14
The fan speed increases in the following instances:
·
When the heatsink is above 86°C. Comparator U5B’s output switches low when the
heatsink-mounted temperature sensor’s output voltage increases to 860mV (86°C). The
output of U5B is coupled to the fan controller via D80. Note: hysteresis for the overtemp
detector is supplied by R177 and R179, so that the heatsink temperature must decrease
to 61°C (610mV) before the circuit resets and the fan begins to run slow.
·
When the amplifier is playing loud enough
and
the heatsink temperature is above 60°C.
When the temperature is above 60°C, the voltage at U4-7 is above 600mV. This “opens”
output U4-1. When program material is sufficient to drive U4-5 above 5V (D75 and D76
charging C70 through R193) output pin U4-2 also opens, and R174 is allowed to pull this
line high. With U4-10 high, the output of U4-13 goes low, allowing the fan to run fast.
AMPLIFIER MUTING
The amplifier is muted in the following instances:
·
At turn on. C79 is low at turn on and is charged by R195. When U4-8 goes above U4-9
(after about 4 seconds) output pin U4-14 goes low, turning on the power amps.
·
When the heatsink rises above 86°C. U5-2 goes low and this is coupled to C79 Via D79.
4 seconds after the heatsink cools to 61°C, C79 charges enough to again un-mute the
amps.
·
When the AC supply voltage goes below 65% of nominal. At high voltage levels D67 is
able to charge C60 enough to reverse bias Q68. As the AC input voltage is reduced to
65% or less, R155 is no longer able to offset the current through R154. This turns on Q68
which again resets C79 and mutes the amps.
·
When the power transformer’s core gets above 130°C. A thermostat in the transformer
shorts to ground (J2-2). With J2-2 at ground potential, Current flows through R156 which
turns on Q68 and mutes the amps.
fan control continued.