Weirdest wiring issue ever: level changes

[Marc Alan Goodman]

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

-marc alan goodman

[TomVan]

Hey Marc,
Always love a challenge but glad its you and not me...

Have you tried plugging a mic cord directly into your console/mic pres and so forth and see if it happens that way?

Also your cables through the wall. Any chance they got penetrated by a nail/screw?
Peace
Tom

[Marc Alan Goodman]

Hey Tom,

Thanks for taking a look. I've tried plugging a mic in directly. However the problem is so intermittent that it's no proof. It only happens on some channels at some times, usually minutes to hours apart from each other.

As far as the nail question it's unlikely. I ran the wire myself and everything was screwed in before the cabling was installed. There's a lot of solder points to consider, but that still doesn't explain why it slowly ramps up and down.

Willing to try anything. let me know.

-marc alan

[lilith_envy]

Could you post an audio file of it happening.
Also start documenting the signal flow each time it happens, there has to be so common element between the faults.

It sounds like a capacitor that on the way out, a dodgy Op amp and dry joint.

My next thought is fluctuating power??

[Marc Alan Goodman]

No, the sound remains good. Usually I just take the section where the level is higher and drop it by about 10db in pro tools. The parts always wind up sounding fine.

My first thought was power but that doesn't seem logical to me. First off it doesn't happen across the board. Second off when you starve a power supply it just drop the level, it starts to sound like crap.

Someone suggested a belt dsub pin. I still don't get it since it seems to slowly ramp up and down over 5-10 second intervals instead of just jumping, but I'm gonna test the resistance of all my cabling tomorrow.

I'll also try to find some sound clips to post while I'm at the studio, maybe take some screen shots.

Thanks for all your help!

[Marc Alan Goodman]

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!

[John Sayers]

wow - amazing Marc - glad you got it sorted

[Aaronw]

I hate patchbay gremlins! They can be absolutely aggravating!

My first guess though from your description, you need to recap some pre's or components that the issue's are happening on. You might check them anyway. Do they look swollen at all?