Soundtracs MX Story...

Well...I've been doing a lot of work on this thing...and I haven't kept up on the posts and so there's going to have to just be a truncated summary here of what's happened and what's to come.

At present the MX is very much disassembled, partly for cleaning, but mostly to address/improve the ground scheme.

Got the extruded cross-members cleaned up (meter bridge housing and armrest). They still show battle scars but I'm okay with it...in a way it reminds me of how even good tools get dinged up when they get used, but it doesn't mean they don't work just as intended.

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Another thing that's going on here is that I've decided to turn the thing into a true split layout with some input strips to the right of the master section. I wish it wasn't so complicated to get it into a 16 x 8 x 16 layout, and it could be done but the backplane layout wouldn't match up because the base mixer in the MX-series was a 24 x 8, so the backplane follows suit. The 32 x 8 had an 8-channel backplane/bottom panel section added on and the 40-channel another, so its easy to just take that 8-channel chunk and put it over to the right of the 24 x 8 section. I've got to reconfigure the ribbon cable interconnect that goes from channel to channel and do some rework of the meter bridge but its nothing permanent...and much of the tear-down involved in doing this has to be done anyway because of the grounding issues. So it'll be a 24 x 8 x 8 mixer...and actually that makes more sense in my room because a 16 x 8 x 16 would have the master section too far off-center with respect to the monitor speakers...I don't want to have to keep bobbing my head back and forth when mixing. And also it is pretty likely that for the most part the tape machine will be setup as a 1" 8-track, so having the 8 "return" strips instead of 16 to the right of the master section will make perfect sense most of the time.

Here is the separating of the backplane/bottom panel sections:

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I've drilled out a whole load of rivets to get the jack PCB's out which is necessary to properly execute the grounding mods for the jacks...here is the mixer over a week ago...

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The master section comes apart:

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XLR jack shells...these all had to be de-riveted...53 of them...

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Here is a closeup of one of these shells...at first I thought these jacks were pretty cheesy...jack bodies were falling out of the shells and others wouldn't come out...I didn't get it. With the grounding issues and earlier issues I addressed with the power supply I was starting to feel like it wasn't as substantial a mixer as I thought:

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Well, turns out these jacks are pretty neat. They are identical to these only Neutrik branded. Good quality stuff.

I found a good source for bulk by-the-foot 40 conductor ribbon cable so I got enough to replace the munched cables...working on getting a shopping list of hardware needed to remount the jacks and reassemble the backplane/bottom panels.

The grounding issues are too numerous for me to mention here just yet, but I've got parts on order to replace the power interface cable and chassis-mount connectors and I feel I've got a real solid handle at this point about what is not right as well as some good solutions.
 
Forgot to mention that I repaired the 5 broken pots in the master section. I didn't have any spare pots in the same values but I had the same type of pots, so I took the 5 pots apart and transplanted good shafts into the pots with the broken ones...had to get creative with the brokan PAN pot for group 8 as none of the available transplant shafts had the metal band to provide the center detent, and are also a slightly different design so I had to modify one shaft to accomodate the band pulled from a broken shaft. BUT, all's well that ends well and the pots are all repaired and installed.

Did some work on the meter bridge related to the reconfiguring of the board layout...I could have left the meter bridge as stock with the L-R, SOLO and groups 1~8 meters at the far right of the meter bridge, but I wanted those meters over the master section's new location.

The lense in the meter bridge is a single piece plexi-glass assembly with the meter PCB mounted behind...solution? Mark some measurements and cut it up on the chop saw. :eek:

Came out just fine. The cuts were nice.

I also got the pieces polished up to reduce the years of scuffs and scratches on the plexi-glass.

I bought some tile-board coupling molding made out of PVC that I'll use to couple the sectioned meter bridge lense. I cut pieces today and painted them black. I think it'll look nice.

Then the only other wrinkle is that the ribbon cable to the meter PCB from the master section will need to be longer, but that's not a problem since, like I mentioned in another post, I found some available by the foot, and the connectors can be transplanted.

I have some mil-spec style connectors on the way as well as some new cable to make a new PSU interface cable. This is related to addressing the ground scheme issues.
 
It's great that you're liking the mixer, Cory and see lots of progress ahead. Is it just me or do you also appreciate the modular LEGO-like built of this thing? I mention that fact often but it's a mostly overlooked perk of pro gear. You can grow to really hate something where you have to perform major dis-assembly just to take out a channel on lesser equipment, for instance. I know it's a lot of work still, with the Soundtracs, but it's nice nevertheless. :)
 
Your attention to detail is staggering, Cory! I can't wait to see this bad boy all cleaned up, back together, reliable and sounding great! +1 on cjacek's comments, I look forward to having an old-skool modular console too.


