James HE

I screwed up my mixer
« on: January 21, 2007, 12:38:58 PM »
I have a Soundcraft spirt sx 20 channel board. I opened it up a few weeks ago and gave it a pretty good cleaning. Got it used and it was pretty bad as far as dirt and grunge on the pots and sliders. I poked around to see what was up on the inside as far as chips and caps. Mostly smt componets. Tl072's on the channels and 5532's in the master section. So I had a bright idea  :roll: .

(cue long story you might get a good laugh out of)

I have some OPA2604 soic's that I got by mistake instead of DIP - hmm what if I just swap out 4 of the TL072's for the 2604's? Sweet I'll try it. So I pack the board up and bring it to my house from my rehersal space. I forgot to bring the powersupply like a dumbass. So I have the board, the chips I want to swap in, all set up on my "bench" at home, but I cant really test it. Hmm I should just wait till tomorrow, but I was really looking forward to diving into it at the time. OK what ever I'll just swap them out anyways. Practiced my smt skills on an old PCI card, learned how to desolder and solder them, got confident enough to just go for it. I had to revome the HPF switch and a ceramic cap on each channel to have room to work. Took those out. swapped out the soic chips, put the switch and caps back in, closed up the board, went to bed. Took the mixer back to my rehersal space next day and tried out the channels.

no go, nothing coming out of those channels, except for some really bad sounds when I screamed into the mic and had the gain cranked. Aw shucks.  I knew I was too much of a newbie to just pull this off, but I just wanted to learn anyway. And i love making rash, impulsive decisions!

A few days later I bring soldering station and all my tools to my space (and a light so I can actually see!) and started to figure out what the problem was. First thing I measured was the voltage going to the opamps. I measured a few different ones. Looks like the opamps work on somewhre around +/- 3 volts (it varied a lot depending on where I measured, from like 2.5 to 3.9). Should have measured that before! minimum voltage for the 2604's is +/- 4.5 V. Well that probably explains why the channels just did not work. I just assumed the board would be putting out more juice than that. Does this seem odd or wrong to anyone?

So I put the TLo72's back in. ONe channel at a time. The first channel I did came out perfect - or rather sounding as crappy as always. NOw here's where I knew I'd have some problems - cause I pretty much destroyed some of the ceramic caps when I pulled them out. I just wanted to get it back together, working somewhat, I can live without 4 channels, but I needed to have it back together as soon as possible and to confirm I didn't absolutly ruin it. I completly destroyed one of the caps (leg came off) I had some random ceramic caps from Radioshack and paralleld a few together to get near the right value of that cap. So instead of 680p I have like 640p but with some really crappy ceramic caps from Radio shack on one channel. That channel has twice as much gain and is reall really noisey. well, obviously I need to source some new caps, but I needed the thing back together so lets move on. The last two channels I redid work and sound ok but the gain is really low, not sure what the problem is there at all. As of right now I'm just not using those 3 bad channels. at least I got one back right!

Before I go back in I need a few caps - the ones I had to take out were 680pf ceramic. Honestly I don't know what they are for.. lol. It would be intresting to trace the board to figure out exactly the schematic, but I'd have to remove a ton of componets to really do that effectively - which I am not intrested in at all. I have a soundcraft 1600 schematic, (got it from here somewhere) and this looked similar, but it's not exactly the same. No problem I'll get some new caps.

Also I'm thinking I'll get new opamps. These have been unsoldered and resoldered now, all ham-fisted by me, and might be just fine, but I'd like to take the chance of one of them being damaged out of the equation. I could just put TLO72's back in, but that's no fun and makes all of this work absolutly pointless! So I need a replacement opamp. Something that will work with low votage (or upgrade the power supply - that might make the whole board better, but I'm not sure I want to undertake that - I'd rather just swap out some componets) I'm thinking about MC33078.
minimum voltage rating +-2.5V.
Is this fine? What other options do I have?

Samuel Groner

    Z├╝rich, Switzerland
  • Posts: 2935
I screwed up my mixer
« Reply #1 on: January 21, 2007, 02:11:04 PM »
The +/-3 V sounds wrong to me, and much more so if it varies a lot.

If you have damaged the decoupling capacitors one might be a half-way short which causes excess current draw and hence makes the supply drop. Check the impedance from both supply lines to ground and to each other.



I screwed up my mixer
« Reply #2 on: January 21, 2007, 02:18:09 PM »
that voltage is way wrong, you should be above +/- 12 volts for sure, likely around +/-15v.

