Film-Tech Forum ARCHIVE: QSC DCA2422 repair issue

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Posted by Frank Cox (Member # 6258) on 06-12-2019, 02:53 PM:

I mentioned here a while back that one of my amps had quit, so I bought a new one to replace it and sent the one that quit in for repair.

For a couple of years before it quit completely, that amp had the left side clip light stay on at about half power. When first powered on, all of the lights on the front panel went on and off again as they should, but the left channel clip light, instead of turning off completely, remained on at about half of full brightness.

So when I got the apparently repaired amp back the first thing I did was take it out of the box and plug it in and turn it on. And it did exactly the same thing as it did before I sent it in -- the left clip light remained on at about half of full brightness.

Without testing it any further I just put it back in the box and sent it back again right away since it was obviously not completely fixed. I was told that they hadn't seen the clip light on when they had it in their shop, which I find difficult to understand.

Now I've just received another email stating that they still don't see that clip light on.

What could possibly be happening here? I can't imagine that it somehow fixes itself on the way there and gets un-fixed again when it's shipped back to me, and that light is right on the front panel where it's not difficult to see. I can't see any reason for anyone to be lying to me about seeing that light on (or not) so could there be some difference in something else between my projection room and their workshop?

Again, the light stays on at about half-brightness when it's plugged in and turned on, with nothing else connected to it at all other than the power cord back to the wall outlet.

Posted by Steve Guttag (Member # 268) on 06-12-2019, 06:17 PM:

Check your load on the bad channel. If it is low, it could do that.

Posted by Sam D. Chavez (Member # 1841) on 06-12-2019, 06:22 PM:

I would switch the speaker leads between left and right and see if the problem follows.

Posted by Frank Cox (Member # 6258) on 06-12-2019, 07:08 PM:

Ahem.

quote: Frank Cox
the light stays on at about half-brightness when it's plugged in and turned on, with nothing else connected to it at all other than the power cord back to the wall outlet.

There were no speakers or any other kind of load connected at all when I tested it upon its return from the repair shop.

The new amp works fine plugged in exactly where and how I removed the old one from when it quit. And the clip light is not on.

Posted by Sam D. Chavez (Member # 1841) on 06-12-2019, 10:54 PM:

I was following Guttag's logic. Sorry, didn't read back to the start.

Posted by Marcel Birgelen (Member # 6801) on 06-13-2019, 06:17 AM:

I guess the repair shop which is handling the case is just a bit incompetent or it's just a case of bad luck. I call it the dentist phenomenon. You know, when you've got this toothache that's slowly killing you, but once in the dentist's chair, it's magically gone and you're wondering why you're there again, together with all his/her torture instruments.

But... To be sure, there is nothing in your power feed that's causing it (although, whatever it is, seemingly doesn't affect any of your other units...), you can still plug it into a UPS, if you have one lying around. Make sure the batteries are full and disconnect it from the main grid, so you're sure it's not just feeding you the grid in bypass, but it is creating a feed of its own. If the problem still prevails, it's not being caused by your grid either.

Maybe it helps if you record a video of your problem and send them a link to it...

Posted by Frank Cox (Member # 6258) on 06-13-2019, 11:37 AM:

I guess that at this point all I can do is wait for the to either find the problem or not, and then send it back to me. I'll plug it in again when I get it back and see what happens. And if the clip light comes on I'll take a picture of it and send it to them.

Posted by Paul Finn (Member # 10726) on 06-14-2019, 09:18 AM:

Another possibility that I have seen before with similar hardware: That channel or a stage in part of that channel in the amplifier may be oscillating. It may even be at a frequency that is inaudible to human hearing. If the condition still exists when you get the amplifier back, turn the level controls to 00. Also connect a speaker/load to the troubled channel output terminals and see if either of these removes the partial clip indication issue.

Paul Finn

Posted by Frank Cox (Member # 6258) on 06-14-2019, 11:17 AM:

I just got an email stating that they have now replaced a couple of transistors and this appears to have resolved the issue.

So hopefully they're right. I'll find out when it gets back here.

The only way that I can think of to test it is to wire it back into my sound rack in place of one of the amps that is currently in use and then run a sound test dcp. Is there an easier way to check it or is that my only option?

Posted by Mark Gulbrandsen (Member # 72) on 06-18-2019, 11:47 AM:

DCA Amplifiers are incredibly reliable and well designed. I have had so few failures out of hundreds of them that I honestly can't remember if it was 3 or 4 failures, one had dirty input pots and was fixed in the booth. I have also seen the heat sinks literally packed with dust bunnies to the point where the heat sinks are totally plugges up and there is basically no air flow. Yet the amplifiers keep on working.

Mark

Posted by Scott Norwood (Member # 30) on 06-18-2019, 01:23 PM:

quote: Frank Cox
The only way that I can think of to test it is to wire it back into my sound rack in place of one of the amps that is currently in use and then run a sound test dcp. Is there an easier way to check it or is that my only option?

The quick-and-dirty test would be to get yourself a CD player or ipod or anything that plays music and one or more loudspeakers (something from a home stereo or boom box would be fine) and test it on the bench. There is nothing magical about a D-cinema server and processor as playback devices, and it's probably better to blow a $50 speaker than your actual cinema speakers if anything is bad (unlikely, but you never know). If it passes that test, then you might consider putting it back in your sound rack to test it more thoroughly.