QSC CX Series, Service Manual

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CX Series
T r o u b l e s h o o t i n g
Current Draw
(Circuit breakers and fuses blow. Burning smell or smoke)
Symptoms:
-
Excessive current without signal present
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-
Fast current draw
-
Medium current draw
. Slow current draw
-
Runaway current draw
Possible Causes:
-
EXCESSIVE CURRENT WITHOUT SIGNAL PRESENT (Above 2A and less than 4A)
1. The amplifier draws high current when the AC supply voltage is first applied. This symptom
may mean that there is a short in the power stages of the circuit.
2. This could also be a misadjusted bias setting. See calibration procedures in this manual for
setting bias, Check for VR511, VR61
1 (CX6/T,
CX
1
2/T), R512, VR612 (CX4/T) or open
d i o d e s D505, D506, D605, D606 (CX6/T, CX
1
2/T), D504, D505, D604, D605 (CX4/T).
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FAST CURRENT DRAW (increases rapidly at only a few volts AC):
1. Reversed or shorted main bridge rectifier BR501, BR601 (CX4/T, CX6/T), BR501, BR502,
BR601, BR602 (CX
1
2/T). Check the bridge rectifier by setting DMV to diode mode. To verify
a good bridge rectifier, connect the negative lead to BR+ (BR = bridge rectifier) and check for
0.4 to 0.6
v
at both AC terminals. Connect positive lead to BR- and check for 0.4 to 0.6
v
at
both AC terminal.
2. Both supply clamping diodes shorted or reversed D512, D513, D612, D613 (CX6/T, CX
1
2/T),
D510, D512 (CX4/T).
3. Shorted or reversed C509, C510, C511, C512, C609, C610, C611, C612 (CX
12
/T),C511,
C512, C611, C612 (CX6/T), C511, C512, C513, C514, C611, C612, C613, C614 (CX4/T).
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MEDIUM CURRENT DRAW (increases slowly, can go to 30 VAC before current becomes excessive.):
1.
Single polarity driver or output short.
2. Open or missing bias diodes D505, D506 (CX6/T, CX
1
2/T), D504, D505 (CX4/T).
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SLOW CURRENT DRAW (above 60 volts AC before current begins to increase, amp may pass signal):
1. Severely misadjusted bias circuit or defective D505-506 (CX6/T, CX
1
2/T), R512, R612, D504,
D505 (CX4/T).
2. Severe oscillation causing current drain. Check all feedback components and high frequency
snubber circuit
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RUNAWAY CURRENT DRAW (30-40V AC before current begins to increase or runaway):
1. A reversed filter capacitor: caution, may vent explosively.
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