QSC K10.2, Service Manual

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41 TD-001517-01
K.2 Series Service Manual
8.4 Power supply MOSFETs and repair
Power supply failure is often related to shorted power MOSFETs Q13 and Q14. Both MOSFETs often fail together, but just
one can fail in some instances. Unfortunately most power supply MOSFET failures cause catastrophic damage to PCB traces
or cascading damage to other components down/up stream. Check all components in the schematic below before deciding
to perform a component-level repair.
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.
(verifiable from the top of the PCB, with the clamp off - Figure 8.4.1)
Repair notes:
• First consider replacing the AMP/PSU board with a new one. Repairing
the power supply at a component level is not recommended due to
the complexity and small components in the circuit.
• Use the highlighted schematic in Figure 8.4.2 as a component check guide. Replace components in red and check com-
ponents in blue.
• Q13 and Q14 don’t always fail in pairs, but always replace both when one has failed.
• A failure of Q14 could cause direct damage to the PWM controller at U15. If this occurs, replace the board with a new
one instead of repairing.
• Check gate drive components (resistors and diodes) tied to gate signals QM-DR and AUX_OUT.
• If the LEDs or LCD screen 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.
• 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 8.4.1 - Measuring PSU MOSFETs from top
DNI
CURRENT SENSE
(RC) CLAMP MODE
56K FOR PASSIVE
1/10W
10.0K
56.2K
1 PCNT
1W
0.68UF
5 PCNT
250V
220PF
1 PCNT
1UF
MMBTA56
2.00K
1 PCNT
25V
10 PCNT
1/10W
1 PCNT
1N4148
10 PCNT
RB751S40T1
10
0.0022UF
10
RB751S40T1
1/16W
STP11NM60FD
25V
1/10W
1 PCNT
20 PCNT
1 PCNT
10.0K
1/8W
1 PCNT
1/16W
RB751S40T1
10 PCNT
1UF 22.6
250VAC
STP45N65M5
250VAC
R121
R52
R123
R150
T2
C116
Q13
D40
Q18
R164
D35
D27
C52
C124
Q14
R67
C128
T3
D51
T1
C137
T3
R157
AUX_OUT
QM_DR
PRI_LO
XF-MID
PSU-VSW
3
2
1
3
1
4 3
6
4
B2 B1
PRI 1
PRI 2
Figure 8.4.2 - Some components to check if the MOSFETs fail.