Hi Everyone,I purchased a QSC CX Power amp some time ago. I was told it had a fault. What it is, the amp when connected to speakers has a loud continuous hum. When i connect a mixer to it, both channels are working ok,with no distortion,just this loud horrible hum at a constant volume. Increase or decrease in signal makes no difference to hum. Any help would be appreciated.So far i have not had much help. Schematic is available on the web. These amps are a class H Thanks Michael
You may get more help over on PA forum where Enzo watches pretty closely.
Probably there is a bad connection somewhere. I'd take a sound probe, which is an amp + speaker with a clamp in the input to prevent overloading, then probe from front to back to see where the hum starts. Sound probe Gain should be set high in the front end, low at the back end of the amp. The clamp on a sound probe is a .047 uf 400 v cap, followed by a 4700 ohm resistor, followed by two red, green, or yellow LED's line to line from + to minus. The same cap and resistor go on the minus probe as on the + probe. Use an amp with a line level input, not a phono input. Some PA amps have different speaker return voltage for the two different channels, so a case ground doesn't work for the minus of the sound probe. You can alternatively use an AC voltmeter, with a FM radio tuned to a rock station. If you see beats in the AC voltage, you are seeing the beats of the rock music (and that part is not hum or oscillation)
So whereever the hum starts, then you look around there for bad solder joints, burnt up components, or oxidized IC pins and sockets, gappy volume pots, oxidized source selector switch contacts, whatever. Remember ribbon cable is a high failure component, where the connector pinches the wire - but they can be removed and replaced to remove the oxide. As can all connectors.
Don't be surprised if you find incinerated components in the front end. Roadies and musicians have to set up on stage in the dark, and sometimes the 17 vac line out of the guitar or PA amp goes in the wrong socket, like the input of the PA amp. The 1/8 watt input resistors on one of my PA amps were so burnt I couldn't tell whether they were resistors or caps.
If the amp is not distorting, I'd tend to say the back end is okay. However towards the end do a full power check on resistors using an analog VOM to read the voltage (power) out. Also do a final Idle bias current check on the output transistors, see the service manual.
Warning to newbies: Voltage over 24 from one hand to the other can stop your heart. One hand at a time only. Use alligator clips for the other meter or sound probe lead. Also voltage over 1 through a metal piece of jewelry can burn your finger or hand to a cinder. No jewelry, not on hand wrist or neck. String or leather macrame is okay.
If you get into back end problems, use a light bulb box in series with the AC of the amp to limit the current until everything is sounding okay. I use a $1 FM radio to exercise the amp, not a $600 cell phone. DC out the input jack can ruin your cell phone. DVM are great for reading DC levels, but AC levels, they produce random numbers on music except for the $180 RMS versions. The $180 RMS versions find ultrasonic oscillation invisible, so I don't suggest buying one of those anyway. I use a Sear craftsman non-autoranging dVM, and a $200 Simpson 260-6XLPM analog meter for Ac measurements. A .047 uf cap in series with the analog meter probe is required to stop it reading AC if a DC voltage is present. $30 analog meters can be used with 25 VAC scales on all but the first stage at very high gain. The earphone jack of a FM radio will put out 5 VAC so that is okay for a 25 vac VOM. Scopes are nice but if you step on the probe it is $50. Also, scopes are full of e-caps that go bad in 20 years, whereas my simpson meter has zero e-caps and hence is 30 years old working fine.
Happy hunting!
Probably there is a bad connection somewhere. I'd take a sound probe, which is an amp + speaker with a clamp in the input to prevent overloading, then probe from front to back to see where the hum starts. Sound probe Gain should be set high in the front end, low at the back end of the amp. The clamp on a sound probe is a .047 uf 400 v cap, followed by a 4700 ohm resistor, followed by two red, green, or yellow LED's line to line from + to minus. The same cap and resistor go on the minus probe as on the + probe. Use an amp with a line level input, not a phono input. Some PA amps have different speaker return voltage for the two different channels, so a case ground doesn't work for the minus of the sound probe. You can alternatively use an AC voltmeter, with a FM radio tuned to a rock station. If you see beats in the AC voltage, you are seeing the beats of the rock music (and that part is not hum or oscillation)
So whereever the hum starts, then you look around there for bad solder joints, burnt up components, or oxidized IC pins and sockets, gappy volume pots, oxidized source selector switch contacts, whatever. Remember ribbon cable is a high failure component, where the connector pinches the wire - but they can be removed and replaced to remove the oxide. As can all connectors.
Don't be surprised if you find incinerated components in the front end. Roadies and musicians have to set up on stage in the dark, and sometimes the 17 vac line out of the guitar or PA amp goes in the wrong socket, like the input of the PA amp. The 1/8 watt input resistors on one of my PA amps were so burnt I couldn't tell whether they were resistors or caps.
If the amp is not distorting, I'd tend to say the back end is okay. However towards the end do a full power check on resistors using an analog VOM to read the voltage (power) out. Also do a final Idle bias current check on the output transistors, see the service manual.
Warning to newbies: Voltage over 24 from one hand to the other can stop your heart. One hand at a time only. Use alligator clips for the other meter or sound probe lead. Also voltage over 1 through a metal piece of jewelry can burn your finger or hand to a cinder. No jewelry, not on hand wrist or neck. String or leather macrame is okay.
If you get into back end problems, use a light bulb box in series with the AC of the amp to limit the current until everything is sounding okay. I use a $1 FM radio to exercise the amp, not a $600 cell phone. DC out the input jack can ruin your cell phone. DVM are great for reading DC levels, but AC levels, they produce random numbers on music except for the $180 RMS versions. The $180 RMS versions find ultrasonic oscillation invisible, so I don't suggest buying one of those anyway. I use a Sear craftsman non-autoranging dVM, and a $200 Simpson 260-6XLPM analog meter for Ac measurements. A .047 uf cap in series with the analog meter probe is required to stop it reading AC if a DC voltage is present. $30 analog meters can be used with 25 VAC scales on all but the first stage at very high gain. The earphone jack of a FM radio will put out 5 VAC so that is okay for a 25 vac VOM. Scopes are nice but if you step on the probe it is $50. Also, scopes are full of e-caps that go bad in 20 years, whereas my simpson meter has zero e-caps and hence is 30 years old working fine.
Happy hunting!
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