Mackie FR Series M.1400, Service Manual

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M•1200/M•1400 SERVICE MANUAL
CIRCUIT THEORY
10
THERMAL MANAGEMENT
Refer to schematic on page 29.
The M•1200/M•1400 T-Design Heatsink/Fan cools output devices evenly and does not
collect dust on the circuitry. The fan operates at two speeds, controlled by the
amplifier. An LM35DZ mounted in the center of the heatsink provides temperature
information to the fan control and overtemp circuits. There is also a thermal breaker in
the power transformer.
FAN CONTROL CIRCUIT
The fan voltage comes from a feedback regulator circuit formed by Q91 and Q92. The
output voltage is programmed by the ratio of R209 to R211// R207 (//=parallel). When
the unit is powered up C83 charges through R241 providing a 40 second delay before
the fan starts. As C83 charges, U6D’s output switches to -16V, starting the fan. Then
U6A’s output switches to open, allowing U6B to increase the fan speed, if triggered. R240
and C82 give the fan extra voltage when starting.
The fan speed is increased by U6B adding R211 in parallel with R207. (Q91 wants it’s
base a diode drop above ground, so Q92 doubles its Emitter voltage). U6B is triggered
by U4D to increase fan speed if;
•
Channel 1 SOA protection is activated (U5C), OR
• Channel 2 SOA protection is activated (U5D),
OR
• The heatsink is above 60 deg C (U5B)
AND
program is detected (U4C),
OR
•
The heatsink temperature is above 80 deg C (U5A).
U6C and Q89 are used to reset the circuit on power down. Under normal operation
C50 is charged via D57, Holding Q89 off. When power is removed C50 discharges and
Q89 switches on as the rails collapse. When the voltage on Q89’s collector falls below
ground U6 switches to the negative rail discharging C82 and C83.
THERMAL SHUTDOWN
Extreme heatsink or transformer core temperature will activate OVERTEMP. U5A
activates overtemp if the LM35DZ detects heatsink temperature above 80 deg C.
Feedback resistor R137 rescales U5A’s voltage reference so that the heatsink must cool
to below 50 deg C before overtemp is deactivated. Overtemp can also be activated
by the power transformer thermal breaker (via Q89). In either case, both channel are
muted via D69 and D66.