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Author Topic: Revisit JBL PRX615M scratchy until warmed up  (Read 697 times)

Jim Layton

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Revisit JBL PRX615M scratchy until warmed up
« on: February 27, 2017, 08:48:20 am »

We discussed trouble-shooting a while back. Thought I had it sorted out but no.

I have four PRX 615Ms. One of them (labeled to make sure it is the same one every time), will often randomly emit a quiet "scratchy" sound...like radio interference. It will do this with no XLR connection, just power, at several venues. The speaker is set for "monitor", which I think means there is an automatic gate. I tried that setting while using it as a mains speaker for a speech. I could hear the audio get cut off as the voice trailed off. Switched to mains and it went away.

Once the monitor has been on a while it seems to go away. When the event gets going I don't have time to go up and listen. I listen from FOH through headphones and don't hear anything.

The noise is pretty "gentle" and not harsh. I "think" the noise level is completely independent of the gain level on the amp but I am not sure. I'll have to try it next time.

I can pull the amp off and take a look. Any ideas?

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Will Knight

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #1 on: February 27, 2017, 09:01:12 am »

We discussed trouble-shooting a while back. Thought I had it sorted out but no.

I have four PRX 615Ms. One of them (labeled to make sure it is the same one every time), will often randomly emit a quiet "scratchy" sound...like radio interference. It will do this with no XLR connection, just power, at several venues. The speaker is set for "monitor", which I think means there is an automatic gate. I tried that setting while using it as a mains speaker for a speech. I could hear the audio get cut off as the voice trailed off. Switched to mains and it went away.

Once the monitor has been on a while it seems to go away. When the event gets going I don't have time to go up and listen. I listen from FOH through headphones and don't hear anything.

The noise is pretty "gentle" and not harsh. I "think" the noise level is completely independent of the gain level on the amp but I am not sure. I'll have to try it next time.

I can pull the amp off and take a look. Any ideas?

With the amp off, check the wiring between the amp and the speakers.  Sometimes depending on extended usage, the speaker connection may loosen just a tad due to the frequent cycles of heating and cooling.  Would also check the actual Monitor switch for any deterioration/oxidation due to temp changes over time.  Does the scratching also occur when / if you turn the gain knob up or down? If so, consider a VERY light spray of DeOxit. I have 612's where one exhibited a similar behavior and a bit of cleaning (air+DeOxit) did the trick. 

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Jim Layton

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #2 on: February 27, 2017, 02:47:28 pm »

With the amp off, check the wiring between the amp and the speakers.  Sometimes depending on extended usage, the speaker connection may loosen just a tad due to the frequent cycles of heating and cooling.  Would also check the actual Monitor switch for any deterioration/oxidation due to temp changes over time.  Does the scratching also occur when / if you turn the gain knob up or down? If so, consider a VERY light spray of DeOxit. I have 612's where one exhibited a similar behavior and a bit of cleaning (air+DeOxit) did the trick.

I will do this. Thanks!
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Bob Kidd

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #3 on: February 27, 2017, 04:18:07 pm »

We discussed trouble-shooting a while back. Thought I had it sorted out but no.

I have four PRX 615Ms. One of them (labeled to make sure it is the same one every time), will often randomly emit a quiet "scratchy" sound...like radio interference. It will do this with no XLR connection, just power, at several venues. The speaker is set for "monitor", which I think means there is an automatic gate. I tried that setting while using it as a mains speaker for a speech. I could hear the audio get cut off as the voice trailed off. Switched to mains and it went away.

Once the monitor has been on a while it seems to go away. When the event gets going I don't have time to go up and listen. I listen from FOH through headphones and don't hear anything.

The noise is pretty "gentle" and not harsh. I "think" the noise level is completely independent of the gain level on the amp but I am not sure. I'll have to try it next time.

I can pull the amp off and take a look. Any ideas?

Definitely sounds like a dirty pot. I have had it occur on some of my speakers. Good cleaning should resolve it.
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Matt Shep

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #4 on: September 21, 2017, 01:48:09 am »

We discussed trouble-shooting a while back. Thought I had it sorted out but no.

