Topic: Anyone ever seen this? Wierd slow recording level fluctuation

[landshark]

Hi all -

I was taping The English Beat at a local venue here in Boston, and my setup exhibited some very strange behavior.  I set levels at the beginning and all looked fine.  About 2/3 of the way through, I went back to check levels, and now the right channel was running significantly lower than the left, like 20db lower.  I thought, "Oh crap" and increased the right channel gain on the preamp to being the levels back up to equal to the left channel.  When I got home and reviewed the take, I found that after I did that increase in gain adjustment, about 10-15 mins later, the recording level on the right channel began to rise, eventually getting to the point of clipping.  Ugh. 

So, over a 2hr15min period, my right channel started out ok, then after about 30-40mins lost about 30db of amplification, then after another hour, regained that 30db.  Both the fall off and rise in recording level were pretty smooth over at least a 20 min period.

The slow nature of the rise and fall makes me think of some sort of screwiness going on with a capacitor charging and discharging, but I have no idea where in the signal path such activity could have occurred. 

The rig:

2x AKG 463's, the caps were bought new from B&H, the bodies bought used (and arrived broken, grr) but sent to AKG / LAND Audio for repair ($200, grrr).

2x 10db barrel attenuators

1x Shure FP24 (rebranded MixPre, transformer-based pre) bought used on Ebay, in great cosmetic condition, told by seller had seen low use

2x 10db barrel attenuators

1x Korg MR1, bought new, set to 0db of gain, line input

Both the Korg and the Shure were powered by a Tekkeon 3400, running 12v to the Shure and 5v to the Korg.

Anyone ever run into a similar problem?  Or have any ideas about where in the signal path I'm running into problems?  Any ideas welcome!!!

Mike

[landshark]

Well, I sent the mic bodies and caps to Richard Land for testing.  He must have got them this morning, and called me about 11am with the diagnosis.  Kudos to Richard for an awesome turnaround time!!!!

Keep in mind I bought both ck63 caps brand new from B&H...

Cap #1, the likely source of the problem, Richard said the mylar on the diaphrahm had pulled back a little from the edges, so his hypothesis was they had some issue with moisture, causing the falloff in volume, and then dried out, causing the return of the volume.  Guess I'm gonna have to stop using my mics to stir my cocktails <grin>....

Cap #2, he said had a lot of distortion, looked like a quality control issue, but needed fixin'.  Grrr AKG....

Anyway, I was able to get B&H to fax Richard a copy of my receipt (took like 5 mins, was SO much easier than I expected!), so he's able to complete the warranty service work, and he said he'd get them done today so he could 3day them to me (AKG only covers 3day under warranty) for arrival Thursday.  This was critical to me since I need them to tape Matt Nathanson at The Roxy here in Boston on Saturday.  I'm bummed about what seems to be questionable quality control on the caps, but heartily impressed by the turnaround time and service from Richard at LAND Audio!


Mike

[boojum]

I would be quite grateful that this has been cleared up so quickly and easily.  True, it was an inconvenience.  But you have had the mics looked at by a real pro and will have them back soon, better than new.  You are one lucky guy.

Cheers

[landshark]

No kidding Booj - I am grateful.  Of course, I'm crossing my fingers that this situation was the culprit.  Richard indicated his hypothesis was just that - an hypothesis. 

Now if I could just go back in time and re-record that English Beat show....<grin>

Again, Kudos to Richard at LAND Audio, incredible customer service if you need anything done with your mics (and not just AKG).

Mike