PA troubles - HELP!

[ivanthetrble]

We have been having a problem with our PA that is making us pull our hair out. Here is the basic set up:

 

Mackie 16 channel board -> Stereo graphic Eq -> Feedback reducers -> Mackie i1400 amps (x2) -> 15' three way mains and 18" subs (x2).

 

The problem is an intermittent drop out of one or both of the mains while the subs continue to work. The Mackie amps have a built in crossover where if you are using two i1400 amps you can have one for the mains and one for the subs. We have taken the amp for the mains in and the tech says it is fine. We even changed that amp out for a new one and the problem remains. Sometime it is one channel that will go out, sometimes it is the other, sometimes both. For some reason turning the amp for the mains off and then back on seems to help at times. Really at a loss where in the signal chain we should be looking. Seems the amp is the likely cause but having the same problem with two different amps, one brand new, seems odd. Any thoughts would be greatly appreciated.

[picker13]

the only thing I can think of is a bad cable or connector. You might consider trying it without the feedback inhibitors. Is this only at high volume? They are not going into protect mode are they? Since you switched out the amps, they are likely not the problem. Can you borrow someone's active crossover and try that instead of the included one in the amps?

[ivanthetrble]

Our next step was going to be change out all the connectors. Good idea about taking the feedback reducers out out of the chain. Main problem is this is something that may or may not happen. When we get things set up before a gig we run a CD player thru the PA for background music and we never know which channel will be out or if everything will work. One time when it happened we switched the main cabs and the problem reversed so it seems like the speakers themselves are not the problem. Thanks!

[W. M. Hellinger]

One time when it happened we switched the main cabs and the problem reversed so it seems like the speakers themselves are not the problem. Thanks!

[ivanthetrble]

For example, one time the right main may be out but the left one was working (subs always work on both sides and are running mono by the way). We reversed the speaker cables on the amp and now the right main was working and the left wasn't. Seems to indicate that the speaker is not the problem. It is not isolated to just one channel of the power amp either. Sometimes one, sometimes the other and sometimes both. Sucks!

[gadgetx23]

You should be able to narrow it down by watching your signal LED's on each piece of equipment when it cuts out. (e.g. if the signal light on your amp is lighting up, then the problem is with the amp or connection to the speakers; if your eq doesn't even display a signal, then it's the EQ, the mixer, or a cable in between) Finding the first piece of a equipment that doesn't get a signal should help point to the problem.

[ivanthetrble]

Good thought! I can't remember off the top of my head what LEDs have and have not been on when things are working and if there were some that weren't lit when the problem occurs.

[DJ Swydez]

For example, one time the right main may be out but the left one was working (subs always work on both sides and are running mono by the way). We reversed the speaker cables on the amp and now the right main was working and the left wasn't. Seems to indicate that the speaker is not the problem. It is not isolated to just one channel of the power amp either. Sometimes one, sometimes the other and sometimes both. Sucks!

 

This sounds like a bad speaker cable or two. You may want to consider using the speaker cables you use for the subs to power the mains and see if that resolves your problems. If it does, you can attempt to fix the old ones or just go out and buy two new speaker cables.

 

Good Luck!

[W. M. Hellinger]

I believe those Mackie amps have a history of having something funky with internal wiring (ribbon cable?).

 

So... did you quiz the tech who checked out the amp as to exactly what he checked out, and did he know what he was looking for?

 

And... is this drop out thing happening at high volumes or basically all the time?

[Al Poulin]

Now, this is a longshot, but does this happen only at high volumes? With 2 1400s running full steam, with one being bridged... are they on the same circuit? Is it possible they're not getting enough juice, and this is the amp's way of coping, or would the breakers trip first? I'm far from being an expert on electricity but is this a possibility? I used to have a Peavey CS1000 that had one channel that would sometimes work and sometimes not. It almost seemed to depend on the venue. I'm just guessing here of course... Al

[Al Poulin]

 

I believe those Mackie amps have a history of having something funky with internal wiring (ribbon cable?). So... did you quiz the tech who checked out the amp as to exactly what he checked out, and did he know what he was looking for? And... is this drop out thing happening at high volumes or basically all the time?

