFE, and certainly not from a Youtube video, or with someone on an AV forum giving you remote advice. Your "FIFO electrician or auto sparky mate" is not the person to solder or terminate tone arm wiring either :-)
Given the age and obvious signs of no preventative maintenance, the unit did need a full service (mechanical and circuit.) The first obvious point was the fluctuating platter speed. Upon opening, what really peaked my interest was the motor pcb, as I had seen the same 3rd party motor pcb on several other brands that utalise the FG (Frequency Generator) servo control system.
A common design is for the platter speed to be controlled by frequency or voltage of the AC motor. Not as stable, this design is prone to many influences including external ones.
Offering a far superior stability to platter speed, the FG servo-control system generates a constant frequency, which the platter speed is compared to, and then the servo applies a current to the motor which corrects any difference between the two (FG reference signal and platter speed)
Having encountered this motor pcb previously, I was familiar with the circuit having to document, create a diagram and essentially reverse engineer to a point the first time I encountered it, due to there being no schematics. Seeing none in that particular models service manual, and noting the manufacturer was not one that retailed audio equipment, I knew I would see it again some day, and has turned up in several other turntable brands since.
Having this document on hand ensured a prompt repair and replacement of the necessary components. Other workshops would've resulted in what has become known as the "Liquid Mike Approach" - replacing 80-90% of the components, due to the one performing the job having no qualifications, industry training or understanding of circuit theory. The "refurber" method would still take a good 2-3 hours given the densely populated pcb, components added to the labour cost.
Circuit repaired, calibrated and tested, remaining preventative maintenance performed and returned to the lucky owner of this iconic turntable. Whilst this circuit repair was necessary in this case, JZ Services always recommends mechanical and circuit preventative maintenance.
This comprehensive maintenance procedure is never to be skimped on. Turntable labour costs are charged in half hour increments after the first hour at JZ Services, hence the $150 plus a hand full of components for basic belt and direct drive turntable service procedures is well worth your money, given the cost of a comparable Pro ject or Rega turntable starting at $500 to purchase new.