Power Supply, Mosfet Failure (Bop) - QSC K8 Service Manual

9.6 Power supply, MOSFET failure (BOP)

The most common PSU failure is a failed Q13 or Q14 MOSFET. The failure can cascade down further to gate-drive compo-
nents. In some cases, the PSU will not start due to a PWM controller U15 failure.
Symptoms:
• no power
• The PSU attempts to start but does not succeed.
Failure verification:
• Q13 or Q14 shorted between any two pins of gate, source, or drain.
Repair notes:
• Use the highlighted schematic in Figure 9.9 as a component check guide. Replace components in red and check compo-
nents in blue.
• Q13 and Q14 don't always fail in pairs, but they should both be replaced in the event of a failure.
• A failure of Q14 can put a huge current pulse through C112 and C113 while Q14 is failing; check for the proper rectified
AC voltage across the primary caps if the power supply appears to be dead.
• Check gate drive components (resistors and diodes) tied to gate signals QCL-DR and QM-DR.
• If the LEDs blink but the module won't start, suspect a failure to the power supply controller, U15.
• AC current draw at idle should be 0.1—0.3A at 120Vac and 0.1—0.2A at 230Vac. If the idle current is not in this range,
there is a problem.
BR1
KBU1005
R185
243
1 PCNT
1/10W
C130
0.022UF
10 PCNT
500V
LINEUV = 1.27V
R148
6.34K
1 PCNT
1/10W
R143
63.4K
1 PCNT
1/10W
R149
63.4K
1 PCNT
1/10W
Figure 9.9 - Components to replace and/or check in the event of a power supply MOSFET failure.
36
C132
0.022UF
10 PCNT
500V
R186
243
1 PCNT
1/10W
PRI_HI
- +
R152
82.5K
1 PCNT
1/8W
R172
C112
82.5K
470UF
1 PCNT
20 PCNT
1/8W
400V
C113 R188
470UF
82.5K
20 PCNT
1 PCNT
400V
1/8W
C114
R189
0.022UF
10 PCNT
82.5K
500V
1 PCNT
1/8W
PRI_LO
OC = 26.1A = 0.75V
C117
220PF
5 PCNT
50V
U15
16 7
VIN CS
15 13
LINEUV OUT
1 12
RTDEL AUX
2 4
RTON VREF
3
RTOFF
R151
8
RSLOPE
10 2.21K
SS_SD
10 PCNT
1 PCNT
5 9
SYNC FB
1/10W
UCC2893
C21
C115
0.1UF
10 PCNT
1.0UF
25V
20 PCNT U3
7
16V
MOC205R2V-M
PRI_LO
EE42 3C90
100X40AWG LITZ PRI
100X40AWG LITZ SEC
22AWG AUX
C116
R144
0.68UF
5 PCNT
11K
250V
5 PCNT
1W
Q13
STP11NM60FD
R145 QCL-DR
11K
PSU-VSW
5 PCNT
1W
4
B2
T1
R146 B1
3
11K
5 PCNT
Q14
1W
STW26NM60
QM-DR
R147 R150
11K 29.4K
5 PCNT 1 PCNT
C124
1W 1/10W
0.0022UF
20 PCNT
125VAC
D25
R153
2
IMBD4148
768
R155 R158
A2
1 PCNT
2.87 1.00K
1/10W T1
1 PCNT 1 PCNT A1
1/10W 1/10W
1
PRI_LO
D27
BAS21 R57
R157
21
QM-DR
1 PCNT
C118
1/10W
10
0.1UF
1 PCNT D23
1/8W
MBR230
25V
R168
10
1 PCNT
1/8W
D24
MBR230
3
P3
T2
P1
1 7
T2
S7
S10
10
S12
C52
R166 12
100
220PF
10 PCNT 5 PCNT
250VAC 2W
C128 C137
1 4
1.0UF 1.0UF
T3 T3
20 PCNT 20 PCNT
16V 16V
3 6
D35
IMBD4148
PSU-VSW
D40
BAS21
R165
QCL-DR
10
1 PCNT
1/10W
R164
1.13K
1 PCNT
1/10W
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