 

 

 

I would have thought the ribbon cable too but he says he switched with a new amp and it did the same thing. My Mackie 1400 did something similar to me but just once. For a few minutes, one channel was much louder than the other, even though both gains were at the max position. I don't remember if I hit it, tapped it or rubbed it right, but it was okay after that and never did this again. Even after falling a few feet and slamming on my concrete basement floor. (there are now a few LEDs that stay lit all the time now though...) Al

[6Imzadi]

 

Here is the basic set up: Mackie 16 channel board -> Stereo graphic Eq -> Feedback reducers -> Mackie i1400 amps (x2) -> 15' three way mains and 18" subs (x2).

On a day where you have some time try this:

Remove the EQ and feedback reducers.

Then get a crossover, even if you have to borrow or rent one for a day.

DO NOT use the crossover in the amps.

Let it run in the basement or garage for a couple hours.

Watch the mixer for wierd lights.

Watch the crossover, if it has lights.

Watch the amp's lights.

The problem is an intermittent drop out of one or both of the mains while the subs continue to work.

When this happens, turn off the sub amp and unplug the subs. Does the symptoms continue? If it does, swap your left and right cables both at the amps and the speakers.

By this time, you should have an idea which lights are not acting right.

If the system does not act bad while testing, suspect the EQ or the feedback thing.

[W. M. Hellinger]

 

Now, this is a longshot, but does this happen only at high volumes? With 2 1400s running full steam, with one being bridged... are they on the same circuit? Is it possible they're not getting enough juice, and this is the amp's way of coping, or would the breakers trip first?

 

Al:

 

I don't know about Mackie 1400's, but my experience concerning browning out the AC supply: Amplifiers usually shut down and then cycle back on once the AC level drop below a certain threshold. IOW: The whole amp shuts down, not one side or the other. It's pretty much like pulling the power cord out of the socket and plugging it back in.

[agedhorse]

Doesn't sound like a power supply problem to me.

 

Which amp is running in bridge mode?

 

What exactly are the speakers you are using?

[ivanthetrble]

Doesn't sound like a power supply problem to me. Which amp is running in bridge mode? What exactly are the speakers you are using?

 

 

OK, a little more information. The Mackie amps are run with the amp for the mains in stereo and the one for the subs in bridge mono. The levels on the amp for the mains are turned up all the way and the level on the amp for the subs is about 75%. The main level on the board is only about 30-40% so I don't think the amps are being driven all that hard. Never see the LEDs on the amps ever approach clipping. Doesn't seem to be a full volume only problem since it usually is first noticed at fairly low volume with our CD player providing background music before we start. Speakers are EV 15" three way mains and the subs are EV 18" single speaker cabs. We have tried different cables and I think we have ruled that out as a problem. I'm still thinking it is something with the amp since turning the amp for the mains off and back on sometimes fixes the problem. We have wondered about temp as well. It seems to be more of a problem at start up and after things get warmed up it may work fine. Main flaw in that theory is it still happened when we swapped out the amp for the mains.

[boomerweps]

Are these older Mackies? Mackie had a run of bad ribbon cables that would intermittantly shut down a channel, like a cracked solder joint it would "jump" the signal once warm and at higher signal levels. My buddy had one of the earlier problem M1400s but got screwed on the fix because one side of the ribbon cable was soldered in place.

 

Boomerweps

[agedhorse]

Which model EV subs? There are 3 different drivers used in their products. If the driver is an EVM-18B or DL-18B you will want to be careful in bridge mode since those drivers are best powered by about 400 watts RMS each. If it's the EVX series driver you are ok.

 

Read carefully the instructions for gain control position on bridge mono mode. Each manufacturer is different depending on how they derive the inverting signal inside the amp. Some require that you use only one channel's input control and the other must be set to zero (fully down). Otherwise, I recall an anomoly of one channel latching up and the protect relay not closing (grounding the binding post output) but the amp will still work but only 1/2 of the bridge will be functional. This is one reason why I tend to discourage bridge mono mode unless you are really familiar with it's little quirks.

