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Author Topic: QSC PL224 issue  (Read 983 times)

Mike Caldwell

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QSC PL224 issue
« on: August 26, 2018, 05:47:08 pm »

This is not a low AC voltage issue, anyone ever had a QSC PL224 (basically a PLX in an all black chassis) get stuck in power up loop, power on, red protect LEDs come, relay clicks, starts over again, in all about a six second cycle.
Worked perfect one day, next day had this issue up on the initial power up.
Amp has the issue when not connected to anything.

I'm calling QSC, just wanted to see if anyone else has seen this.


I just wanted to add that out of the 26 QSC power amps I have This is only the second one to up and quit, the first was 10 or so years ago.
« Last Edit: August 26, 2018, 07:30:47 pm by Mike Caldwell »
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Tim McCulloch

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Re: QSC PL224 issue
« Reply #1 on: August 26, 2018, 06:15:57 pm »

It's detecting a fault during start up.  Why it's cycling depends on the fault and fault handling of the protection circuit design.
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Frank Koenig

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Re: QSC PL224 issue
« Reply #2 on: August 26, 2018, 08:06:19 pm »

This is not a low AC voltage issue, anyone ever had a QSC PL224 (basically a PLX in an all black chassis) get stuck in power up loop, power on, red protect LEDs come, relay clicks, starts over again, in all about a six second cycle.
Worked perfect one day, next day had this issue up on the initial power up.
Amp has the issue when not connected to anything.

I'm calling QSC, just wanted to see if anyone else has seen this.


I just wanted to add that out of the 26 QSC power amps I have This is only the second one to up and quit, the first was 10 or so years ago.

When a PLX detects an input fault it, apparently, opens the AC power relay. Just the other day I accidentally plugged a cord with phantom power on it into an input on an operating PLX1804. It went click and the fan stopped. I immediately pulled the cord out and the amp came back on. I later figured out why. Not your exact problem but evidence of how an input fault, and likely others, present. -F
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Tim McCulloch

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Re: QSC PL224 issue
« Reply #3 on: August 26, 2018, 08:32:25 pm »

When a PLX detects an input fault it, apparently, opens the AC power relay. Just the other day I accidentally plugged a cord with phantom power on it into an input on an operating PLX1804. It went click and the fan stopped. I immediately pulled the cord out and the amp came back on. I later figured out why. Not your exact problem but evidence of how an input fault, and likely others, present. -F

DC present at either the inputs or outputs will trigger the protection circuit in most amps built in the last 30 years.  QSC probably has more spots in the signal chain and PSU that are monitored for various fault conditions.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Frank Koenig

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Re: QSC PL224 issue
« Reply #4 on: August 26, 2018, 10:09:41 pm »

DC present at either the inputs or outputs will trigger the protection circuit in most amps built in the last 30 years.  QSC probably has more spots in the signal chain and PSU that are monitored for various fault conditions.

Good to know how much better things have gotten in the last 30-40 years -- which was about my hiatus from audio. When that thing went click I was sure I'd blown something up. RPBGs were dancing in my head. As it turns out, the speaker, amplifier, and USB audio interface were no worse for wear. In my defense this did not happen at a gig but rather "in the lab". Y-cords were involved... I shall retreat to the lounge.   :-\  --Frank
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Mike Caldwell

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Re: QSC PL224 issue
« Reply #5 on: August 27, 2018, 07:34:54 am »

DC present at either the inputs or outputs will trigger the protection circuit in most amps built in the last 30 years.  QSC probably has more spots in the signal chain and PSU that are monitored for various fault conditions.

Some years ago I had a one of my QSC PL's jump right into protect when I powered it on and stay there. The compressor feeding the amp had about 1.5 volts of DC on it's output.
Thank you QSC protection!

Pat Semeraro

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Re: QSC PL224 issue
« Reply #6 on: September 05, 2018, 10:47:27 pm »


I just wanted to add that out of the 26 QSC power amps I have This is only the second one to up and quit, the first was 10 or so years ago.

Its actually remarkable how reliable that platform has been.  QSC sells that same amp in several slightly rebranded cases.  The PLX and CX versions are the same as the PL versions with the exception that they cut the second power supply stage caps value in half.  (470uf in PLX/CX 1000uf in PL - doesn't sound like much but actually makes a pretty big difference in use.  QSC sells the 1000uf caps and its an easy upgrade if you can spare an hour.)

Your fault sounds like IGBT failure in the smps.  A lot of protections are built into that platform.  Some (like the 14 amp continuous current draw limit) limits total output power, but protects the amp from silly mistakes and catastrophic faults. 
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Mike Caldwell

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Re: QSC PL224 issue
« Reply #7 on: September 06, 2018, 08:57:31 am »

Its actually remarkable how reliable that platform has been.  QSC sells that same amp in several slightly rebranded cases.  The PLX and CX versions are the same as the PL versions with the exception that they cut the second power supply stage caps value in half.  (470uf in PLX/CX 1000uf in PL - doesn't sound like much but actually makes a pretty big difference in use.  QSC sells the 1000uf caps and its an easy upgrade if you can spare an hour.)


Your fault sounds like IGBT failure in the smps.  A lot of protections are built into that platform.  Some (like the 14 amp continuous current draw limit) limits total output power, but protects the amp from silly mistakes and catastrophic faults.


Actually it turned out to be shorted outputs in channel 2. It happened while the amp just setting idle shortly after power up.

Debating repairing a 17 old amp or replacing with a PLX2502.

I always heard it was the CX series that had the smaller power supply cap values than the comparable models in the PLX series.

Pat Semeraro

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Re: QSC PL224 issue
« Reply #8 on: September 06, 2018, 10:34:21 pm »


I always heard it was the CX series that had the smaller power supply cap values than the comparable models in the PLX series.

The PLX has the castrated power supply and no monitoring, CX adds monitoring circuits and PL has the full power supply. 

Also worth noting that CX502/PLX1602 share main board and transformers with CX1102/PLX3402 but the high voltage rail has no components.

The PLX 3402/CX1102 has 82v low rail and 134v high rail.  The PLX1602/CX502 82v lower rail only.  If the PLX3402/CX1102 isn't quite powerful enough, then bridging a pair of the PLX1602/CX502 gives you a stereo amp with 165v rails plus double the cooling capacity.    That may not sound like much but if you're using those amps to drive subs, a pair of the smaller bridged amps in practice is quite a bit more powerful with more output current down low than just one of the PLX3402/CX1102/PL236. 
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Re: QSC PL224 issue
« Reply #8 on: September 06, 2018, 10:34:21 pm »


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