Amplifier Mosfets - QSC K.2 Series Service Manual

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:
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".
42
Take good care during the reassembly process when replacing the power MOSFETs. Apply a fresh
+5VA
R18
5.62K
R13
1 PCNT
10 1/16W
1 PCNT
1/16W
R19
LF_+5VA
5.62K
1 PCNT
U1
C25 C27
1/16W
10UF 0.1UF
1 16
VAA CSH
20 PCNT 10 PCNT
6.3V 16V LF_VB
2 15
GND VB
LF_IN- LF_HO
3 14
IN- HO
LF_COMP
4 13
COMP VS
LF_CSD
5 CSD VCC 12
C22 C26
RB751S40T1
LF_LO
0.1UF 10UF
6 VSS LO 11
10 PCNT 20 PCNT
16V 6.3V
7 10
VREF COM
LF_-5VA
R20 DNI
8 9
CSLO DT
10K
R14 98-1073PBF
1 PCNT
R15 R17
10 1/16W
1 PCNT 5.62K 0
1/16W 1 PCNT NA
1/16W R16 NA
-5VA
3.57K
1 PCNT
D10
1/16W R113
ES1D
2.87
1 PCNT
1/10W
R53
2.87
1 PCNT
1/10W
Figure 8.5.2 - Components to check if amplifier MOSFETs fail (LF shown)
Figure 8.5.1 - Measuring amplifier MOSFETs from top
VDD
L5
LF_VDD
C79
1UH
47UF
20 PCNT
100V
A1
D3
BAS21WS-V
R23
10K
1 PCNT
1/16W
R55
63.4K
1 PCNT
C33
1/10W
22UF
DNI
10 PCNT
Q2 C75
R54
25V
0.1UF
IRFB4227PBF
C93 10 PCNT
2.87
C43 200V
22UF
1 PCNT
10 PCNT
TP14 1.0UF
1/10W
25V
10 PCNT
250V
LF_VSW
DNI
R93
Q3
1
R57
IRFB4227PBF 1 PCNT
1/10W
2.87
R108
1 PCNT
10K
1/10W
1 PCNT
C34
1/16W
22UF
10 PCNT
LD1
25V
GREEN
D13
ES1D
L6
LF_VSS
1UH
C81
47UF
20 PCNT
VSS
100V
A1
C35 C42 C57
100.0PF 1.0UF 1.0UF
5 PCNT 10PCT 10PCT
500.0 VDC 100.0VDC 100.0VDC
A1
R60
D17
10
ES3G
1 PCNT
1/10W L2
36UH
73MW NOM
D18
C36 R58
ES3G
100.0PF 63.4K
5 PCNT
1 PCNT
500.0 VDC
1/10W
15MW NOM
R61 R59
10 10K
1 PCNT 1 PCNT
1/10W 1/16W
C37 C58
1.0UF 1.0UF
10PCT 10PCT
100.0VDC 100.0VDC
A1
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