Qsc usa 900 power amplifier coming on but no signal.what can be the problem.I check all outputs

The red clip lights on the QSC RMX2450 indicate that the amplifier is receiving a signal that is too strong and could cause distortion or damage to the speakers. The green lights indicate that the signal is within a safe range. When there is no input signal, the amplifier defaults to the red clip lights as a precautionary measure. Once a signal is received and it is within the safe range, the lights turn green. This is a normal behavior for this amplifier.

If you are experiencing an issue where the QSC RMX 1450 amplifier is not giving any sound when connected to the main output of your Mackie 16 OK mixer, but works when connected to the aux send, here are some possible causes and solutions:

1. Check the main output level on the Mackie mixer. If the level is too low or muted, this can cause the amplifier to not produce any sound. Increase the main output level on the mixer and check if the amplifier produces sound.
2. Check the cables connecting the mixer to the amplifier. If the cables are damaged or not connected properly, this can cause the amplifier to not produce any sound. Ensure that the cables are connected securely and are not damaged or broken.
3. Check the amplifier settings. Ensure that the amplifier is set to the correct input source and that the input gain level is set correctly. You may also want to check if there are any protection modes or limiters enabled on the amplifier.
4. Check the speaker connections. If the speakers are not connected properly or are damaged, this can cause the amplifier to not produce any sound. Ensure that the speakers are connected correctly and are not damaged.
5. If none of the above steps work, there may be an issue with the mixer or the amplifier. You may need to contact a qualified technician to further diagnose and repair the problem.

Remember to always follow the manufacturer's instructions and safety guidelines when working with audio equipmen

You have a high power amplifier and the work you propose leaves no room for either error or compromise. If, as you suppose, the output is by complementary pair, the transistors must be matched pairs providing equal gain characteristics.

Unless you have seen inside a complete amplifier of the type you are only guessing at the output stage(s) that could be by module or a multiple transistor output. There also might be other driver stage(s) sharing the same heat sink.

The least you will need is a service manual and an internet search produced a number of results on the below link.

https://www.google.co.uk/search?rlz=1C1CAFB_enGB673GB676&ei=PtG_W56NM6mIgAaYwaqIDg&q=qsc+mx1500a+service+manual&oq=qsc+MX+1500a+amplifier&gs_l=psy-ab.1.3.0i71k1l8.0.0.0.32505.0.0.0.0.0.0.0.0..0.0....0...1..64.psy-ab..0.0.0....0.D6yNqkAW5nw

it is not passing it's self test. check the usual suspects (shorted output transistors, blown mini fuse, and that annoying pcb mount fuse in the power supply (looks like a resistor with one stipe)) if they all check out, test the relay coil for continuity. if it is open then replace the relay, but if it reads below 200 ohms the relay is ok, and i'd bring it to the shop for more extensive testing and repair.

Hi,
this power amp uses 2SA1962 and 2SC5242 output transistors. You can download the schematic diagram from this website.
Thanks.
gylacz

http://www.retrevo.com/support/QSC-RMX-4050HD-Amps-manual/id/23107ci658/t/2/

Page 7...

CLIP: red, to the left of each Gain control.

Normal indication: illuminates whenever the amplifier is driven beyond full power. The LED's brightness indicates the amount of distortion. Distortion that causes only brief flashing may not be audible.

During muting, the indicator fully illuminates. This occurs during normal "On-Off" muting.

Abnormal indication:

•Bright red illumination while the amp is being used indicates either thermal muting or a shorted output.

•If the amplifier overheats, the fan will run at full speed, and operation should resume within one minute. Allow the fan to run, and make sure the amplifier ventilation is adequate.

•A shorted or overloaded output circuit will cause excessive Clip flashing and possible overheating.

•If distortion is audible without a Clip indication, the problem is either before or after the amplifier. Check for damaged speakers or overloaded signal source. The amplifier Gain control should be in the upper half of its range to prevent input overload.

It is likely that additional components were damaged when the output transistors (2SC5200, 2SA1943) were shorted. Check the driver transistors which preceed the outputs in the circuit as well as resistors in the area. Also, there should be a bias transistor mounted on the heat sink that needs to be checked as well.

Keep us posted,
Dan

This is a very precise questions as QSC amp are pretty rare and high end electronics equipment. There is no way getting an answer other than getting the amp at a sound shop for repair, wich might go for a visit at QSC anyway. From a sound shop repairman...

In the order you state them.

Receiver, EQ, Amplifier,

These graphic eq's and the amplifiers are pin 2 hot on the XLR by default (which is the norm) unlike some other makes. Handy to know when making up cables from receivers like this.

The weaker signal caused by leaky capacitor and change value resistor is need to replaced with a new one so it's works normal again. Your tech did is re soldering the cold solder he did not replaced some defected parts.

Hope it may help you;

Regards;
VOTIT