Thermal Management System; Updates And Corrections; Stripped Heat Sinks - Alesis MATICA 500 Service Manual

speed will still be modulated by the drive to the amplifier. If the heat sink gets hot enough to trip
the 80deg. C. breaker, then the relay circuit will disconnect the load and also turn on Q25 and
Q26. This will increase the fan to its highest speed and cool the amplifier. Upon cooling below
65deg. C. the amplifier will resume normal operation.

1.80 Thermal Management System

The large extruded aluminum heat sink in the center of the chassis is the heart of the
amplifiers heat dissipation scheme. It is a forced air cooling system. In the A8 the heat sink has
a thermal resistance of .075 deg. C. per watt when the fan is at full speed. This is equal to about
800 watts of heat. The air from the fan is forced into the center of the side of the heat sink with
fins and the air flows around the sink and out over the amplifier circuitry. It has an air intake at
the front of the amplifier and exhaust at the left rear. As explained in the fan speed control
section, the speed of the fan is determined by the demands on the amplifier. If for some reason
the fan should stop but not fail there is enough dissipation in the main heat sink as well as the
four TO-220 stand up heat sinks on the PCB to allow operation indefinitely while at a quiescent
condition.

2.00 Updates and Corrections.

2.10 Stripped Heat Sinks

Occasionally when tightening down the heat sink clamps one of the screws will strip the
threads out of the heat sink itself. Because heat sink is a rather expensive and bulky item, a way
was found to reuse stripped heat sinks. Use a long (at least 1 1/4") machine screw from the
clamp side, and a threaded hex standoff inserted into the heat sink fin side. Do not use just a
hex nut, as it will probably not be able to hold the clamp pressure any more then the heat sink
did. See diagram on next page.
Alesis A4/A8 Amplifiers Service Manual------ 5 03/07/03