Have you dated this console? You know, flowers, chocolates, dinner, movie...lol...actually just curious when it was made?
 
It's great that you're liking the mixer, Cory and see lots of progress ahead. Is it just me or do you also appreciate the modular LEGO-like built of this thing? I mention that fact often but it's a mostly overlooked perk of pro gear. You can grow to really hate something where you have to perform major dis-assembly just to take out a channel on lesser equipment, for instance. I know it's a lot of work still, with the Soundtracs, but it's nice nevertheless. :)

I really do like the fully modular design, and I like the fact that they designed the fastening of the modules to last, having the module screws fasten to those yellow zinc oxide plated threaded bars in the armrest and meterbridge housings. For all intents and purposes I never have to worry about one of those screws stripping out.

And the XLR jacks with the quick release shells...I mean, how often am I going to need to remove an 8 channel XLR jack PCB? Hopefully never, but if it takes 1 to 2 minutes to remove 8 channel modules (to access the PCB) and just seconds to unlatch the jack bodies from the shells (with a small screwdriver) you could easily get one of those things out of the mixer in under 3 minutes if it needed repairs...even the meter PCB...maybe 5 minutes to have the meter lense out of the mixer with full access to the PCB? These are extremes that will likely never need attention but I'm just making the point that it doesn't stop with the modules you know? Master section can be out and free of the mixer in...maybe...under 5 minutes? And all the internal connections are either low friction slip connectors or latching type...the connectors for the interconnection ribbon cable from channel to channel are even gold-plated. Little things...I'm not saying this mixer has got it all or is unique in these things...it was a lower-end mixer in its day, but IMO had good bang for the buck. Its just unique in my experience with the equipment I've had and so it becomes remarkable to me.
 
Your attention to detail is staggering, Cory! I can't wait to see this bad boy all cleaned up, back together, reliable and sounding great! +1 on cjacek's comments, I look forward to having an old-skool modular console too.


Have you dated this console? You know, flowers, chocolates, dinner, movie...lol...actually just curious when it was made?

Thanks, Brian!

And, yeah, you know you are gonna owe us some pictures of that "bad boy" when you get it. ;)

Dated this console...heheh...that's funny :)...Soundtracs introduced the M-series in about 1982 I think, but I'm not sure when this one was made...hmmm...trying to see if the numbers on the opamps tell me anything but I'm not sure...one of the 5532 chips says "414XB", and one of the TL074's says "8939BR"...so...I dunno. But this isn't one of the early M-series boards...the early ones were a little different cosmetically and actually were very different in terms of the input strips...different layout on the PCB's and component differences. I figure its safe to say this is a mid-80's unit.
 
The grounding issues...

So I thought I'd put some detail of the grounding issues up in one post and I think the easiest way to organize this is to state the best practice and then state how that hasn't been followed in this mixer.

Pin 1 grounding problem: in the case of this mixer (and with most audio devices AFAIK) the best practice is to strap pin 1 of each input or output jack to the device's ground reference (usually the chassis) as close as possible to the jack. Substitute "shield" for "pin 1" if we are talking about a TS or TRS jack, but the point is that we are talking about the ground conductor of the audio carrying jack and "pin 1" typically seems to be used in the industry when referencing an audio interconnect relative to this whole grounding thing. The reason for this being a best practice is that the pin 1-to-ground reference connection is a source of inductance; an antenna for garbage interference present in the environment. A steel chassis will not keep all of it out, and while much of it is beyond the human hearing range its artifacts are not...audible noise. So my mixer breaches this best practice by employing a star-ground for every pin 1 connection. That means there is a lug on the backplane next to where the power connector is to which all audio grounds (and so much more) connect. I estimate the total length of wire going between the ground lug and the pin 1 connections on the backplane to be well in excess of 12 yards...

