Hitachi HA-3700 Repair

The HA-3700 is a reliable amplifier, however, should you ever have to replace the audio power output transistors, then they are expensive due to limited stock. There is of course the possibility of using equivalents after a little modification; however, an engineer will charge a premium for this type of mod.

If you are an engineer, then you might be pleased to learn that this amplifier uses the famous HA12002 protection IC. Pin 6 of this IC receives the overload trigger; however, this IC also detects AC power, neural point potential, and temperature. These parameters then provide a cue to the relay driver for controlling the RY901 relay. When this amplifier is working correctly, there is usually a click sound 10-15 seconds after depressing the power switch. If you do not hear that, then it is likely that there is an overload or a short in the amplifier circuitry.

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