With that low voltage, either your new opamps are attempting to pull a lot more current or you have them hooked up wrong.  I would guess that there are current limiting resistors for those opamps and your new ones are draining more current than can be supplied and your voltage falls.

That value of cap sounds more like a filter cap than anything else.  I doubt they are the cause of your gain issues.
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James HE

I screwed up my mixer
« Reply #3 on: January 21, 2007, 02:47:40 PM »
I measured the voltage on many of the opamps, not just the ones I had swapped. Or is it that if one of the caps were damaged when I measured, it could have caused a voltage drop through out the board? I just have a crappy DMM and attributed the variances in the voltages to that and the fact that I was having a really hard time holding the prongs on the pins of the soics. I was measuring between each supply pin and ground. When I open the board back up I will remeasure with the caps out of the circuit.  I couldn't trace the circuit like I wanted to without taking a lot of stuff off the board, but I didn't think the 680pf were supply decoupling caps. There are some 1uf caps (look like those yellow AVX metalized poly film) close to the opamps that I thought were supply decoupling. The board sounds the same as it always did, except for the channels that are not working properly.

I forgot I also did one of the stereo channels, which is absolutly fine after putting the TLO72's back in. The 3 channels that aren't working properly I did in a bit of a hurry cause I had a friend coming to the space to remix some of his stuff - I don't mix though the board it just handles rehersals, headphones duty, mic pre's for toms when I need it,  and is used as a monitor controller of sorts.

I was very surprised that the supply lines were such a low voltage, and thought at first something must be wrong. Then I considered the build quality of the board and figured that's just how it is. I will try and suss out if there is a problem more thoruoghly when I open her back up. I will see if I can get a schematic from Harmon.


I screwed up my mixer
« Reply #4 on: January 21, 2007, 02:49:58 PM »
I guess you learned to do one channel at a time, I hope you can salvage this thing,I do not know if it can stand another rework, from a pc board point of view. Some of those old , is it Japanese?, PC boards can get pretty funky with age, the weird solder they used, etc.

Fix one channel and you fix them all. You are pulling down your power supply. Who ever was running your board at the last big gig, probably put a 50 amp fuse in there so you would either finish the show or go down like Great White.

Please do not leavre it plugged in un attended.
The pwr trans is going to emit a foul smell in about ten minutes, followed by your hopefully installed smoke alarm going off on Sunday morning, not good. Mosat of the people in Richomd probably have a pretty good hangover right now.
I am trying to humor you, because when stuff like this happens to me, I usually break down and cry like a baby.

Ceramic caps are not the problem.

I don't think. If there the regular 0.01 types across the rails.

Check your pin 4-pin 7,8 schematics and data sheets to make sure they jive. Sometimes you can swap a dual opamp in for a single and run into trouble.

Do ohm the power positive to gnd, like the guy said up above.

The low output is the +/- 6 volt supply.

I am going with Payton.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

James HE

I screwed up my mixer
« Reply #5 on: January 21, 2007, 03:05:54 PM »
I'm trying to remember if I measured the voltages before or after I removed the 2604's. I think before and I didn't check them again after removing them or after putting the 072's back. Me no so smart.  :shock: I'm pretty sure I put the opamps in the right way - at least the dot was in the right position - that always marks the same pin right? I'm not sure what manufature the 072's are - they just say 072 and have no other markings.

Svart yeah I couldn't figure out why the cap could effect the gain. I suspect I just did something too sloppily and it's causing problems. This is my first smt experience so defintly a possibility! I feel like (now at least) I can handle smt, I sort of like working with it. I just need to go back in and troubleshoot some more.

James HE

I screwed up my mixer
« Reply #6 on: January 21, 2007, 03:32:37 PM »
The powers only on when I'm there - except when I go up for a smoke break. The space is in the basement below a music store and studio in a building downtown. It at one point was a fallout shelter - pretty cool. You go down 2  flights from the back entrance to get to the rehersal spaces. I love my little Bunker, mildew smell and all. (except when the rockers next to me are rehersing when I wanna record!) So yeah fire would be bad - destroying my studio, 5 other rehrsal spaces, the inventory of a music store, and the contents of a pro studio. Thanks for that image! :shock:

I doubt the board is that screwy, I think I'm just doing something wrong and overlooking something obvious. Hopefully the board can take another rework (or 3) I haven't pulled any pads or anything. Gone through a lot of desoldering braid though. lol.

I wish I could source the componets I need locally but I'm going to have to wait and get a mouser order together.

Payton all the way!


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