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Solution: since all the jacks are PCB-mounted and there are some complexities in how the grounds run on those PCB's, the solution that has been chosen is to bare a spot to the ground trace as close as possible to the middle of each jack PCB, get a wire soldered up to it really well and then drill the backplane and fasten each ground wire for each PCB to its own lug as close as possible to each PCB. Naturally this will involve drilling the backplane for each jack PCB, and there are 20 in all, and baring the paint and using either star washers or keps nuts to ensure a good contact with the chassis. There is the possibility of having the XLR jacks strap directly to the chassis through the connector shell...there are 4 pins from each XLR jack, pins 1, 2 and 3 as well as a ground pin that connects to the shell so in theory I could just link the ground pin and pin 1 together and then make sure that the shell has good contact with the backplane, but because of the way the trace is run on the PCB (it covers about 85% of the PCB) it opens the door for some complications in trying to isolate those, so it is more sensible to just strap each XLR jack PCB to the chassis with as short a length of wire as possible.

The above solution covers the jacks on the backplane, but there are issues with the talkback mic input jack and the headphone output jack as well which are mounted on the master section chassis dress panel. The talkback mic jack gets its pin 1 reference back at that star ground lug, so I'll need to bond that to the master section dress panel. The headphone jack actually gets its pin 1 reference back at the power supply chassis. Yep...that's right...through the mixer, through the 10' unshielded power umbilical and round and round all the 0V wiring in the supply and ultimately back to the PSU chassis which is a lame grounding anyway...more on that below. So I'll need to bond that to the master section dress panel as well. In both of these special cases I have additional modification to do to ensure that the dress panel is properly grounded as well (and this goes for ALL the module dress panels) because as it stands, each panel gets its grounding through an improper path AND there is high potential for ground loops...again, more on that just below.

Module chassis dress panel grounding problem: the components of a device's chassis should all have as low an impedance path to the common ground reference point on the chassis. On the Soundtracs MX the input strip panels actually get their grounding through the master section jack PCB's. The ground reference goes from the star-ground lug to the master section jack PCB's through ribbon cable interfaces that connect the jack PCB's to a large connector PCB that goes under the 10 master section PCB's. That's where the master section gets its grounding, and from there the looooong ribbon cable that interconnects all the input PCB's carries that ground to a separate pin of the 40-pin IDC connectors used on that interconnect, and then that pin goes up to the dress panel on each input strip on a dedicated trace through the shell of one of the pots. This is the same way that the master section panels get grounded too. There are all sorts of things wrong with this. Though the panels are all painted, there is the possibility for additional grounding where the panels fasten to the frame which is metal to metal with metal hardware. This opens the potential of ground loops within the system which are an open door to environmental interference noise. The other issue is that it is not the lowest impedance path to the chassis. Channel 1 is at the end of 10' of ribbon cable. Back to the master section for a moment to highlight another wrinkle...that master section dress panel is actually two panels riveted together. Only one of the panels has the path to the chassis star-ground through the shell of one of the pots...the other is supposed to get its grounding through 1 or more of the 7 rivets that fasten the two panels together...there are a couple areas around two of the rivet holes that are bare metal, but the way they put the rivets in the rivets don't really touch the bare metal...Soundtracs was just trusting that there'd be enough contact through paint and hopefully some contact with those two bared metal spots to provide for grounding. The bared metal spots show some intentional thought into grounding, but the assembly makes the grounding appear as a non-critical afterthought.

Solution: all pots that are presently in contact with their respective chassis dress panel need to be isolated via nylon washers (as all the rest of the pots already are), and EACH dress panel needs to be strapped directly to the chassis. I'm going to accomplish this by baring metal under the TRIM controls and putting ring connectors in between the panel and the nylon washer and then having the wire from the ring connector go to screws I will put in on the underside of the meter bridge housing. There is an extremely good conductive path from the meter bridge housing to the metal left and right panels of the mixer frame and I will be improving the path from those to the backplane. I will put a spade-type disconnect on each wire from each chassis dress panel to the meter bridge housing so that it is still easy to get the strips out. Lastly I'll strap the isolated terminal of the 40-pin IDC connector in with the rest of the grounds. Superfluous I suppose but a quick spot of solder on each channel PCB will keep the ground scheme consistent and not have a dead-end path going down the ribbon cable.

Additional chassis ground continuity problems: as mentioned earlier, each part of the device's chassis should have as low an impedance path to the chassis' ground reference as possible. Right now the main chunk of the bottom panel and backplane (which covers 24 of the 32 channels and the master section) has a slam-dunk grounding as that's where the chassis ground lug is located (right by where the power comes into the mixer...which, by the way, is exactly where it should be). Between every other part of the chassis there exists paint (the 8-channel add-on back plane and bottom panel, the forward bottom panel of the primary section, the end panels for the frame, etc.) There was a passive approach to continuity here...metal wasn't bared and I think the ideology was that popping the rivets in would be "good enough". I'm not comfortable with that and the best practice validates my discomfort.

