Crown ce 4000, Service Manual

Page: 15 / 140

Theory of Operation 3-5
130485-1 Rev. B
CE 4000 Service Manual
©2002 Crown Audio, Inc.
are attached to the amplifi er output heatsinks (one
per channel). R711 is attached to the primary side
power supply heatsink and R713 is attached to the
power transformer. The NTC sensors feed the fan
control circuit. Normally, the fan runs at very low
speed and R325, R42, R508, R509, R333, R334, and
R86 limit the voltage applied to the fan. For medium
speed, Q105 conducts, shorting out R333, R334,
and R86. For high speed, Q104 also conducts which
shorts out the other dropping resistors.
“TLC” is a feature that allows the amplifi er to decrease
the amount of power it is dissipating if the power
devices or transformer are climbing towards an
unsafe temperature. This circuit uses the NTC-derived
voltage to control a varying power limiter. This circuit
only comes into play if forcing the fan into high speed
is not enough to keep the unit at a safe temperature.
The TEMP IN signal is delivered to U125-A’s non-
inverting input while a reference voltage is sent to
its inverting input. U125-A scales and inverts the
temperature signal while U125-B inverts this new
signal. U112-B and U112-C compare the temperature
information to the input signal amplitude. Normally,
the output of U112-B&C is open collector but if
limiting is required, this output is forced low and is
sent to the compressor circuit as TLC 1.
The NTC voltages controlled by R339, R74, R712,
and R714 respectively are diode-OR’d to U115-D.
U115-D compares this voltage to a reference formed
by R337 and R336 to determine if, as a last line of
defense, Timing Cap 1 should be pulled low until
the amplifi er cools off.
In the rare event that DC is produced by the output
stage, U113-C and U113-D form a window detector
whose output goes low in that condition. This signal is
latched on permanently by U113-A and U113-B and
R304. This signal, which is high for a fault condition,
is sent to U115-4 and will also turn off the modulators.
To reset this latch, the power must by cycled to
the amplifi er.
3.3 Power Supply Operation
3.3.1 Power Entry and Filtering
AC power enters the amplifi er through a power cord
equipped with an IEC (unplugable) connector. It then
passes through the EMI fi lter to remove switching
noise caused by the amplifi er and power supply. The
parts on the primary side of the power distribution
circuitry (including those on the EMI PWA) have all
met stringent regulatory safety requirements and,
if they fail, must be replaced by the correct part as
called out in the PWA documentation.
3.3.2 Fuse
In the rare event that a power supply failure draws
excessive current from the mains and the power
supply itself cannot reduce the current by shutting
down the controllers, a fuse will blow on the EMI fi lter
PWA. This fuse is accessible through a slot in the fi lter
shield and is the same value regardless of the mains
voltage that the amplifi er is used with. Always replace
the fuse with the same type and rating!
3.3.3 Low-Voltage Supplies (Flyback)
Several things happen in parallel when the power
switch is fi rst turned on. The line current fl ows through
PTCs R1 and R2 to prevent high inrush currents.
The current is then rectifi ed by D1 and fl ows through
L1 and D2 to charge energy reserve caps C1 and
C6. As soon as suffi cient voltage is detected at the
“Vbulk” node, the low voltage “fl yback PWA” supply
starts running. It is a buck supply that self-starts by
free running (not synchronized to any clock) at about
80 kHz. This supply produces all of the required low
voltage supplies except the Vp gate drive power
mentioned above in the “Output” section. All of the
secondary supplies are isolated from the primary by
a transformer. After the low voltage supplies are up
and running, the clock circuitry runs and generates
a synchronizing signal for the flyback PWA that
overrides the free-running circuit and forces the
fl yback to run at 125 kHz.
3.3.4 Turn-On Control Circuitry
As mentioned in the previous section, while C1 and
C6 are being charged through the PTCs, Q11 is
held off and C47 charges up in approximately two
seconds. U10-A’s output then goes high which turns
Q8 on forcing U13-B’s output high which turns on the
relay via Q9 and shorts out the PTCs and allows full
line voltage to get to C1 and C6. As Q8 is turning
on, Q3 turns off causesing C24 to discharge through
R9 and, after about 15mS, forces U13-A’s output low
which turns Q14 and Q15 off. When Q14 and Q15 are
on, they ground the soft-start pins of controllers U2
and U4 and this prevents them from running.
Three conditions can prevent the node at U10-A’s
output from going high. U24-A will hold this node
low through D45 if there is a fault detected by the
audio output stage or if the power supply detects an