SA-GX370 Cooling Fan

The SA-GX370 and SA-GX470 amplifiers have an interesting cooling system, which consists of a passive heatsink that manages the temperature of the power amplifier ICs majority of the time. However, they also have a cooling fan close to the mains transformer, which operates at high power output levels only. Usually the fan does not operate until the user increases the volume for greater power output. The operational condition of the fan is integral to the amplifier starting. If the fan does not start for whatever reason, then the amplifier will not start either. When the system requires the cooling fan, the built-in microcontroller performs a power on self-test of the fan motor. If the result of the test indicates a fault, then it displays the "OVERLOAD" message, and goes into that subroutine.

After some research, I managed to find some useful information that shows how to test the fan, which is a simple DC motor. I suspect that eventually almost all of these amplifiers will go into protect mode displaying "OVERLOAD", because the motor is the one main component that can wear out after many years use.

Using a digital multimeter, you can measure the resistance across the motor terminals, which has to be between 20 Ω and 30 Ω. If the resistance is below 5 Ω, then the motor is short, and if the resistance is over 1 kΩ, then the motor is open circuit. A pair of copper strips on the power supply circuit board forms spring contacts that mate with the contacts on the motor.

The best way to gain access to this assembly is to remove the motor, which uses three plastic retaining tabs to remain secured to the back metal panel. The contacts can also develop oxide over the years so it might be a good idea to clean it away to ensure proper electrical contact.

Just as an experiment, I decided to see what the internal programming would do if I switched ON the amplifier without the cooling fan. Without any speakers connected, without any input signal, and the volume control knob set to the minimum end, I switched it ON and there was no "OVERLOAD" message. However, as I began to turn the volume control, it tripped into "OVERLOAD" at the maximum end. This indicates that the firmware tests for fan operation every time it is required, and if it cannot detect a fan for whatever reason then it goes into the "OVERLOAD" routine.

Three plastic locking tabs secure the fan assembly to the back panel. One simply has to push them in to release the fan.

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