Topic: Right channel fading sensitivity

[datbrad]

Last night I was taping David Grisman Sextet (3 members of the old DGQ + 3 new guys) and I had a strange problem at the show. As it went on, I noticed my right channel was fading gradually, down to about 10% of the gain level of the left channel by the end. Really odd. It had rained earlier in the afternoon, but by showtime it was sunny. Muggy, and high 80s, but clear.
 
I was running my old AKG 460s with the CK61 card caps that have not had any issues before, a pair of Sound Devices MP-1 mono preamps, XLR line into the WMOD 661. The cables are all ProCo star quad, not very old and in great condition. The preamps are about 6 years old, but also like new condition. Finally, everything was powered by freshly charged Duracell 2650 nmh batts that are 18 months old, and have never failed.
 
So guys, do I assume it's the mics or a cap going bad, the preamps having some issue, or the recorder?

Here's a jpg of the Wavelab window with the waveform.

[drewloo]

My first guess would be bad batteries in the fading preamp (since it's a cheaper fix than a new cap  ).  Was low-battery/power light in plain sight?  Could it have turned to red without you noticing it?  I've had Duracell, Maha, and Energizer batteries all crap out eventually.

Have you been able to repeat it again at home?

[datbrad]

I thought about the battery, but both preamps still showed full power at the end. (On these SD mono preamps the power light is green for full charge, yellow when getting low, and red when about to go). I hope that it's the batteries, but I am afraid it's the caps since humidity could have been a factor (based on what I have read about similar situations and damp conditions).

I guess I will go get some new batteries and hope this does not happen again next week for Umphrey's.

If it does, hello new Milabs.

[drewloo]

You may be right about it being the cap--just saw this in the mp-1 manual:
Batteries
The MP-1 is designed to operate on two AA alkaline cells for approximately 26 hours with typical signals
(without phantom power).  The audio performance of the MP-1 does not vary throughout the life of
the batteries.
Though I always thought that an analog device's performance will slowly decrease as the power source goes below the minimum necessary voltage.  Maybe in the mp-1 it doesn't since it has output xformers? 

[page]

Though I always thought that an analog device's performance will slowly decrease as the power source goes below the minimum necessary voltage.  Maybe in the mp-1 it doesn't since it has output xformers? 

some will, but I think it varries by device (for example: with the old sonosax m2 units, you lost headroom, not signal as you lose power).

I'm going to cast a vote for caps/bodies. If you listen to it, does it get quieter, or does it distort horribly as it's going down? Where I'm headed with this is with my beyers, I could remove the bodies and still achieve circa 30 mv/pa because the pad is at the cap, not the body. With the 460s, the pad is on the body, not the cap right? So if there isn't any distortion/popping/etc, but a reduction in mv/pa output, I'd look at the body to see if that was going. If there's distortion, I'd be inclined to look at the cap. Not fool proof logic by any means, but that's where I'd start.

If you still have your caps attached to the bodies, or at least can pinpoint which was connected to which, you could switch them and see if the problem moves or stays with that channel. A loud stereo at home should be able to quickly tell that one. Sorry to hear about the trouble.

[Walstib62]

The only thing any of us out here in cyber-land can do is guess on this one. You will need to set everything up and test it out to see what is happening. Change out 1 component at a time if the problem comes back to see which component is causing the issue. If you cannot reproduce the problem with everything running on good batteries, you may want to go as far as to run 1 of the pres with low batteries to see if that will cause the issue you are seeing. At least then you will know what symptoms a low batt. condition will cause.
Good Luck

[F.O.Bean]

Setup at home and keep switching the caps and bodies back and forth and see if anything hapopens or is consistent

Best of luck!

[datbrad]

My plan is to run the mics next week into a buddy's extra channels on his R44, no preamp. I have to think it's the mics, so while this test may or may not show any issues, I guess it's worth a try. Also shooting an email to Richard Land about this.

If it's the CK61 caps, I might just skip repairing them and pick up a brand new pair since they are only $300 each. I think after almost 17 years of recording, I have gotten my money's worth out of the original investment.

[itook2much]

I had the exact same thing happen when the 9v in the battery box got too low.  Looked just like yours.
May not be your issue, but a battery is the cheapest step.

[goodcooker]

When I ran an MP2 for a little while, I had the same resulting waveform when my rechargeable batteries went dead on me...of course in both channels though, since it's a stereo preamp.

Test some fresh batteries with a meter then do a test at home is the cheapest place to start.

[datbrad]

Update: Both Doug Oade and Richard Land think one of the caps is the issue.

Doug said a cap that is not screwed on tight could have caused what I experienced, as well an internal screw within the capsule could be loose. I know the caps were on tight, so I don't think that was it.

 Richard points to the probability that one of the caps is becoming sensitive to humidity, which points to a capsule rebuild.

Since he charges $200 per capsule to rebuild, I am leaning towards a brand new pair of capsules instead.

I contacted Sound Devices to find out if a failing battery could cause this with their MP-1 preamps, but according to their tech, the performance of the preamps do not decline as the battery life does. The preamps will perform to spec until just before they shut down.

(updated after talking to SD)

[Hypnocracy]

Agree with having Richard replace both caps and be done with it...cause if they are like my 460's the caps are pushing 20 years...


Richard just did the Cap Replacement on my CK61's...they sound perfect and levels within one db...

[datbrad]

Agree with having Richard replace both caps and be done with it...cause if they are like my 460's the caps are pushing 20 years...


Richard just did the Cap Replacement on my CK61's...they sound perfect and levels within one db...

I thought about that, but for another $100 each, I can buy brand new caps. I am torn on which way to go.