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Author Topic: QSC PL380 issue/problem... troubleshooting..  (Read 803 times)

alan hamilton

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QSC PL380 issue/problem... troubleshooting..
« on: October 27, 2019, 10:54:48 pm »

EDIT: I'll leave this post here, but also see the third post in this thread because I was able to recreate the issue, and eliminate some variables that I was not able to do until I could recreate the issue. I still don't know what the problem might be though.

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I'm trying to troubleshoot a problem with one of our QSC PL380's. I want to make sure I'm not overlooking something.

It has had a problem with one channel twice in the past couple of months. So that it part of the problem... very hard to even replicate the issue.

The first time it happened we were using only channel 1 on the amp. 4 Ohm load. Amp powered on normally. But- No signal, no output. Confirmed signal was definitely getting to the rack...

There are bananas to NL4 connectors on the front panel. Moved the NL4 from channel 1 to channel 2, the amp acted like it had been repowered, clicks, protect circuit (both red lights) engage like it normally would on an off/on, and side two works. Switch back to channel 1 and now it works.

Suspected the NL4 cable so just replaced it for the show. No further issues. At the shop the cable tested fine, one connection was slightly loose though so tightened it and the cable still tested fine. Ran some signal to the amp, switching sides with inputs and outs, on and off, no problems.

Did several shows with no issue, then this Saturday, again was using only one channel, and exact same problem resurfaced. Tried turning the amp on/off/on and no difference. Always powered on normally but no signal on channel 1 and no output. Switched NL4 and input to channel two and channel 2 was fine... put them back into channel 1 and nothing. Switched NL4 back to channel 2 once or twice again (ch 2 always worked, channel 1 nope... until...),  lo and behold- Upon connecting the NL4 to channel 2 either the second or third time- Clicks, red protect lights for a moment (basically the same routine as a normal start up would be), ch2 still works... and switching back to channel 1 and now channel 1 is working fine too.

And continued to work fine. I tapped on the rear switches... no issues... Saw no issues with the banana to NL4 on the front panel of the rack.

As far as common denominators... different NL4 cable this time. Even different speakers.

I took it out of the rack and brought it home with me for some Sunday night homework... and of course it's working perfectly fine tonight. I can't recreate the problem.

But since the problem has happened twice...whatever it is... I need to solve this. I'm not sure there's enough information with this fault mode, and the rarity of the problem, that even if I send it to the factory they can chase this down unless there's some known issue/potential fault that this description triggers.

And I'm all ears if there's something external to the amp that could be a cause or needs checked.

Ideas?
« Last Edit: October 28, 2019, 02:42:59 am by alan hamilton »
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Kevin Maxwell

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #1 on: October 27, 2019, 11:13:29 pm »

Is it wired up to anything in the rack? Are you using the Binding posts? I would suspect a short on the binding posts or on the connectors that the binding posts are wired to, if it is wired to anything.. Look very carefully to see if there is a very thin strand of wire where it shouldn't be. Just a shot in the dark. 
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Bob Stone

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #2 on: October 28, 2019, 12:08:52 am »

Okay that post was incredibly hard to follow....if I understand right, your amp is going into protect on channel 1 sometimes? that's what all that rambling boils down to?

If so, Kevin's suggestion of a short somewhere makes the most sense to me as well.
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alan hamilton

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #3 on: October 28, 2019, 12:27:09 am »

It's hard to explain because it's weird and I'm trying to hit the points.
It's NOT going into protect and keeping a channel from working.

It turns on normally, which is the protect lights come on for a few secs, and then the amp is on. Twice now (once in August and once in Oct), channel 1 has not worked though. It's not in protect, it's just not showing signal and no output.

I can swap to channel 2 with the same I/O cables and channel 2 works fine.

Both times, once in August immediately and this weekend after a couple of times, when I swap the NL4 to the channel 2 output, the amp will act like it's been powered off and back on. It will go through its startup routine and then it has worked perfectly fine on both channels.

It is never staying in protect. If there was a short between the bananas to the NL4's on the rack's output panel then shouldn't that put the amp into protect and it stay there?
I've checked for that anyway and see nothing, unless there's something inside the female NL4. But we're still back to a short should put the amp into protect. And why would connecting the NL4 to channel two, clear the problem on channel 1? And why does it cause the amp to go thru it's startup routine?