[Craig Vecchione]

The description really sounds a lot like there's a cold soder joint in the amp. If it the problem occurs but you're able to 'jump' it back to life by raising the gain, and it suddenly cuts in...it's almost certain to be a cold joint. Of course, finding it then becomes the issue.

 

I've read about guys like Mark H. firing up the soldering station, a nice big magnifying light, and his favorite music and settling in for many hours of resoldering a big mixing console. I'd imagine this to be similar if less time-intensive. All you need is some cash and a good tech who also agrees this is your amp's problem....

[ivanthetrble]

 

Are these older Mackies? Mackie had a run of bad ribbon cables that would intermittantly shut down a channel, like a cracked solder joint it would "jump" the signal once warm and at higher signal levels. My buddy had one of the earlier problem M1400s but got screwed on the fix because one side of the ribbon cable was soldered in place. Boomerweps

 

 

I think the amps are about 4-5 years old. A cracked solder joint has been high on our list of suspects but it is a pain to find. IIRC when the amp was taken in before they let it run for a couple of days and it behaved itself. The local shop wanted us to bring in the entire PA including cabs and set it up and go thru it. Good idea but a pain and expensive. I'm just the lowly bass player in our band. I gave our guitar player the link to this thread and hopefully he will add his $0.02 as well.

[ivanthetrble]

Humm, here is a thought. What if both subs keep working only because we are running them in bridge mono. I would think that only one side of the siginal has to get thru the main's amp to make both subs work in bridge mono mode. If it was a problem with the amp for the mains and we changed the sub amp to stereo as well then in theory when the problem occurs we should loose one side of the subs as well. Seems worth a try.

[6Imzadi]

Just pay Mark to come out to troubleshoot your system. Seems like he is not that far away.

[W. M. Hellinger]

 

The local shop wanted us to bring in the entire PA including cabs and set it up and go thru it. Good idea but a pain and expensive.

 

Well... "at the gig" is not the place to be trouble shooting... 'cause IME: that's more of a pain and could be more expensive.

 

I suggest taking the local shop up on their suggestion if you don't have the chops to DIYS.

 

If it were me... I'd set the rig up in the garage, pop the top off the amp and do a tap and wiggle test... a Sharpie works good for the tap and wiggle test. But... that's me. I know how to do that sort of thing without Liz resorting to CPR or calling 911.

[Craig Vecchione]

 

I IIRC when the amp was taken in before they let it run for a couple of days and it behaved itself. The local shop wanted us to bring in the entire PA including cabs and set it up and go thru it.

 

 

 

But leaving it running for days sounds like the last way to get it to act up. Sounds like it fails at 'cold' startup and then possibly kicks in and works. They would need to shut it off a lot, let it sit, maybe put in it a cold room, etc.

[6tring]

I would have thought the ribbon cable too but he says he switched with a new amp and it did the same thing. My Mackie 1400 did something similar to me but just once. For a few minutes, one channel was much louder than the other, even though both gains were at the max position. I don't remember if I hit it, tapped it or rubbed it right, but it was okay after that and never did this again. Even after falling a few feet and slamming on my concrete basement floor. (there are now a few LEDs that stay lit all the time now though...) Al

 

 

Hey Doc, remember there have been a few times when we would turn the amp off and on again and the volume was present but very low. Does not happen a lot but sounds like what Al is referring to.

 

A clarification to the make and model of the mains we are using. They are Celestion CR Series.

 

[W. M. Hellinger]

 

But leaving it running for days sounds like the last way to get it to act up. Sounds like it fails at 'cold' startup and then possibly kicks in and works. They would need to shut it off a lot, let it sit, maybe put in it a cold room, etc.

 

I'm kind-of thinking that selenoid delay switches are being flaky. It seems to me like an iffy ribbon cable (on what I believe was undersized PCB mounted pin assemblies on those amps) would do partial or real hit and miss dropouts... and would be somewhat insensitive to the relative "warmth" of the amp. Of-course, bench testing with (probably) a dummy load doesn't replecate live... where there's SPL shaking the amp.