Providing sound for a banquet at a reception hall this past weekend and I ran into some problems.
2 Yorkville U15 run off rmx2450 each channel
2 Yorkville UCS1 run off 2 rmx2450 in bridged mono
Mixer (Mackie SR24)
Effects rack (dbx 2231s, Yorkville unity processors, dbx compressors)
Snake (horizon 16x4)
-Please see attached file
-There was an outlet in the front of the stage and I plugged the two amps for the subs there.
-I plugged the mixer, effects rack, and poweramp for the mains into an outlet at the back of the stage
-So my tech was working on the tops while I worked on the subs. He routed it incorrectly, so Iíll explain the chain.
Tops : Main outs from mixer > Unity Processor (crossover) > Unity Bi-amp/high out > Eq in > Snake A Channel tail end > From snake box, ran a cable back to the poweramp > U15s
Subs: Sub Out from unity processor > Snake C channel > from head of snake, channel C into poweramp > UCS1s via parallel inputs
-I had the tech correct the chain for the tops because he didnít have to go through the snake because the poweramp was right there by the mixer/effects rack. Also had him bypass the EQ.
New Signal Chain
Tops: Main outs from mixer > Unity Processor > Unity Biamp out > Poweramp > U15s
Now, hereís where it gets funky. Had him power up everything. Mixer turned on, effects turned on, and as we are about to turn on the poweramps, we smell smoke and I have him shut everything off. At this point I had no idea what was going on because none of the poweramps were on. My first thought was that I was drawing too much power somewhere. I ran to unplug everything, and the first piece I went to get was the cable in channel C of the snake (which is for the subs). It was HOT, so I left it. I took off the plugs from the outlet.
It was total chaos. Channel A of the snake on the tail end lost its coating (which probably was what the smoke was) and the wire was glowing red (about a foot of cable). Now, if you were following along, Channel A was plugged into the EQ output. A cable on the box end was originally patched to the poweramp (the one in the back for the mains) but it was pulled out. So, Channel A on the box end had nothing connected to it.
I still cant explain what happened.
What caused Channel A to short and heat up like that? It was only connected to the output of the EQ. And it only melted from the jack to about a foot in.
Why was channel C hot? Matter of fact, I touched the entire snake cable and it was really warm. My guess is that the heat from channel A was heating all the cables? Or was it channel C that was causing the problem?
None of the poweramps were on. Mixer and effects were on for about 20-30 seconds.
So after everything cooled down, I used another outlet for the mixer, and used separate outlets on opposite walls for the poweramps. Not knowing how the circuit is split up, I took my best guess. Powered everything back on and nothing smoked. Channel C on the snake passed signal for the subs. Other channels on the snakes worked as well. Had a flawless night with speech and presentations, and two hours of pumping music at levels barely clipping.
What happened? Iím glad nothing serious, like fire, started. But something DID happen, and itís serious and I donít want it to happen again. Please input, and Iíll try to answer questions if I left things out.
I think you may well have a NEUTRAL/GROUND swap at AC outlets powering gear at one or the other end of the snake. If it were a HOT/GROUND swap, I believe the results would have been much more spectacular! The bad news is that these are hard to find ... the $10 tester shown in this thread won't find it because it only looks for 120 VAC on the proper prong and, since both N and G are bonded back at the main panel, it shows OK as long as they're both connected. However, it makes the full load current on the mis-wired outlet flow in the ground conductor (in this case the shield of the snake) which will likely just cause some serious overheating. If you're lucky, you can open up the outlets and check the colors of the wires connected: green (or bare) is safety ground, black is "hot" 120 V (to the narrower slot), and white is "neutral" (to the wider slot) are the correct connections. I've seen this several times but usually it just causes very serious hum issues. "Bootleg" grounds, where there's a connection between neutral and ground somewhere besides the main panel can cause serious hum problems but I doubt this much current in the snake because it puts a section of ground and neutral wiring effectively in parallel, sharing the load current.