Cutting to the chase... When the problem rears its head, channel one doesn't work. Nothing showing on the input and no output. Channel two will work fine though. The amp powers on normally.
After a time or two of switching the load to channel 2, the amp will appear to reset for some reason, go thru what then seems like its normal startup routine (the protect lights normally come on for a couple of secs for the normal start up), and THEN channel 1 and channel 2 will function normally.

I'm not sure how to explain it any better than that.

« Last Edit: October 28, 2019, 12:40:47 am by alan hamilton »
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alan hamilton

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #4 on: October 28, 2019, 02:40:28 am »

Ok, I can add a whole lot to this now. I found a way to recreate the problem. The problem is not anything to do with the I/O panel on the amp rack or any of the speaker wiring. All of that is out of the equation now. It's definitely something to do with the amp.

I can recreate the issue on the bench fairly consistently by turning on the amp with no load. Part of the time it will power on and the protect lights will come on for a second (as normal), but there's actually no sound of a relay. There will be no input, no output, and the fan is not spinning. Other times, there will be the sound of the relay, the fan will start, then spin up to a higher RPM and then quit. If I then connect a speaker to the channel 1 outs, nothing happens. But if I then connect that same speaker to the channel 2 outputs, the amp will appear to reset, the fan will spin up, the protect lights will come on for a second, and the relay will click (just as it would with a normal startup routine). And I then I can get sound from either channel.

Sometimes, powering up with a speaker connected only to channel 1, it will work normally. But not always. For example, it worked perfectly to begin with tonight for a few power cycles.

It doesn't appear to have any issues though if I power the amp up with a load on channel 2. That would be a common denominator between the 2 times it's acted up- In Aug and then this weekend, when the rack was powered up there was no load on channel 2. We were only using channel 1 and that was the only channel connected when things were powered up. The majority of the time we do use both channels and that would appear to hide the problem. So that would explain why we didn't have any known issues from the time it happened in August until this weekend (the only two times not using channel 2 so no load on channel two).

But of course, it shouldn't be doing this. It's indicating a fault somewhere.

Both times there was an identical amp rack connected the same way on the opposite side of the stage with absolutely no issue like this. In fact, this isn't even the only PL380 in the same rack, and it's not behaving this way either. And it's definitely the same amp that has had the problem each time because the racks are marked.
So it's not some quirky PL380 bug. The others aren't doing this.
« Last Edit: October 28, 2019, 07:49:25 am by alan hamilton »
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Chris Grimshaw

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #5 on: October 28, 2019, 03:38:01 am »

Since the amp appears to work normally when it is working, my bet would be an electrolytic cap on the protection board that's just about drifted out of spec.

I'd also check things like ribbon cables - reseat those, and a bit of contact cleaner wouldn't hurt either.

Chris
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Bob Stone

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #6 on: October 28, 2019, 04:52:58 pm »

Since the amp appears to work normally when it is working, my bet would be an electrolytic cap on the protection board that's just about drifted out of spec.

I'd also check things like ribbon cables - reseat those, and a bit of contact cleaner wouldn't hurt either.

Chris

Agreed, checking the ribbon cables that bring signal between the channels might help.

Also, the PL380 isn't that old of an amp...do you still have warranty coverage for it? If so, I'd really suggest going that route.
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alan hamilton

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #7 on: October 28, 2019, 06:28:36 pm »

Agreed, checking the ribbon cables that bring signal between the channels might help.

Also, the PL380 isn't that old of an amp...do you still have warranty coverage for it? If so, I'd really suggest going that route.

I think at this point all of them are past 3 years old.
I put in a call to QSC today and got a case number... awaiting an RMA via email.
I was mainly concerned with not being able to replicate the problem initially, that either I was missing something, or someone would recognize the problem to give me something to look for and/or point the service center in the right direction. Nothing worse than them turning it on and everything seeming to work properly each time. But I can now tell them how to replicate the problem, and by replicating it on the bench, that also confirms it's at the amp and nothing external in the I/O panel and wiring of the rack.

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Re: QSC PL380 issue/problem... troubleshooting..
« Reply #7 on: October 28, 2019, 06:28:36 pm »


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