Solution: simple...make sure there is bare metal between all chassis members, and I'm going to go the additional step of applying conductive grease to all bared metal surfaces to improve continuity and stave off oxidation. I've also drilled out and removed the rivets that fasten sections together where I plan on baring the metal and I'll be using machine screws and keps nuts to ensure a tight bond at these locations.

PSU chassis/mixer chassis/0V power rail bonding problems: best practice here is that the PSU chassis and mixer chassis should be directly bonded and to NOT have that bond be co-mingled with DC power rail 0V references until you get to the mixer chassis. The chassis ground interconnect should also provide the shielding in the power interconnect. I've got a little mess here with regard to this. At present the PSU and mixer chassis are bonded together, but the DC power rail 0V references are tied in the PSU AND in the mixer chassis. Not only that but the connection from the ground prong on the power input socket and the PSU chassis is on, like, a 20 or 22AWG wire to a VERY thin and flimsy lug...so its like grounded and I want it to be GROUNDED you know? To make matters worse there is a ground lift switch...I'm sure this was needed but with a proper ground scheme and a properly grounded source of power a ground lift should never be needed...remember that the headphone jack gets its pin 1 reference all the way back at the PSU chassis? Yep...engage the ground lift and now pin 1 of the headphone jack has no ground reference, but it DOES have a 12' antenna on it. As I mentioned, ALL of the DC power rails (+/-17V, +24V and +48V) have their 0V ground reference strapped to the chassis in the PSU AND in the mixer, but at least one of those rails has to go back to the PSU and then back to the mixer chassis to get its reference bond in the mixer chassis. There are two wires in the power interconnect that carry grounds...one is the 0V for the +17V rail, chassis interconnect and one other 0V (not sure which one yet), and the other is (again) the 0V for the +17V rail, and one other 0V (the +24V and +48 power rails share a 0V reference so there are three 0V references altogether). That second one does NOT provide a direct chassis interconnect...its the one that goes to pin 1 of the headphone jack...so everything gets referenced to both chassis ulitmately but it is a mess. And the power interconnect is NOT shielded.

Solution: for starters I'm scrapping the power cable and its connectors. I can't find pins for this thing which has the same plug configuration as a current Hirose connector, but the pins aren't compatible. So, not being able to get the pins out or get new pins it really becomes a mess to try and convert the current cable for best practice operations. 8 conductors are utilized at present on the unshielded cable, and I need 12 minumum over a shielded cable. The cable is fully populated with 12 pins/sockets and conductors, but the chassis connectors on the PSU and the mixer are only populated with 8. I'd need to be able to get the pins out of the chassis connectors at least the ones carrying grounds, and populate the rest of the empty spaces...again, can't get pins, can't get the current ones out, and on top of it all I'd have to get some metal braided sleeving and put over the cable and then find some way to bond that to the chassis at each end. It was easier to get new connector sets and find new cable. I suppose I could just get two panel-mount versions of those current Hirose connectors but I think that would cost me about $30 and there is no guarantee that the pin layout and pin/socket size on my cable would interface appropriately with the new chassis-mount Hirose units, and I'd STILL have to deal with the shield and chassis interconnect. I found these connector sets for a good price...they are import and surely aren't going to be the same quality as Cannon or other genuine mil-spec connectors, but I figure they will be better than what's on the mixer, and size-wise they'll drop right in. I found 12 conductor 20AWG cable with braided shield for $1.28/ft. All in all the new cable will run me a little over $40 for the parts including shipping, but with the metal connectors and the braided shield in the cable I'll have a proper and healthy chassis-to-chassis bond, and then I will be using the extra conductors to properly run the 0V references for the DC power rails (two conductors for each taken straight off the outputs of the PCB in the power supply instead of all mixed together with the chassis, and then all bonded to the chassis in the mixer). I'll remove the ground lift switch, or rather convert its function to a disconnect for the 12VAC rail which just powers a couple Littlite sockets, one on each side of the mixer. Also, I'll put in a proper sized lug and wire for the strap from the ground lug on the input power connector on the PSU to the PSU's chassis. The other piece that will be added is a passive noise filter inside the mixer chassis...planning on using 2,200uF low-impedance electrolytic caps with a parallel film cap in between each power rail and its 0V reference before those 0V references strap to the mixer chassis.

