35
TD-000487-00
K Series Service Manual
9.5 Amplifier section, MOSFET failure (BOP)
While not a common failure due to the DSP limiting and protecting the amplifier section from being over-driven, the MOS-
FETs in the amplifier section can fail in certain conditions. For example, if the LF or HF transducer has a short across the pos-
itive and negative terminals, high-current can flow through the MOSFET, ultimately causing failure to the amplifier section.
Timing issues with the gate drive circuit can also cause MOSFET failures.
Symptoms:
•
BOP 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—Q5 shorted between any two pins of gate, source, or drain
Repair notes:
•
Use the highlighted schematic in Figure 9.8 as a component check guide. Replace components in red and check compo-
nents 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 components (resistor and diode) tied to +12VLOW. If the gate driver IC fails, these components usually fail as
well.
•
A quick check of the amplifier feedback path can be done by measuring resistance from the switching node (Q2 source
and Q3 drain, for example) back to the comparator inverting input; it should be about 11.3k
Ω
.
•
You may check for amp switching by looking for about 7.5 Vdc on the U2/U4 pin12 comparator output (0-15V at 50%
duty cycle).
•
Q16 can fail if the gate drivers and/or amp FETs fail. Check for shorts across Q16 collector-emitter, shorted D37, shorted
D36, shorted D14/29, or opened R12/15. A failure with the gate drive supply can also take out the -85V rectifier, so
check for a shorted D34.
•
Check the transducer associated with the failed amplifier section (LF or HF) for a short across the positive and negative
terminals.
1 PCNT
5.62K
25V
1 PCNT
1/10W
1/10W
21
1 PCNT
1 PCNT
3.65K
10UF
20 PCNT
25V
2.87
1/10W
IRFB4020
1 PCNT
1/10W
21
21
1/10W
1 PCNT
20 PCNT
10UF
1/10W
1 PCNT
1.13K
1 PCNT
10PF
14.0K
1/10W
IRS20957
1 PCNT
1 PCNT
1 PCNT
150K
50V
5 PCNT
1/8W
10UF
30UH
19.6
200V
5 PCNT
120PF
200V
120PF
5 PCNT
19.6
1/8W
1 PCNT
4.53K
20 PCNT
25V
1/8W
BAS21
1/10W
BAS21
150K
1.13K
1 PCNT
1/10W
IMBD4148
ES1D
1/10W
10.0K
1/10W
1 PCNT
IRFB4020
1/8W
1 PCNT
1/10W
1/10W
1 PCNT
2.21K
6.34K
1 PCNT
R62
L2
C30
R63
C31
R50
D12
R47
D10
R45
R43
C14
R54
Q4
Q5
R55
C18
R41
U7
R37
C11
R34
R35
R31
R51
D6
C24
R73
R27
R15
D16
+12VLOW
CH1-SD
+VCC
-VCC
9
2
16
8
14
3
10
7
15
11
1
6
13
4
12
IN
+15V_A
A1
CSH
VB
HO
VS
VCC
LO
COM
DT
OCSET
VREF
VSS
IN
CSD
VDD
A1
Figure 9.8 - Components to replace and/or check in the event of an amplifier section MOSFET failure.
Summary of Contents for K8
Page 2: ...2 QSC Audio Products LLC...
