QSC PowerLight Series, Technical & Service Manual

Page: 18 / 100

16 QSC Audio Products, Inc.
TD-000083-00
(from between the diodes and the heat sink); it is extremely
fragile, so it is best to replace it during reassembly with a
new one.
Then remove the 16 screws that attach the MOSFETs to the
printed circuit board (PCB); if the MOSFETs are blown, some
of the mounting screws may be melted or damaged. The new
MOSFETs come with new mounting screws with captive
washers, so there is no need to save old ones for reuse.
Remove the other four mounting screws so that the heat sink
assembly is free from the circuit board. Set the hardware
aside.
Find where the twisted wire lead from the heat sink’s thermal
sensor attaches to a pair of pins on the board, and unplug it.
Detach the gate drive cable, which is an 8-conductor ribbon
cable that plugs into an 8-pin header. Lift the heat sink and
MOSFET assembly from the board. If the strip of fishpaper
(Figure 2.11) remains stuck to the PCB, leave it there; if it
comes loose, set it aside for re-assembly later.
2. Remove the MOSFETs from the heat sink. Thoroughly clean the
old thermal grease off the heat sink. Apply fresh thermal
grease to the new MOSFETs (QSC part # QD-000188-00) and
attach them to the heat sink.
Make sure the new MOSFETs are correctly oriented. One end
of each MOSFET has a mounting hole, while the other has an
open-ended slot (see Figure 2.9). Each MOSFET must be
mounted so the slot end faces
away from the diode end of the
heat sink (see Figure 5.11). Try to get the MOSFETs evenly
spaced and as close to perfectly in line as you can. The silk
screen print on the circuit board (see Figure 5.12) shows the
correct orientation.
Tighten the MOSFET mounting screws to 32–35 lb-in (3.6–4.0
N-m) torque. Do not over-torque them.
3. Remove the four diodes on the circuit board (D1, D2, D7, and
D8). See Figure 2.12.
4. Check the value and rating labeled on capacitor C20 (see
Figure 2.13). If it is not a 0.1 µF 250V capacitor (early modules
will have the original 0.047 µF 400V component instead),
replace it with QSC part #
CA-410009-00 .
5. Remove resistors R5 through R8 (see Figure 2.11) and replace
them with 4.75
Ω
¼-watt resistors (QSC part # RE-.47501-10 ).
6. Set the MOSFET and heat sink assembly upside-down and
place the PCB on top of it. Make sure all the threaded holes in
the heat sink and the MOSFETs line up exactly with the holes
in the board. If any MOSFETs don’t line up, reposition them so
they do because misaligned mounting screws can damage
them when tightened.
Make sure the fishpaper strip is in place, and then reattach
the MOSFET and heat sink assembly to the PCB. Make sure
you use the correct screws and washers in each location.
Tighten the four heat sink screws to 32–35 lb-in (3.6–4.0 N-m)
torque and the MOSFET mounting screws to 12 lb-in (1.4 N-
m). Do not over-torque them.
CAUTION: Overtightening its mounting screws will destroy a
MOSFET. When tightening a screw, use the split lockwasher
as a visual guide; as soon as it is flattened, the screw is tight
enough.
Plug the thermal sensor lead onto its two pins on the circuit
board.
7. Clear any solder from the circuit board holes for the four diodes.
Apply a thin coating of new thermal grease to both sides of
the alumina strip and set it in place on the heat sink.
Insert the four new diodes (120V models: QSC part #
QD-000183-00 ; 230V models: QSC part # QD-000182-00 ) into
their holes in the PCB, but do not solder them in place yet.
Then set the two mica insulators in place over the diodes
(they will overlap in the middle), and clamp and fasten the
diodes to the heat sink; tighten the screws to 32–35 lb-in
(3.6–4.0 N-m) torque. Do not over-torque them. As you do
Figure 2.10: The heat sink assembly, with all four MOSFETs
attached. The “diode” end of the heat sink is at right; the slots in
the MOSFETs would face to the left.
Figure 2.11. The silk screen on the circuit board shows the
correct orientation of the MOSFETs. This photo also shows
the four resistors (R5–R8) to be replaced.
“Diode”
end of
heat sink