QSC RMX 850
RMX calibration procedures
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