At the end of all this I expect to have a mixer that holds up well against the radio station down the street and all the other digital noise in the environment.

I'll be doing the "cell phone test" when its all done: plug all your cables into the mixer for your normal setup but disconnect them from their sources or destinations, strap a 150ohm resistor across pins 2 and 3 of the balanced connections at the ends of the cables, turn all trims up to, say, 2:00, all channel faders to unity, route all channels to the main buss, main buss faders at unity and then turn the output to your monitors all the way down...have a friend initiate a cell phone call about 10' away and slowly turn up the monitor output level...if you can turn it up to your normal level and you don't hear any cell phone interference then have your friend start slowly walking toward the mixer. Keep your hand on the monitor out level so you can cut it quick if you need to. If your friend can walk right up to the mixer with everything setup as listed above then congrats. You've got GOOD grounding.
 
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Mixer dating: I looked at more of the opamps last night...some of the 5532's have that "414XB stamped on them but a majority of the chips, including a number of the 5532's and ALL the 074's have numbers beginning with "89" and some even "90". I believe this is a 1990 vintage mixer.

Chassis grounding update: got key areas of the backplane stripped of paint as in these examples:

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Here is a shot of the polished meter bridge lense pieces, now three pieces as opposed to the original one-piece affair to allow flexibility in placement of the meters with my reconfiguration of the board into a 24 x 8 x 8 from a 32 x 8:

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Here is some of the tile board coupling material I used to make couplers for the three lense pieces (stock material on left, trimmed and painted material on right):

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And here is an example of one of the fabbed pieces joining two lense sections. The coupler was trimmed to fit relatively precisely in the opening of the meter bridge housing so it looks a little too short here:

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Got the master section chassis dress panels cleaned up too:

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Other stuff...I figured out how to get the faders completely disassembled without removing the tiny push nuts which is great because I CAN'T FIND THOSE PUSH NUTS ANYWHERE!! Not that small anyway and they are quite a challenge to remove without damaging the fader. I'm glad to get them apart because these versions of these popular ALPS faders don't have a rubber gasket at the top to keep contaminants out and there was quite a bit of lint/fuzz/hair stuck in the nylon glides...they feel a little gritty and have more drag than they ought to and I figured out how to even get the little bearing glides off the "carriage" (the thing that holds the wipers) and get everything thoroughly clean. I've done one fader so far but now that I know what to do I'll be able to get the rest of the master section faders cleaned up which is probably the biggest task to tackle before I can put the master section back together. I also got all the hardware and connectors ordered today to do all the new ground strapping and get the frame put back together and the jacks mounted.

Here's another thing...Soundtracs put these thin low-grade plywood strips along the bottom of the mixer, sort of a sacrificial set of skids that can be beat up or whatever. you can see them in this picture. Well, the problem is that the backplane and bottom panel assemblies are NOT structural with respect to supporting the mixer when it is sitting on a horizontal surface...the panels cover a pretty large area and are made from metal that is 0.050" thick...not sure what guage that is but it isn't heavy which is why the bottom panel is all tweaked up and dented. It would be one thing if those skids provided stiffening as well, but they DON'T. The structural members with respect to supporting the mixer when it is on a horizontal surface are the 1/4" thick ground aluminum end plates...and the sturdy extruded aluminum channels that make up the armrest and meter bridge housing fasten quite firmly to those end plates and the master module and all the channel modules hang on those. The backplane and bottom panel sections merely provide parallel strength to the rectangle formed by the end plates and extruded cross-members preventing it from becoming a parallelogram...very important of course, but not in terms of holding up the guts when it is sitting on the table...so I'm scratching my head as to why there aren't skids that go underneath the end plates. I can understand some skids to protect the bottom panel sections but those are also acting as feet and the only thing stopping the bottom panel from deflecting if pressure is applied would be the PCB's inside...that's why some of the ribbon cabling got buggered up. I'm going to look at modifying the idea they employed to place skids all the way outboard so that they are underneath the end plates which are what's holding everything up, and maybe also something in the middle, but it'd have to be stiff so that it can actually protect what's inside and prevent the panel sections from deflecting and damaging stuff...hmmm...
 
Nice to see this progressing Cory! :)

Hopefully we can do another glamor shoot when its done!

Cheers! :)

You bet, Ghost! :)

Updates:

Here is some extruded aluminum channel I found with a PVC cover...it is about 4" x 1" cross-sectionally, and the piece I have is about 12' long. There are purpose-built end caps for the channel and I believe I have a lead on some. I will paint the PVC black and cut lengths to go across the bottom thus replacing the thin plywood strips. The aluminum channel will actually provide structural support and protection for the bottom panels where the plywood did not.

