Weirdest wiring issue ever: level changes

[Marcocet]

Okay, I've held off on posting this for a while because the problem has been intermittent over the last year or so and it seems completely impossible to me so I'd convinced myself it was all in my head. However I ran into some seriously annoying proof this week and it's time to get it worked out.

In short: Sometimes while tracking the levels will slowly fluctuate up or down about 10db. It will suddenly ramp up at a random time, and then at some point later suddenly it ramps back down. Sometimes it stops happening. Sometimes it doesn't. It will only happen to one or two inputs at a time, not all the way across the board. For information's sake I'm running PT HD3 on a power mac through Apogee AD16X and Rosetta 800 converters with a Antelope OCX clock. It has continued to happen across multiple clock sources.

Here's where things get weird. If I mult the signal to multiple tracks it will still come up across both tracks, even across different converters. That led me to believe it must be happening before the conversion. However it happens with random microphones (both my tube U47 and sennheiser e602 dynamic were doing it last week). And it happens across any of my pres (Avedis in a Purple rack, Sage SE-1s, AND the pres in my 1608 console via their direct outs).

To make things even weirder it happened while I was tracking the other day with the U47 through the Avedis into a Pultec EQP1-A. The Pultec has a very distinct sound to it when the input is being clipped, and when the signal would ramp up this would happen.

SO: Somewhere between the microphone and the mic pre the signal is randomly ramping up and down. The only things between them are the microphone cable, the cable running through the wall, and the patchbay. The patchbay is a redco branded dsub bay. The mic lines, pres, and converters are all on separate bays. There's no other issues (not crackling, no sudden cuts, nothing like that). Does ANYONE have ANY idea what the hell could be going on? I really am starting to think there are ghosts in the cabling.

Thanks so much.

[FastlanePro]

Sounds like you *could* have some cable leakage somewhere. This would mean you have some sort of high-ish resistance connectivity between lines in a cable somewhere. could be mic cable - could be interconnect.

If you have a simple cable tester - it won't likely show up. you need to be checking resistance between pins of your cable at a high R range with a multimeter. The resistance should read either 0 or really high (i.e. Megaohms). If it's somewhere in the middle (500 - 2000 ohms), it may be leakage somewhere.

Of course - this doesn't mean it has to be in the cable. It could be something in a piece of gear too (i.e. bad solder joint or something). I just know the cable leakage thing can be "intermittent" and also dependant on signal/voltage levels.

Hope that helps. And I hope I'm not leading you down the wrong path.

Cheers,
Lane

[Thumper]

Is it a slow volume change, or an instant change?

If it's instant, the first thing that comes to mind is an intermittent connection on one leg of a balanced cable. This will cause a 6 db change in signal.

Go grab a cable tester. I'd recommend the sniffer/sender from Rat Sound. $45 I think, and it will make you feel a lot better about life knowing that all of your wiring is correct, and there aren't any polarity reversed signal paths in your room.

[dubrichie]

fluctuations in your AC power?

[Marcocet]

it's a slow ramp up and down, over usually about 5-10 seconds. No sudden jumps ever.

My first thought was power but that's just not how power works. If the power supplies weren't getting enough juice it wouldn't just drop the level but keep a clear signal.

The fact that it's a slow ramp is what's really throwing me. I'm gonna have to test out all the cabling of course.... Thanks for the tips on measuring the resistance. I'll get to it tomorrow.

[Thumper]

I would use a signal generator, and run a sine wave from your mic pockets straight to a converter input. See if there are any level differences between mic lines. If one or more lines show up 10db quieter, then there is an issue with those lines (duh I guess...)

It's far more likely that you're loosing 10db and it randomly "fixes itself" than getting a 10db bump in gain somewhere.

You sure you don't have an assistant that fecking with you?

[Sean240]

Quick question - do you find you have to crank the gain on your pre's a little higher than normal? By that I mean, if you were to plug the same mic into the same pre but with a short mic cable bypassing your permanent cable plant, would the pre gain be way too hot? That simple test would indicate a high resistance joint in your cable system.

Given where you have determined it is happening, it sounds more like a high resistance joint than a mysterious voltage being induced randomly.

Have you ever seen an incandescent light bulb as it is dying suddenly come back on for a brief period? It too will start to glow slowly (ramping up) and then burn bright for a while, then dim slowly again...

What happens in that case, and may be similar to your problem, is that a voltage is arcing across a high resistance joint (in your case a short, a hairline break, broken multi-stranded cable...). Over time, that HRJ heats up, and begins to conduct better. This can be the ionization of the air gap between the break, or the metal expanding and closing the gap, or even the same condition removing a short that may be partially grounding your signal? Eventually the heat once again causes the joint to fatigue and go back to it's higher resistance state.

I've seen even more bizarre things happen due to the effect humidity can have on nylon in carpets, bad earthing (grounding) of building power systems causing ground voltages to rise, highly conductive rodents and insects, heavy machinery operating nearby causing vibrations...

But +1 for the cable test. Rule no 1 in solving electrical problems - first look at things that move. In your case, non fixed wiring, patchbays that get fatigued from years of insertions and removals, patch leads.

[Kingtone]

id actually wager it is possibly your apogee converters.
i have a friend who was having similar weird issues with his wiring. when he explained it to me i suggested he check out the converters. he did, and his apogee was having issues.

when it happens.. take note what converter channels it is happening on. you may be able to narrow it down to a few channels. either way... this is just another option.

[kafka]

Originally Posted by Marcocet

it's a slow ramp up and down, over usually about 5-10 seconds. No sudden jumps ever.

My first thought was power but that's just not how power works. If the power supplies weren't getting enough juice it wouldn't just drop the level but keep a clear signal.

The fact that it's a slow ramp is what's really throwing me. I'm gonna have to test out all the cabling of course.... Thanks for the tips on measuring the resistance. I'll get to it tomorrow.

I still think it's power. It's the one thing that all of the equipment that is experiencing the issue has in common. What is your facility like? How old is the building? What kind of businesses are your neighbors?

[DCtoDaylight]

Have you ever been able to capture the phenomenon in a recording and give it a close listen? It's conceivable that the"boosted" part could have some RF or other external signal sneaking in that might be audible (which could be happening, as Fastlane notes, because of leakage somewhere).

Does it ever happen on line-level inputs?

You've gotten good advice about troubleshooting; I'd just add that being methodical is the ticket. Start with nothing going into the converter and add things one at a time.

Good luck!

[Marcocet]

Alright, the problem is FINALLY solved. Check this out

So I tracked it down to the patchbay that has my mic lines coming up on it. Though on about two dozen previous tests the problem had not happened when using the lines in the control room all the sudden it did once. That was all I needed: definitely the patchbay.

I called Chris at Redco up and here's what happened. Apparently in their first run of dsub bays a few years ago a very small number of them had a problem where a few of the assembly screws were too large (by a fraction of a mm). As the patchbay heated up and cooled down (which happened a lot seeing how they're next to my apogee converters) the screw would gap and then touch the circuit board for the ground switching, shorting something out.

Chris was amazing as usual and overnighted me a new bay, as well as sending a shipping label to send the old one back. Obviously since the problem is intermittent there's no way to be ABSOLUTELY sure yet, but if it's not this I kinda give up.

Thanks to everyone who gave suggestions, and thanks to Chris and Redco for helping me out so fast. I only wish I'd called sooner!

[teo]

That's something I would have never thought of...not in a thousand years....

[KRStudio]

Redco just got a new customer! After hearing the way they treated this situation I'm all in. Good business gets my business every time.
Jeff