Sunday, December 21, 2014

QSC RMX 850 CIRCUIT DIAGRAM- BIAS ADJUSTMENTS AND CALIBRATION

          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.

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

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