eight channels, a couple issues

[andywho]

I've just finished eight channels (two channels of each). in putting them through their paces, I've noticed a couple things...

1) one of my A12s outputs a badly distorted signal. I've triple checked the voltages (I'm running the SC25s both at 18V) and switched out the op amps between the two to no avail. for the life of me, I can't find any difference between the good A12 and the bad one (except for I have no header at J6 on the bad one--the kit didn't come with one, and I figured it wasn't necessary seeing as I wasn't going to be doing anything with J6 anyway(?)). I'm not sure where to start looking for a fault. I've posted a couple pics here:
http://www.andywho.com/ssa/A12_01.jpg
http://www.andywho.com/ssa/A12_02.jpg

2) two of my trim pots are quite noisy (one N72, one J99). I figured it would go away with a few full turns of the knob, but it didn't. as recommended in the instructions, I did use a dab of adhesive on all eight channels to keep the trim pots tight to the PCB, and even though I was careful not to use too much glue, maybe somehow it got inside the pot? I would assume that they're sealed, though, so I don't see how that's possible. it's especially bad on the N72--turning it past 3 o'clock or so cuts the signal completely. is there anything I should try other than simply replacing the part? I posted an audio file of me turning one of the offending pots here:
http://www.andywho.com/ssa/N72dirtypot.aif

any help would be much appreciated. digging what I'm hearing so far from the healthy ones...

[tpryan]

How many is a few turns? Rotate the pots through their full travel at least 10 times and see if the situation improves.

[andywho]

turn each sucker at least fifty times, and here's where I'm at:

N72 situation didn't seem to improve.

J99 cleaned up for the most part, but if I turn it all the way CCW, it jumps back to full gain. interestingly, the OTHER J99 works perfectly, but I noticed that when I turn the trim pot CW I can hear (mechanically, not in the signal) a faint scraping sound. thoughts?

[andywho]

I'm thinking I may just bite the bullet and replace the pots on the two channels, unless anybody has any ideas about how to fix the dirty ones that are already there.

in the meantime, back to my original question: do these pots sound the way they do because of the adhesive I used? or anything else I may have done inadvertently? or did I just get unlucky?

that J99 is almost livable--but I'd like my unit to be 100%.

as always, thanks.

[tpryan]

It does sound like the pots have been damaged. What sort of adhesive did you use?

[bradb]

seems like when your J99 jumps to full gain you may have an error in your input attenuator resistors.

If you see my a12 post "Low Gain" I quote Tim's test of checking the input resistors. Your values between A12 and J99 won't be the same, but you do the same procedure and check between good and bad J99s (you have a good one too right?)

[andywho]

here's a quick update on my various issues for anyone interested: turns out I used the wrong kind of adhesive for the trim pots. it seems to have outgassed too much when setting, and corroded the metal in the pot. I used a radio shack brand; tim recommends this:
http://www.maplin.co.uk/Module.aspx?C=SEO&U=SM&ModuleNo=29013

I ordered new pots from onlinecomponents.com (they had the parts and no 25 piece minimum like mouser), popped them in, and the channels work perfectly. should be noted that I ordered several extras for all the channels in case the same thing pops up in the future.

as far as the A12, after hours of checking voltages and troubleshooting, I narrowed it down to the cinemag output transformer. jiggling and/or pressing on the wires entering the top of it caused the distortion/short/whatever to come and go. currently in tim's shop being replaced. I'll post again to confirm when I get tim's final verdict.

turns out I also had one more thing--hum in my N72s--as mentioned in this thread:
http://seventhcirclestudios.com/SCA/phpBB/viewtopic.php?t=564

tim agreed that it was a power thing--the C84s suck a lot of power, and with a full box, the PS02 can't quite keep up. he gave me a very good "upgrade" price on a PS03, so I can't complain. I'll post when I receive and install that.

[andywho]

swapped in a PS03, and everything's working close to perfect. 7 out of 8 channels are sounding brilliant.

one little thing: my N72's are in slots 1 and 2, and the one closest to the power supply has a little more hum than I'd like. I noticed it first when micing up a kick drum with a sennheiser e602. it's loud when the gain is cranked, and actually noticeable at normal levels when I solo the track.

I cranked the gain on both N72's and recorded several different dynamic mics (e602, SM7B, SM57), then A/B'd the tracks (same cables, converters otherwise):

http://andywho.com/temp/N72_AB.mp3

I've noticed some of my dynamic mics in my studio space can get pretty buzzy, and this N72 seems to only add to the problem. is it because it's closest to the power supply? would I be better off putting a different flavor of preamp closest to the PS? all I know is this: channel 1 (N72) = buzzy, channel 2 (N72) = medium buzzy, channels 3-8 (A12X2/J99X2/C84X2) = super quiet.

thanks again for all the help.

[tpryan]

Did you get a new mu-metal divider as well as the PS03? If not, send an e-mail to sales@seventhcircleaudio.com and we'll get one out to you ASAP. In the meantime, try swapping the A12s for the N72s to see if that makes a difference.

[andywho]

I have a divider, but it's from my PS02. didn't get a new one with the PS03 as far as I could tell. if there's something different about the one that comes with the PS03, I'll give it a shot. in the meantime, I will make that switch.