Lf And Hf Amplifier Leds; 12Vlow Voltage Rail Failures - QSC K.2 Series Service Manual

8.6 LF and HF amplifier LEDs

Each amplifier section has an LED that is driven at LF_LO and HF_LO output of the gate driver (pin11). If the amplifier is on
and switching, the LED will constantly illuminate green, which is a good indication that the amplifier circuit is working. The LF
amplifier section uses LD1 and the HF amplifier section uses LD2. These two LEDs can give us a good information about
the behavior of the amplifier circuit.
LED behavior notes:
• During normal operation, LD1 and LD2 should always be on
together. If just one LED is turned on, then the LED that is
turned off most likely has a problem in the correlating amplifier
section.
• During standby operation, LD1 and LD2 should always be off.
In standby the DSP shuts down the amplifiers.
• If both LEDs are turned off and the amplifier is in normal opera-
tion, look at the +12VLOW rail and the secondary power rails
(VDD and VSS).
• If both LEDs are randomly turning off and on at the same time,
an issue with the +12VLOW rail could cause this behavior.
Check the components in the AUX OUTS section in the power
supply. Also, this issue may be related to service bulletin K2-
002.
• Recycle AC power to start the amplifier in normal operation, or, input a low amplitude signal to keep the amplifier run-
ning in normal mode.

8.7 +12VLOW voltage rail failures

The class D amplifiers require a steady voltage from the +12VLOW rail to correctly function. If this rail fluctuates, the class D
amplifiers could pulse off and on. If the rail is too low, the class D amplifiers will latch into a shutdown mode and not allow
audio output. +12VLOW is generated on the secondary side of the transformer T2 in the AUX OUTS circuit.
Symptoms:
• All 3 signal LEDs (Ch.A, Ch.B, and Ch.C) turned on
without signal input
• Chirping or buzzing sound heard in speaker, especially
when coming out of standby mode.
• No audio, especially when coming out of standby
mode.
+12VLOW notes:
• If the chirping/buzzing sound is heard, or there is no
audio when coming out of standby, please refer to the
latest Service Bulletin. The AMP/PSU board may need
to be replaced with a new one.
• If the amplifiers LEDs are not turned on during nor-
mal operation, the +12VLOW rail could have failed.
Immediately check the -15V rail on the input/DSP
board at J11 pin3.
• The +12VLOW and -15V voltage rails are generated after fuse F1. If +12VLOW has a shorted component, it will proba-
bly cause F1 to blow. The -15V rail will also measure 0▲Vdc if F1 is open.
• SMD MOSFET Q16 could be the culprit. Q16 is located on the bottom side of the AMP/PSU board. Also check the
Schottky diodes in the circuit as highlighted in Figure 8.7.1.
TD-001517-01
K.2 Series Service Manual
Figure 8.6.1 - The amplifiers are active if the LEDs are on
TP1
+15V
F2
D31
ES1D
5A
R68
24V
6 243
C91 C72
1 PCNT
1000UF 0.1UF
1/10W
C49 20 PCNT 10 PCNT
SEC 2
25V 25V
120PF
5 PCNT
200V
7
C50
120PF A1
5 PCNT C92 C74
SEC 3 200V
T2
1000UF 0.1UF
20 PCNT 10 PCNT
R69
25V 25V
243
8
1 PCNT
F1
D32
1/10W
ES1D -15V CURRENT:
5A
24V MC33078 = 5.5MA
-15V
TP2
L79L05 = 6.25MA
-5VA CURRENT = 22MA
TOTAL = 33.75MA
+15V
R56
2.00K
1 PCNT
1/10W
R71
2.00K
1 PCNT
D36
1/10W
SMD110PL
Figure 8.7.1 - Components to check if +12VLOW rail has failed
D61
D62
MBR230 1N4148
U7
L78M05
C94
1
IN OUT
1000UF
GND
20 PCNT C15
25V
0.1UF
10 PCNT
25V
C4
DNI
LM79L05AC
C82
0.1UF
10 PCNT
22UF GND
7
VIN4
10 PCNT 25V
R73 6
DNI
25V VIN3
C83
3
VIN2 VOUT
22UF
2
0
VIN1
10 PCNT
NA DNI
25V
D1
NA D12 U4
MBR230 -5VA CURRENT: 1N4148
2X IRS2092 = 11MA * 2 = 22MA
TOTAL = 22MA (187MW)
NOMINAL -15V RAIL IS AT -13.5V
WITH ZERO COPPER THETAJA = 180C/W
JUNCTION AT 25C=33
A1
+12VLOW CURRENT:
HF + LF = 89MA (MEASURED)
R156 Q16
HF DEADTIME = 948UA
2.00K IRLML2502
2X LO LEDS = 500UA * 2 = 1MA
1 PCNT
TOTAL = 91MA
C95
1/10W D29
+12VLOW
SMD110PL
1.0UF
R70 C56
10PCT
1000UF
2.00K
100.0VDC
20 PCNT
1 PCNT
25V
C125
1/10W
D20 D37
SMD110PL
SMD110PL
1.0UF
10PCT
100.0VDC
+5VA
TP11
3
C19
10UF
20 PCNT
6.3V
C6
10UF
20 PCNT
6.3V
D15
1
SMD110PL
TP12
-5VA
OUT
C86 R99
0.1UF
10K
10 PCNT
1 PCNT
25V
1/16W
VSS
43