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All the backplane/bottom panel sections are now separated and all the holes for new grounding points are drilled and paint stripped.

Here is the main section ready for drilling (white vinyl tape was used to mark the hole locations)...

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Here is an "inside" shot after the holes were drilled and paint stripped (you can see the shiny sets of 4 holes in between each set of 8 input jack sets...I'll have double up on the master section as there wasn't enough room to place holes between the XLR out jacks):

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Look ma, no rivets...I drilled them all out now to get those plywood strips off and ended up drilling out the ones between the main backplane section and the main bottom panel section. You see, those bottom sections are all tweakity and the only really good way to get them straightened out is to be able to work individually on each section, so *poof* went the rivets.

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I also got all the necessary spots bared on the master section chassis dress panels and modified the grounding on the master section PCB's and talkback mic jack and headphone out to interface with the changes to the dress panels...also replaced the 4 local filter caps in the master section. That's as far as I'm going to go with any recapping in the master section at this point.

The power supply has now been rewired with the new ground scheme...6 fat 0V wires (two for each DC power rail) as opposed to 2, and I eliminated the ground lift switch altogether and replaced the +48V phantom power enable switch function with switching the 12VAC rail (which provides power to the Littlite jacks on each side of the mixer...I won't be using those so I thought it better to be able to disable that AC rail if not needed...the current is too low to worry about interference with the DC conductors in the power cable interconnect, but it certainly won't hurt anything to have that AC rail down if it isn't needed). I decided to eliminate the switching function for the phantom power because when OFF it coupled the phantom circuit in the mixer with the chassis of the PSU. That switch should really be up in the mixer but I didn't want to add a switch up there and frankly I can't see the need for that switch at all since each channel has individual phantom power switching. So in summary, instead of "Earth Lift" and "+48V" on the back of the PSU chassis I now have them labeled "12VAC" and "N/C".

I'm still waiting for an order from Parts Express with a whole mess of ring and spade connectors, and also still waiting for the new power interconnect connectors.

So there's progress. There is still some more work to do on the master section PCB's, and I still have most the master section faders to disassemble and clean, but then it will be time to put that back together.

The backplane and bottom panels need straightening and then I can put those back together with the new machine screw hardware I got last week, which also included the hardware to reattach all the XLR jacks.

Once my new power interconnect connectors arrive it will be time two make up a new power cable and solder the chassis connectors into the PSU and the mixer backplane. Can't recall if I mentioned it or not but I have the cable...It turns out it is very nice quality Belkin cable. Just right for the task.

I have to drill and tap a whole bunch of holes in the meter bridge housing for the ground straps for the channel and master section dress panels, but I've got it planned out how I will accomplish that.

Oh...and I have to reconfigure the ribbon cable that goes across all the input strips and the master section because of my reconfiguration of the mixer to a 24 x 2 x 8 x 8 (i.e. remove a bunch of the IDC connectors and repunch them), but as a nice fringe benefit of the reconfiguration I will be able to lop off about 3' of cable since it looks like the length of the cable is one-size-fits-all (IOW they used the same length of cable for the 24, 32 and 40-channel mixers and varied the number of connectors for the specific model...since mine is the 32 channel model and I'm taking connectors from the end and moving them to the middle it leaves that 3' tail...yipee!).

Its worth it.
 
MAN! What a CHORE!

I got all the master section faders cleaned up last night...took hours but I'm glad I did it...been sort of dreading it actually. Now its a bit daunting to realize that there are 32 input faders to do as well but I can do those onesy-twosey as I have time and still complete a mini project whereas with the master section its all 10 or its not "done".

A little pictorial of taking the faders apart:

Here is one of the faders with the element board removed. This one wasn't the dirtiest and not the cleanest. The main issue is the fuzz and gunk built up in the snap-on nylon glides that guide the "carriage" along the upper rail (I have no idea what the proper term is for the "carriage" but that's what I call the thingy to which the wipers are mounted). The stuff get's packed in there causing increased binding with the fader action.

