QSC CX Series Service Manual page 25

CX Series
6. Defective gain control
"RINGING' SOUND WITH NO INPUT TO AMPLIFIER
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1. C518, C519, C523 open or wrong value. (CX4/T) C505, C516.
2. R541, R542, R538, R547 open or wrong value. (CX4/T) R523, R524, R502.
3. Defective op amp U503. (CX4/T) U501
4. Unstable driver transistors (Q501, Q502), or output transistors (Q505 - Q510). (CX4/T) drivers
Q503, Q504, and outputs Q500Q502 & Q505Q507.
OUTPUT COLLAPSES INTO A LOAD
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1. D519, D520, R548 wrong value or open. (CX4/T) D509, D511, R518.
2. Misadjusted current limits (VR502, VR503). (CX4/T) R513, R514.
3. Defective op amp U503. (CX4/T) U501.
4. R501, R502 wrong value. (CX4/T) R516, R517.
VOLTAGE RAILS OK WITHOUT SIGNAL (Collapseswit
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1. C525, C526 leaky or not holding a charge. (CX4/T) C506, C507.
2. Check for capacitance value of C520.
AMPLIFIER GETS TOO HOT WITHOUT LOAD
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1. Bias trimpot (VR501) misadjusted, opened or burned. (CX4/T) R512.
2. Incorrect bias diodes (D505, 506 should be 1 N4934). (CX4/T) D505, D506.
3. R517, R518 open or has drifted in value. (CX4/T) R504, R527.
4. Defective op amp U503. (CX4/ T) U501.
ONE CHANNEL CLIPS PREMATURELY
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1. R517 or R518 may be open or drifted in value. (CX4/T) R504, R528.
2. Misadjusted current limits (VR502, VR503). (CX4/T) R514, R515.
3. Defective zener D511, D514. (CX4/T) D503, D506.
4. Check for open R512, R522, R539, R546. (CX4/T) R513, R525, R503, R526.
5. 680 ohm chargeback resistor R548 may be open. (CX4/T) R518.
GAIN WRONG
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1. Too high: Missing one of the two feedback resistors from output, R538 or R542, (CX4/T)
R524; or input RN902, (CX4/T) RN500 is wrong value. (Faulty input resistors will cause the
amplifier to fail the Common Mode test.)
2. Too low: Above feedback values wrong or shunt resistor R541/C520, (CX4/T) R523 wrong or
open (open causes very low gain with instability. Check circuit trace).
POOR FREQUENCY RESPONSE (Be sure to correct any gain problem first)
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1. High frequency, determined by feedback capacitor C519, (CX4/T) C516 (wrong value also
causes instability)
2. Low frequency, determined by DC feedback rolloff C520, and "butterworth" feedback
compensation network C518/R542. (CX4/T) C502 low frequency rolloff.
STEP CIRCUIT (CX12/T only)
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Check step circuit components U501, U502, Q503, Q504 carefully, note exact output voltage where
step cuts in (are both matched to 1V). Step turning on too late (too high) will hang up at 2 ohms.
Check referent 15V zener voltages D509 and D510. After inspecting all parts, see below:
1. Step distortion: check step filter components R508/C505, R504/C501, and C528.
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