QSC CX404 repair

[sparky94]

I’ve picked up this amp from eBay. It works wonderfully on three out of the four channels. Channel two is dead. All channel clip lights will illuminate then after a few seconds all but twos would turn off. I opened the amp and started checking components. I found all of channel two’s output transistors in full B,C,E short. As a result of course all the emitter resistors were burnt open as well. I also found R181, R176, R180, and R177 burnt open. Strangely Q57 and Q58 passed my meter test but I proceeded to replace them anyway. I highlighted all these in the schematic. I pulled and tested all the components in green and found no problems with them. During my wait for the main outputs, I placed the new drivers in and proceeded to do a preamp test. While I could hear the music at very low levels, it clipped easily. Even when the preamp was passed into an external amp with none of the transistors in, it would still clip and sound bad. With all these components replaced, I am able to power up the amp and all channels come up normal. No DC offset on the speaker terminals. I cannot seem to get a successful BIAS adjustment so I have the pot set to max resistance. I’ve tried copying closely to channel one and the result is the same. I am able to get output, but if I dial the gain up, it clips. I decided to feed a 1Khz tone in and prove with my scope. I get a good wave all the way up to the second opamp in the preamp stage. Once I probe pin 7 of U6:2 I get the same clipping when I turn the gain up. It seems to happen at the same output power regardless of input (ex if IPad volume is halfway up, clipping will happen when gain pot is at quarter. If IPad is at quarter volume, clipping happens when gain pot is halfway.) of course I get the same distortion on the main outputs only at higher levels. The clipping gets worse on the output if I turn the BIAS all the way down. My forte has certainly been in power supplies and much less amps and I’d like to change this. I see there appears to be feed back from the main stage through R135 from “Speaker_Bus-_B”. Would this cause a gain issue with the preamp or is there an issue with the preamp Opamp?

[tbegal]

Frequency generator. Can you feed some signals in?
Take a shot of the output waveforms.
Sounds like circuit on OP Amp might be failing under load.

[CaptDon]

Do I understand you are operating the unit with parts missing, the output transistors? If so, who can say what the result would be.

[sparky94]

The outputs were installed when I ran this test. I ran without them previously to determine if the preamp stage was good but this still happened. Anyway I fed a 1Khz signal in. On pin one of channel two’s opamp, (photo 1)  I get the sine wave in. It does not clip no matter what the gain pot is set to. However on pin 7, (Photo 2) I get distortion at approximately 1.7Vpp and it just continues to rail after turning up the gain. It stays a normal sine wave until the opamp is pushed farther than 1.7 Vpp. All the power rails are present. It certainly does look like the opamp is bad but it just surprises me as I’ve never seen one fail before. With how strange some of QSCs amps are I just wanted to make sure that the output stage isn’t somehow changing the gain of the preamp stage.