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I found that instead of trying to remove the nearly impossible to remove push nuts that fix the rails to one end of the fader chassis (and even moreso to do it without damaging the rails or fader chassis...I've tried multiple methods on a spare fader of this type and it has been duly sacrificed) I realized could flex the ends of the fader chassis with both side panels removed (one of which is the element board) and squeak the wiper carriage out since the rail ends without the push nuts are totally free-floating:

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The nylon glides clip to the carriage with four little clips each which I could gently pry two at a time with my fingernails and remove...this made thorough removal of foreign debris and proper cleaning possible:

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Carriage, glides and wipers all cleaned up ready for reassembly:

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Clean frame, rails and carriage assembly reassembled with fresh grease:

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A sampling of some of the larger pieces of debris removed from eight of the ten faders:

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And here are ten properly cleaned and reassembled faders. BTW I did all this with the element boards still soldered up to their PCB's as well as to each other since there is still a (suspicious from a grounding standpoint) rod that ties the grounds of all the master section faders together...but I was able to do all the disassembly, cleaning and reassembly without disturbing any of the solder joints...

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I also applied DeoxIT to the element boards 2~3x each with gentle wiping in between to remove soilage. The faders are much more smooth and light as compared to the way they were prior to cleaning.

This is a shot of the underside of one of the two master section chassis dress panels that I mentioned in my last post. This is the half that will now be strapped to the mixer chassis ground reference as well as provide the pin 1 ground reference for the talkback mic jack as well as the headphone jack. The labels tell the story...the top bared metal area is one of the aux master pots, but the bodies of ALL the pots are now isolated from ANY ground traces on the master section PCB's, so the bared metal area is of no significance to that particular pot, it just provides for a convenient mounting location for the ground point (close to the meter bridge where the ground straps will attach) as well as mounting method since the pots firmly nut to the dress panel. A 18~22AWG 5/16" ring terminal will fit just right in that spot over the pot body. The talkback mic pin 1 reference used to go all the way back through the master section PCB's to the jack PCB's and then to the chassis star ground which breaks the rule that all pin 1 references should strap to the common ground reference as close as possible to the jack. On the master PCB pin 1 of the talkback mic jack is now strapped to the metal jack body and then the metal body will be in contact with the metal dress panel that will be referenced to the chassis by the link mentioned previously...I'll put a star washer in there with some conductive grease (to maximize contact but ALSO to stave off oxidation of the now otherwise unprotected steel). At the bottom is the headphone jack which was getting its pin 1 reference all that back at the PSU chassis. It will now be proper. I've modded the pin 1 traces on the master PCB to no longer link to the asinine path all they way to PSU chassis and now pin 1 goes right to a short wire that will be attached to a purpose-made ground lug that goes over the face of the plastic-bodied REAN style TRS jacks, and then of course its metal-to-metal with the dress panel...and same thing with the conductive grease.

master_dress_panel_grounding.jpg



That's all for now...
 
Look at the GUNK that came out of there! :eek::eek:


Cory, I have to think this is going to be the best Soundtracs MX in the world when it's done! I'd imagine Tim/StudioSystems uk would be interested by this story.
 
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hi all i buy a sountracs mrx 32/8/2 next saturday i have a problem i want aux 1/2 post fader in internal it s possible to pre or post equal on the soundstacsguy.com i have find shematics channel for mrx. hi sweetbeats thanx for your pictures of internal desk you make a great job .IT IS POSSIBLE TO TAKE A PICTURES OF ALL AUX JUMPER PRE POST ON CIRCUIT BOARDS THANKS ;SORRY FOR BAD ENGLISH
 
hi all i buy a sountracs mrx 32/8/2 next saturday i have a problem i want aux 1/2 post fader in internal it s possible to pre or post equal on the soundstacsguy.com i have find shematics channel for mrx. hi sweetbeats thanx for your pictures of internal desk you make a great job .IT IS POSSIBLE TO TAKE A PICTURES OF ALL AUX JUMPER PRE POST ON CIRCUIT BOARDS THANKS ;SORRY FOR BAD ENGLISH

Aux 1 and 2 are pre-fade only. The jumper changes aux 1 and 2 from pre-eq to post-eq, but aux 1 and 2 are always pre-fade.

Aux 3 and 4 have a pre-fade/post-fade switch by the knobs, AND a pre-eq/post-eq jumper inside.

Aux 5 and 6 also have a pre-fade/post-fade switch by the knobs, but are always post-eq.

Does that help?

Here are the pre/post-eq jumpers for aux 1 and 2, and aux 3 and 4:

Input_Module_Aux_Jumpers.jpg



Here is a closeup of the jumpers..."E" means pre-eq, "F" means post-eq:

Input_Module_Aux_Jumper_Detail.jpg


Congratulations on getting an MRX!
 
