QSC RMX 850
MECHANICAL DIS ASSEMBLY AND REASSEMBLY
Removing the channel modules
1. Disconnect the amplifier from AC power and allow at least 10 minutes
for internal voltages to bleed down.
2. Using a Philips screwdriver, remove the screws that fasten the top
cover to the chassis. Also remove the top cover’s four recessed screws that
fasten it to the heat sinks. As you remove screws, set them aside, but also
make note of where each type is used so you can properly re-assemble the
amplifier.
3. Lift the top cover up at the rear and carefully pull it toward the
back, removing the five hooks on the front edge from their slots in the
chassis.
4. Pull the gain control knobs straight off from the potentiometer
shafts.
5. Tip the amplifier up on its side and remove the four screws that
fasten the heat sinks to the chassis.
6. Set the amplifier back down and remove the screws that mount the
channel modules to the chassis standoffs. There are six screws in the left
module and five in the right one.
7. Remove the four screws that fasten the fan, fan shroud, and fan guard
to the chassis. Lift the fan shroud out from the chassis; this will give you
room to properly remove the modules from the chassis.
8. Disconnect the wire and cable connections to the channel modules. All
of the connections are either detachable headers or ¼-inch quick-connect tabs
that are disconnected by pulling them straight up. No unsoldering is necessary.
9. Slide the channel modules toward the back so the potentiometer shafts
and front panel LEDs are clear of their holes in the front panel. Lift the
channel modules out from the chassis.
10. Re-assembly is the opposite of disassembly.
Removing the AC board
The AC board provides AC voltage selection, rectification of the transformer
secondary
current, and a regulated DC supply for the cooling fan. It seldom
needs to be replaced unless it is physically damaged itself. Most failures
involving the AC board can be repaired through replacement of individual
components.
WARNING: Regulatory agencies require that any operating voltage
conversions from 120 volts to any other voltage be done onlyby QSC’s factory
service. Any other operating voltage conversions may be done only by a
QSC-authorized service center or international distributor.
1. Disconnect the amplifier from AC power and allow at least 10 minutes
for internal voltages to bleed down.
2. Remove the four screws that fasten the fan, fan shroud, and fan guard
to the chassis. Lift the fan shroud out from th chassis.
3. Disconnect the wires that connect to the channel modules. All of the
large single wires attach to the channel modules with ¼-inch quick-connect tabs
that are detached by pulling them straight up. The remaining three black wires
disconnect at the left channel module with a detachable header. If you are
planning to replace the AC board with another, carefully cut each of the
transformer wires connecting to the board just above its solder tab. You must
leave enough slack to allow connection to the new AC board. Remove the old heat
shrink tubing from the wires and strip the wire ends about 0.25 inch or 6.3 mm.
4. Remove the five screws that attach the AC board to the chassis
standoffs. Lift the board out from the chassis.
5. Re-assembly is the opposite of disassembly. If you’re using a new AC
board, slide new pieces of heat shrink tubing over the transformer wires before
you solder them to the appropriate tabs on the board; after soldering, cover
the joints with the tubing and use a heat gun or other heat source to shrink
them tightly.
SETTING BIAS
Always set the bias
> after replacing any output or driver transistor.
> after replacing any diode or resistor in the driver/output circuitry.
> if the amplifier seems to run too hot at idle.
> if the amplifier exhibits crossover distortion.
> after replacing any diode or resistor in the driver/output circuitry.
> if the amplifier seems to run too hot at idle.
> if the amplifier exhibits crossover distortion.
The bias network sets the quiescent base current in the NPN and PNP driver transistors, which in turn sets the quiescent current in the output transistors. The driver transistors should both be slightly “on” at idle so that the transitions of the signal voltage between positive and negative are smooth and free of gaps or glitches. Too much bias current will cause the amplifier to run hotter than it should, especially at idle, while too little will cause noticeable crossover distortion, especially at low signal levels.
The amplifier circuitry must be cool, or at least within a couple degrees of ambient air temperature, and the top cover must be removed. If the driver and output transistors are significantly warmer than the ambient air, leave the amplifier off and let it cool before proceeding. Before turning the amplifier on to set bias on one or both channels, familiarize yourself with the locations of the trimpots (R131 and R231) and the voltage measuring points so you can work quickly but thoroughly. If the amplifier warms up before you finish setting the bias, you will need to shut the amplifier off and let it cool down before you resume.
Tools and resources you will need:
1. Small flat screwdriver (non-conductive) for adjusting trimpots
2. DC voltmeter
3. AC power
2. DC voltmeter
3. AC power
Procedure
1. Turn the amplifier’s gain controls all the way down. No test signal is needed.
2. Plug the amplifier into an appropriate AC source. Turn the amplifier on.
3. Channel 1: While measuring the DC voltage across resistor R146, adjust trimpot R131 to obtain the voltage listed in Table 1.
4. Channel 2: While measuring the DC voltage across resistor R246, adjust trimpot R231 to obtain the voltage listed in Table 1.
After setting the bias, calibrate the positive and negative current limiting; instructions for the procedure follow below
Setting positive and negative current limits
Tools and resources you will need
1.Oscilloscope
2. 2-ohm resistive load (rated for at least 1200 watts)
3. Shorting connector for amplifier output
4. Variable AC transformer (e.g., Variac, Powerstat, etc.) rated for 25A (120V) or 12A (230V). Make sure the AC supply is appropriate for the amplifier.
5. 1 kHz audio sine wave generator
6. Digital multimeter
7. Clamp-on digital current meter (e.g., Fluke 30 Clamp Meter)
8. Small flat screwdriver (nonconductive) for adjusting trimpots
Procedure
1. Set the audio sine generator to 1 kHz at 1 volt RMS and connect it to Channel 1's input. Connect a 2-ohm load and the oscilloscope probe across Channel 1's output.
2. Turn up Channel 1's gain control partway. On the oscilloscope you should see the amplitude of the sine wave increase accordingly.
3. Turn the gain control back down and apply a short circuit across the output terminals of Channel 1. Clamp a current probe either onto one of the brown wires running to the AC switch or onto
4. Turn the gain control all the way up. Adjust trimpots R139 and R140 equally until the current measured falls within the range shown in Table below.
5. Turn the gain control all the way down and remove the short circuit so the channel drives the 2-ohm load. Turn the gain control back up until the output clips. The voltage at which the signal starts to clip should fall within the range shown in Table below. If the clipping is asymmetrical, that is, the signal clips on either the positive or negative side first, adjust R139 to make it symmetrical.
6. Turn the gain control down. If the amp has begun to warm up shut it off and let it cool a few minutes before proceeding with Channel 2.
7. Repeat steps 1 through 5 for Channel 2. Use trimpots R239 and R240 to adjust the current limiting in steps 11 and 12.
8. Turn both channels’ gain controls all the way down. Clamp the ammeter onto one of the amp’s AC wires and check the amp’s idle current. If the amplifier is still at about room temperature, the idle current should match the value shown in Table below.
BIAS TABLE FOR RMX 850
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