QSC K.2 series, Service Manual

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42 QSC, LLC
8.5 Amplifier MOSFETs
During amplifier section failure, both MOSFETs often fail together, but just one can fail in some instances. Most MOSFET
failures can be repaired as long as failure does not cascade past the gate resistors and into the gate drive controllers. Check
all components in the schematic below before deciding to perform a component-level repair.
Symptoms:
• One channel not outputting audio (after verifying signal output is
okay from the codec)
• Amplifier module will not power up - keeps restarting
• one channel not outputting audio (with verified output from the
DSP into the amplifier section)
Failure verification:
• MOSFETs Q2 / Q3 (LF) or Q7 / Q8 (HF) are shorted between any
two pins of gate, source, or drain (verifiable from the top side of
PCB as seen in Figure 8.5.1)
Repair notes:
• The LF and HF amplifiers use
different MOSFET pairs
! Do not mix
them.
• Use the highlighted schematic in Figure 8.5.2 as a component check guide. Replace components in red and check com-
ponents in blue.
• The MOSFETs almost always fail in pairs. Always replace both MOSFETs with new.
• Check the gate resistors tied to the MOSFETs for open circuit.
• The gate driver ICs can easily fail when the MOSFETs short.
• An amp FET failure can also fail the secondary rectifiers; check for shorted D33—34.
• Check the two resistors and diode tied to +12VLOW. If the gate driver IC fails, these components usually fail as well.
• You may check that the amplifier is switched on by simply looking at the small green LED (LD1 or LD2) on the top-side
of the AMP/PSU board. The LED is constantly driven on during switching.
• Check the transducer associated with the failed amp section (LF or HF) for a shorted or open voice coil.
• MOSFET clamping:
Take good care during the reassembly process when replacing the power MOSFETs. Apply a fresh
and thin layer of thermal grease on the MOSFET body. The torque specification on the 2 clamp screws is very critical to
ensure good heat transfer between the electrical component and heatsink. Sufficient clamp force is required to achieve
the best result. Refer to the installation steps in “6.2 AMP/PSU board”.
Figure 8.5.1 - Measuring amplifier MOSFETs from top
DNI
73MW NOM
15MW NOM
DNI
DNI
100.0VDC
1.0UF
10PCT
100.0VDC
1.0UF
10PCT
100.0VDC
1.0UF
10PCT
100.0VDC
1.0UF
10PCT
IRFB4227PBF
1/10W
10
1 PCNT
20 PCNT
47UF
100V
1UH
47UF
1UH
20 PCNT
100V
10
1/16W 1 PCNT
5.62K
RB751S40T1
1/16W
1/16W
98-1073PBF
NA
0
NA
10 PCNT
1 PCNT
6.3V
2.87
1/10W
1 PCNT
ES1D
ES1D
2.87
1 PCNT
1/10W
16V
0.1UF
10 PCNT 20 PCNT
10UF
3.57K
1 PCNT
20 PCNT
10UF
6.3V
1 PCNT
1/16W
10
16V
0.1UF
10 PCNT
10K
1 PCNT
1/16W
25V
22UF
5.62K
1/16W
1 PCNT
5.62K
1 PCNT
1/16W
10 PCNT
25V
22UF
1/16W
10K
1 PCNT
BAS21WS-V
GREEN
1/16W
10K
1 PCNT
1 PCNT
2.87
1/10W
2.87
1 PCNT
1/10W
1/10W
63.4K
1 PCNT
1
1 PCNT
1/10W
100.0PF
5 PCNT
500.0 VDC
36UH
250V
10 PCNT
1.0UF
200V
10 PCNT
0.1UF
1 PCNT
1/10W
10
ES3G
1/16W
10K
1 PCNT
1/10W
63.4K
1 PCNT
ES3G
100.0PF
500.0 VDC
5 PCNT
IRFB4227PBF
25V
10 PCNT
22UF
TP14
R59
R58
R108
L5
C79
L6
C81
R93
C75
D10
C25
R13
R20
R17
R16
C22
R15
D3
R19
R18 R23
C27
R14
C26
U1
C93
C33
R55
C43
C58
C57
R113
LD1
D17
C42
D18
L2
C35
R60
Q2
R61
C36
Q3
R54
D13
R57
C37
R53
C34
LF_VSS
LF_VDD
LF_COMP
LF_VB
LF_HO
LF_LO
LF_CSD
LF_IN-
LF_-5VA
LF_+5VA
LF_VSW
6
13
7
12
15
1
11
3 14
2
9 8
16
5
4
10
+5VA
A1
VDD
A1
VSS
-5VA
CSH
VB
HO
VS
VCC
LO
COM
DT CSLO
VREF
VSS
CSD
COMP
IN-
GND
VAA
A1
A1
Figure 8.5.2 - Components to check if amplifier MOSFETs fail (LF shown)