SSlmixed.com Forum
Tech discussion => 4K,6K,8K Series => Topic started by: jwhmca on October 16, 2014, 09:32:48 PM
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I know there is a huge alignment procedure for each channel strip, but what about the center section? I have a desk that is leaning a db or two...
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There is no level trim on the Quad Bus Mix Amp (82E26). The gain depends only on the precision resistors. You need to check the coupling capacitors, in particular C29/C30. If you inject a signal to the PRE VCA insert returns and it comes out of the main outputs the same on both sides, that confirms the mix amp fault.
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Great tip, Myringwiring!
Thanks!
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Same is also true of the six summing amps in the 688 section of a 6000.
Old electrolytics right at the summing node are a common source of imbalance... If you do one 26 card, be sure to do the other, (or the other three, in the case of a 4000... or the other 5 summing cards in the case of a 6000!) because the caps will all be the same age, and of one is old enough to fail... they're ALL old enough to fail.
It's a virtual earth node, so the impedance to ground is -in theory- zero ohms precisely. Caps with high ESR (equivalent series resistance) will throw the gain off, because it's as if the values of the equivalent resistance feeding the summing node has changed, and therefore the ratio of input resistance to feedback resistance (the precision resistors on the 26 card) is therefore thrown out of balance.
I'm not usually a habitual "recap everything, it's been almost a week since the last recap" kind of guy... but THOSE caps are fairly critical. Use low-ESR replacements of good quality. -At this point in more opinion-based forums, someone usually chimes in with their favourite types. -I look at what's available, what's the right size & shape, and what has exemplary specs on paper. -It's always worked flawlessly.
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Yep,
C9 is it? The 220uf 10V N.P.
Was dried up...
I have having me a goooooood ESR meter.
I replaced them all with Nichy Muse...
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Oh my god, SSLtech is here. Finally! Are you THE REAL SSLtech?
If you are... thank you, thank you, thank you - don't go anywhere! This is your new home.
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Looks like Keith found his way here! :-))))
Welcome to the SSLmixed forum, the spirit of the original 'Prodigy DIY' forum, with a twist of the Moggy Minor Owners Club!
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Good to see you here Keith, welcome!
-Paavo
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Thanks guys. Yep, that's me!
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I have an imbalance on the stereo bus myself. I'm 0.5dB down on the left. This is what meters with a tone patched in to the PRE VCA insert returns. Any ideas?
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i'm in a similar boat, been chasing it for the better part of 6 months. :/
have recapped all applicable cards in center section, cleaned and reseated small ribbons on buss cards, tightened all patchpoint connections (this actually fixed it a bit. there was a bad connection in the PRE VCA Insert Point) and even checked DLs for any bent pins.
Literally don't know what to do next, but on my 80 channel 8k, ~ 71 channels lean right by around .2 db-.5db, which says to me, if i can straighten out this lean, that those other 9 channels are actually the ones that are out of tolerance.
hoping to revive this thread/maybe Keith can shed some light on us? :-[
z
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I assume you checked the outputs with a AP rig or good meter? It could be what you are using is off?
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oh yes, unfortunately, many times. .... :(
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Some versions of the 26 card (center section main mix card) use one Roederstein(?) electrolytic that often gets overlooked in a recap (because it doesn't look like the rest of the 'lytics - looks more like a film cap). I've seen these go bad and cause an imbalance.
Is there an imbalance if you patch signal straight into the mix bus pre-vca insert return? How about the post-vca return? This may help narrow down where it's occurring. Also, try sending signal through a channel's small-fader path, assigned directly to the mix bus through the mix assignments at the top of the channel, with the pan pot switched out, to eliminate the large fader pan pot as a source of error.
We've just installed an SSL 8k with the film pan (discrete center channel) option, which does away with the trim pots that allow you to calibrate "center" on the large fader pan pot. There are several channels that lean a little one way or the other when panned center. Going to have to figure out a way to tweak those...
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I would also look at the signal at the pre vca send, then post vca send. I have also seen the
little square ceramic caps cause distortion and imbalance like the last post said.
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C9 on the CF82E26 card is typically the main culprit.
Also, make sure to pull all the cards and remove any of the shorting black "heatshrink" stuff that is laid all over the solder sides of the cards. That stuff will do all kinds of strange things.
The biggest problems I have had have been the cards right in front of (closest to you) of the 4 26 cards... they had the black tubes causing some really bad/weird problems.
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Definitely agree! The black sleeving originally started life as and insulating tube, but over time it degenerates into a high-resistance, brittle, semi-conducting sleeve that results in pain and suffering! It is worst on cards such as the two monitor selector cards where it passes over FETs used for audio switching . The FETs have been designed into the SSL cards with high-impedance on their gates, and the semi-conducting sleeve results in spurious switching and partial switching. Also check on the 013 channel VCA cards for this horrible sleeving. It's OK when new or soft, but the harder it gets, the more prone to high resistance it becomes. If you find this awful stuff, make sure you thoroughly clean the PCB.... it leaves a nasty residue that is also partially conductive.