Over-Voltage; Dc/Lf; Soft-Start Signal - Crown Macro-Tech MA-5002VZ Service Manual

130446-1 Rev. A
The output of U701A controls the Soft-start. Refer to
Figure 4.6 for a graphic of Soft-start operation.
When the output of U701A goes low, the opto-triac de-
vice U700 turns on. While on, the input triac Q700 con-
ducts. Positor R702 limits peak input current to the tor-
oid to a maximum of 22A peak (with 120VAC mains).
U701A combines the sloped input from the C120 cir-
cuitry on the main module with the pulsed DC. As the
portion of time which the output of U701A goes low in-
creases, the amount of time where AC mains conduct
to the transformer (via Q700 and R702) increases until
it remains on. When the U111C PWR circuit times out,
the relay closes, bypassing the current limiting soft-start
circuit. Soft-start control signals are shown in Figure 4.7.
The upper signal is that produced by C120. At time 0
the amplifier is off. At time 1 the power switch is pressed
(on). At time 2 C120 has fully charged, the magnetic
C120
CHARGE
VOLTAGE
0 1
PULSED
DC FROM
LVPS
0 1
SOFT-START
TRIAC
CONTROL
0 1
©2000 Crown International, Inc.
2 3 4
2 3 4
2 3 4
Figure 4.7 Soft-start Signal
fields have built up in the high voltage supply, and the
main relay closes. At time 3 a protective action occurs;
note that the DC supply remains. At time 4 the condi-
tion clears and the restart begins. Time 5 is akin to time
2, and time 6 is another protective action. The lower
graph shows Q701 operation (high = on).
Any time a protection mechanism has acted and the
condition then clears, this entire process repeats.

4.6.2 Over-voltage

One mode of amplifier protection is a shutdown in the
event of over-voltage on the AC mains. This is sensed
by the pulsed DC signal produced by the full-wave rec-
tifier in the low voltage supply.
R780 picks off the pulsed DC, and U707D will, if the
voltage is too high, shift its output to a low. When this
happens, the over-volt/therm red LED on the control
module lights and signal OV1 goes low
to the main module.
A low (over-voltage condition) on
OV1 causes U211C to shift to a
low output. This low, through D1,
causes C120 to discharge im-
mediately. This in turn causes the
main relay to drop out, soft-start
to reset, and the ±LL and ±LH
to clamp audio drive. Over-volt-
5 6
age is a non-latching fault con-
dition.

4.6.3 DC/LF

The amplifier senses its own out-
put for DC or very low frequency
and will interrupt the amplifier
channel in the event of DC or low
frequency.
The feedback loop is sensed for
5 6
voltage and the current sense
signal provides current informa-
tion. An RC network at the input
to U109C/D will prevent the LF
circuit from activating with nor-
mal audio frequency material,
and will ensure activation with
high level subsonic currents or
voltages.
If U109C/D sense a DC (or LF)
level, it will output a low, which
5 6
will, through D102, discharge
C120 and initiate power supply
MA-5002VZ Service Manual
Circuit Theory 4-11