Possible cable loom fault? URGENT!!!

[c.gymer]

Hey all.

Last night when I powered up our Lab Gruppen/Turbosound rig and played a cd to check the system everything seemed fine. After 15 minutes or so the main L/R FOH died. Nothing at all. I checked the amp and it was still receiving signal (this is a Phonic amp btw that we've been meaning to replace with the spare Gruppen amp for a while). I checked the monitor feeds and they were all fine.

As the Phonic has been known to crap out on occasion I thought I would use this as an opportunity to put in the new Gruppen amp. So after replacing the amp and firing up the rig again........nothing. The amp was showing an input signal but there was no output.

Some head scratching and conversations with the head engineer later and it was decided that the most likely cause was that the main L/R cables from the amp had a fault. As a stop gap we decided to re-patch the monitor feeds and main L/R feeds so that the FOH speakers were fed via 2 of the monitor cables, and the 4 wedges would be run as 2 independent feeds, thus bypassing the affected cable.

After finishing the re-patch and triple checking my connections (I am VERY anal when it comes to this) I powered up the rig and.............nothing. No FOH. No monitors. Nothing.

I took a cable I knew to be working and a wedge I knew to be working, and at the wall box that feeds the FOH and wedges I went through each one to check for signal. Some had nothing at all, some had a tiny bit of hiss, and 1 had a tiny amount of signal (and this was only audible with the whole system running in the red).

So what started as one simple problem has escalated into a complete and endemic cable failure. How can this of happened? The cable loom (16 core) runs visible except for a couple of feet where it disappears into what I think is an old unused ventilation duct. I guess rats could of chewed the cable, but if that is not the case surely the whole loom can't of gone? What could cause this?

I could really do with some suggestions ASAP guys.

Thanks in advance.

[soulviasound]

How did you power the entire system ? FOH console has same GND with FOH PA ? I don't think it's a cable issue (almost never isn't cable issue).

if "cables damaged"
then "no or little signal in power amps"
else "cables are not damaged"

TEST:
test your connections betwen power amp and main speakers.
test if your final stage amp from your power amps are not damaged.
test if your speakers are not damaged.

cheers

[XLR2XLR]

Has the poweramp got volume controls? (Some do) check them.

Could it be faulty anti thump relays?

Are you 100pct sure the outputs of them amp are correctly wired (not shorted etc) and not creating too low an impedance on the amps outputs thus enabling output protection of some kind?

Try playing a low level tone -40 or so into the amp and see what gives?

[XLR2XLR]

My guess is that something (a number of things?) have been disconnected by someone which you are assuming are connected (think about the things you do not think about as being disconnected, don't assume)

good luck, you are probably hearing system crosstalk.

[Thomas W. Bethe]

In a situation like this you need to know what is working and what is not.

Unplug the main speakers. Put a 1.5 to 9 volt battery across the speaker terminals and you should get a thump. If you don't then your speakers are toast. Then start working backwards but putting a tone or a CD player directly into the amplifier that is feeding the speakers and see if that works. (If you use a CD player make sure you come out of the Phones jack with the volume control off and then gradually bring it up so you don't blow the speakers with a loud sound) if that is working then plug the tone oscillator/ CD player into the cable that feeds the power amps from the mixer. If that works then feed the tone oscillator/CD player into the console. You have to know where the signal is getting lost. This is just basic troubleshooting 101.

I once worked on a church's sound system that was having all kinds of problems similar to what you are describing. It turned out that the cables all went under the floor boards from the front of the church to the rear and the only access to them was to go into the meeting hall below and lift up the ceiling tile. The cable that was used for the speaker lines was made from some type of plastic that used peanuts as part of the plastic matrix and the mice in the church found that they REALLY REALLY liked the speaker cable and had eaten parts of it and it shorted together. We had to replace all the speaker cables with something else that was not so tasty.

In another case the system was plagued by all types of problems and I was called in to look at the system and see where the problems were. I did what I am suggesting that you do and worked my way back from the speakers, checked the amplifiers, checked the electronic crossovers, checked the lines running to the house sound rack and checked the console. I found lots of small problems which I corrected as I went. I got out the information that the sound contractor had left and noticed that a lot of things had been changed and that the EQs and the settings on the amplifiers had all been changed from what the contractor had setup and documented. I asked the pastor why all the things were changed and he said he did not know but would ask the AV crew on Sunday. I reset everything to the sound contractors specs and the system sounded great with no feedback and no bass boost. The pastor said thanks and I left. The beginning of the week I get another phone call from the pastor saying that the system was again having problems and could I take another look at it. I came over and found that all the settings on everything had been changed and showed the pastor. I came the next Sunday and talked to the AV crew and they said that nothing was changed and all they do is setup the microphones and turn the system on and run it. A couple of weeks passed and the pastor calls me and tells me that he has found the "problem". He came in late a night to get some notes for a funeral the next day and was surprised to hear the system going full blast in the sanctuary with all the lights on. He went in and the place was JAMMING and there was the custodian buffing the floor. When asked what he was doing the custodian fessed up. He like to have music on when he was working, he did not like the way the system sounded since, in his words, "it was not kicking enough" so he changed all the settings to make it sound like he wanted it to sound. He fancied himself an audio engineer and reset the system to his own "standards". The pastor had me come back over reset the system, told the custodian NOT to touch the system and in fact we put a key lock on the amplifier cabinet that only the pastor had a key to so it could not even be turned on except on Sunday mornings and so far everything is working well.

The moral of this is that you may not know who or what is changing the systems behavior so going from the known to the unknown is the ONLY way to trouble shoot a problematic system,

Best of luck!