I have four PRX 615Ms. One of them (labeled to make sure it is the same one every time), will often randomly emit a quiet "scratchy" sound...like radio interference. It will do this with no XLR connection, just power, at several venues. The speaker is set for "monitor", which I think means there is an automatic gate. I tried that setting while using it as a mains speaker for a speech. I could hear the audio get cut off as the voice trailed off. Switched to mains and it went away.

Once the monitor has been on a while it seems to go away. When the event gets going I don't have time to go up and listen. I listen from FOH through headphones and don't hear anything.

The noise is pretty "gentle" and not harsh. I "think" the noise level is completely independent of the gain level on the amp but I am not sure. I'll have to try it next time.

I can pull the amp off and take a look. Any ideas?

sorry to bring up this topic from the start of the year but did you ever get this figured out.. one of my 615m's has the exact same problem as yours, gain doesn't affect it and once it has been on for a few of minutes and warmed up the noise goes away.

if not has anyone else have any solutions?

TIA
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hiep nguyen

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #5 on: September 21, 2017, 12:47:51 pm »

.. one of my 615m's has the exact same problem as yours, gain doesn't affect it and once it has been on for a few of minutes and warmed up the noise goes away.

TIA

Matt, if it turn out to be a "heat- sensitive" problem. I would use as is.

In my experience in general "heat- sensitive" problem is one of the worst to diagnose to the component level. PLL circuitry is another.

It it does not bother you and it's out of warranty I would not worry about it.

Would the unit do with no input (unplug), no gate compressor .etc...by itself? at home?
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Matt Shep

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #6 on: September 27, 2017, 01:47:34 am »

Matt, if it turn out to be a "heat- sensitive" problem. I would use as is.

In my experience in general "heat- sensitive" problem is one of the worst to diagnose to the component level. PLL circuitry is another.

It it does not bother you and it's out of warranty I would not worry about it.

Would the unit do with no input (unplug), no gate compressor .etc...by itself? at home?

it did actually bother me as setting up at events/venues it would be quiet loud over the silence (as there was no music yet) but it seems as if the noise only lasts a couple of seconds now compared to what used to be minutes, so for now i will take your advice and just leave it as is i think until next winter when the weather cools down (i assume that this is what has shortened the time from minutes)

and yes it would be exactly the same regardless of input or gain.. out at an event or at home didn't make a difference either. no matter what, just with a power cord it would do it. i have 3 other of the exact same speaker, all same condition and age that have never had this issue.

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Tim McCulloch

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #7 on: September 27, 2017, 02:32:18 am »

it did actually bother me as setting up at events/venues it would be quiet loud over the silence (as there was no music yet) but it seems as if the noise only lasts a couple of seconds now compared to what used to be minutes, so for now i will take your advice and just leave it as is i think until next winter when the weather cools down (i assume that this is what has shortened the time from minutes)

and yes it would be exactly the same regardless of input or gain.. out at an event or at home didn't make a difference either. no matter what, just with a power cord it would do it. i have 3 other of the exact same speaker, all same condition and age that have never had this issue.

Back when we could spray chlorofluorocarbons at will it was fairly easy to find *heat* related thermal problems once things warmed up - we'd just blast small areas/components with Freon and see what changed...  You need the opposite, a concentrated blast of "hot", maybe?

Maybe one of our bench repair guys (Moby?) can tell us, but if this sound is like a scratchy potentiometer that eventually fades away... We had an SRM450 years ago that did that and it was "failing capacitors" but where in the signal chain, I don't remember.
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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

Jim Layton

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Re: Revisit JBL PRX615M scratchy until warmed up
« Reply #8 on: October 17, 2017, 09:23:49 am »

sorry to bring up this topic from the start of the year but did you ever get this figured out.. one of my 615m's has the exact same problem as yours, gain doesn't affect it and once it has been on for a few of minutes and warmed up the noise goes away.

if not has anyone else have any solutions?

TIA

I got busy and never opened up the box or sprayed the pot. I have not heard the noise for the last few months. BUT...it was summer and the boxes are in a trailer...so they were "warm" to begin with. It is cooling down here in PA...so the next couple of weeks may tell a different story.
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