Nakamichi AV-8 Power Supply System

The Nakamichi AV-8 power supply system is similar to other Japanese amplifier designs. The small standby transformer maintains power to the main microcontroller, which scans the front keys, and infrared receiver circuitry. Providing there are no shorts in the main amplifier power supply and no errors returned at pin 35 of IC001 (LC865624A), the microcontroller actuates the standby relay (RY407) through pin 5 (power-switch-out). If RY607 actuates, it connects mains power to the main amplifier transformer, which energises the high-voltage split rails, as well as the low voltage power supply for secondary subsystems.

This is the main transformer responsible for the high-voltage split rails required for the amplifier channels. It also has windings for generating low voltage split rails for the amplifier subsystems such as the analogue switch ICs and op-amps.

The low-voltage split rails for subsystems consist of these transistor-based voltage regulator circuits. Whilst I had it all opened, I decided to replace the power supply transistors B1134, D1667, and D1761. If any of my English amplifier friends have caused a short, then these transistors usually become compromised. Since the PCB also had scorch markings due to heat from them, I felt it was better to change them all. It is just as if you were fixing your car and had opened the gearbox; it is prudent to change the clutch plate whilst you are there.

I also made slight modifications to my new transistors by adding some heatsinks to help dissipate the heat better. I also routed the wires out of the way, as before it was a messy crow’s nest over the regulators. When we route the wires better, the airflow also increases, which helps the transistors dissipate the heat better.

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