Mackie M-2600, Руководство по обслуживанию

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via R33 and R8 and compared against the 5V reference at U3A. If the AC line voltage falls to
around 60% of nominal (around 70VAC), the output of U3A goes low, resetting the mute and
short monostables via D5 and D12. If the +15V supply fails, D3 and D4 bias the U3A inputs such
that the amplifier is also muted. In the event of the -15V supply failing, R7 is no longer able to
pull the anode of D15 low, and this also causes U3A to go low and mute the amplifier.
THERMAL MANAGEMENT
The T-Design Heatsink/Fan cools the output devices evenly and does not collect dust on the
circuitry. The fan operates at variable speeds, controlled by output stage dissipation and
temperature data supplied from both LM35DZ’s mounted on the heatsink.
Fan Control Circuit
Referring to the 186 board: The fan voltage comes from a feedback regulator circuit formed
by U3B, Q2, Q1 and associated components. The fan supply is actually referenced to the -30V
supply, so U3A and Q3 act to shift the ground-referenced control voltage “FAST-FAN” to a -30V
supply reference. The output voltage of this regulator is adjusted via R1. D7 and C9 give the fan
an initial full fan voltage “kick” (28V) to insure the fan starts spinning. The “FAST-FAN” control line
varies from -1.6V (Slow) to -15V (Fast). Short J3 temporarily and adjust R1 for a voltage across the
fan equal to 28V.
Referring to the 170 board: The fan is never allowed to go faster than idle speed (Slow) unless
the output stage dissipation is greater than 136 watts and the heatsink temperature is above
45°C. When the output stage dissipation is greater than 136W, U4D goes low. This eliminates D1
from supplying current to U2A. This decreases the reference voltage on U2A pin7, allowing it to
work as a linear amplifier. With D1 conducting, U2A is effectively biased such that it’s output is
high and the resulting fan speed is low.
With 136W of output stage dissipation, U2A is allowed to linearly amplify the temperature
sensor voltage on U1 via R2 and R5. C5, C15, C16, and C17 provide high frequency stability to
this stage. The amplifier will linearly decrease the “FAST-FAN” voltage (increasing fan speed)
between 45°C and 65°C. Above 65°C, U2A is saturated with the fan speed maximized. D11
isolates the Channel-1 and Channel-2 U2A circuits from each other. The connection of the U2A
controllers is such that whichever channel is being used the hardest will determine the ultimate
fan speed.
Thermal Shutdown
Heatsink temperature is sensed by U1 mounted to the main heatsink (one for each channel).
The output of the sensor (10mV / °C) is compared to a 800mV reference at U2B. When the sensor
is over 800mV (80°C) the output of U2B goes low, muting the amplifier via D27 and associated,
and running the fan fast via D24. Hysteresis is built into the comparator via R3, such that the
sensor voltage must reduce to 520mV (52°C) before the comparator goes back high and the
amplifier un-mutes.
A thermostat is built into the power transformer. If the power transformer temperature
exceeds 130°C the thermostat closes and brings the “THERM” line to ground via Q1 and it’s
associated parts on the 171 board. This shorts out the 800mV reference, and the U2B output goes
low, muting the amplifier. Note that overheating of the power transformer is very unlikely but if it
does occur, the amplifier will stay muted for over an hour before the transformer cools and the
amplifier un-mutes itself.