Thanx for pictures sweetbeats normaly i have manual and shematics with the MRX .I have send a message for this modification to tim studio system uk and he tell me it s possible but it s a big job to do,i think i dont do it .I have put a link to tim for this post .Have a good day
 
Yeah...I'd have to look at the schematic and the PCB and see what would need to be done but it would not be simple.
 
i all the soundtracs mrx is at home, amazing warm sound. How cleaning insinde knobs some one are hard to turn very hard /2(i have kf linear contact for electronics is good or not ?) .for aux 1/2 pre or post fader i wait, hi have one other problem in the link of external input connectors and a module channel mono for one channel i try to fix this .i can t upload pictures .
 
i all the soundtracs mrx is at home, amazing warm sound. How cleaning insinde knobs some one are hard to turn very hard /2(i have kf linear contact for electronics is good or not ?) .for aux 1/2 pre or post fader i wait, hi have one other problem in the link of external input connectors and a module channel mono for one channel i try to fix this .i can t upload pictures .

I use DeoxIT D5 cleaner for the pots (knobs). Can you get DeoxIT? Maybe kf linear contact cleaner is good too, I just don't know about it. Remove the module and see if you can spray a little of the cleaner into the body of the pot (pot is short for potentiometer) and work the pot by turning it back and forth. See if that helps.

"link of external input connectors and a module channel mono for one channel"

I think I understand what you are saying...are you saying that one of the jacks (a place where you plug in a cable) for one of the channels is broken or not working? I need more information.

You can upload pictures after you have made 5 posts. There are also internet websites that cost no money and you can upload pictures there and then put links to pictures in posts here.

Hope my post is not too confusing.
 
Here are some more pics of the master section:

IMG_9869_11_1.JPG


IMG_9868_10_1.JPG


IMG_9871_13_1.JPG



A closeup of the new ground lead that will strap the chassis dress panel to the main chassis (as a refresher this used to occur through the main buss PCB through the large connector PCB through ribbon cables to the master jack PCBS and finally to the original star-ground point...now that green wire is connected to a ring terminal which is fastened to bare metal on the dress panel and will connect to a new dedicated ground point underneath the meter bridge housing...)

IMG_9870_12_1.JPG



And I wanted you to see this...wish I had before AND after pics of this, but the nice Neutrik XLR jacks have silver-plated contacts...I've decided to use Tarn-X instead of abrasives to clean them up. The pins used to be almost black on the talkback mic jack...Took seconds to make them look like new. A little DeoxIT D5 to flush and I reckon its literally good as new.

IMG_9874_16_1.JPG



Here's some more detail on what I did to the master section PCB's as far as ground mods:

1. I changed the headphone jack to get its ground as close as possible to the jack rather than through about 4 yards of cabling to the PSU chassis. To do this I cut out a bit of the ground trace near the jack to isolate it from the factory ground path, and I also removed a small piece of ground trace that was in between the two places where the ground terminal in the jack is soldered to the PCB...if I didn't remove that second little bit then there could still be two paths to the wire I added and we only want one path...the added wire and ground lug is the last part of the mod. I soldered a wire to one of the two ground terminal points on the PCB and then soldered the other end of the wire to a purpose-made ground lug that goes over the jack, and then when the PCB is mounted that lug is in direct contact with a newly bared metal spot on the dress panel...I put some conductive grease on that bared metal spot to protect the bare metal from the environment and to enhance conductivity.

HP_jack_mods.jpg



2. I changed the talkback mic jack to get its pin 1 grounding through the jack shell rather than through the the mixer guts to the central star-ground point. To do this I isolated the pin 1 land on the PCB from the ground trace by removing a bit of trace, and then soldered in a jumper between the pin 1 land and the formerly isolated land that goes to the jack body. The jacks have 4 terminals, pins 1, 2 and 3 as well as a fourth that goes to the jack body. Isolate pin 1, connect pin 1 to the jack body, done. A spot was stripped on the dress panel where the mic jack fastenes and I used star washers and conductive grease to ensure good and lasting contact.

talkback_mic_jack_mods.jpg



3. There were a bunch of places where the ground traces on the master section PCB's would propogate to the bodies of potentiometers. The way Soundtracs designed the dress panels to get their grounding was through select pot bodies. To ensure no possibility of ground leakage through any of these places I isolated those pots from the ground trace. The problem was only present on pots that have a little braket to keep them stable. The bracket is metal and solders to the PCB in 4 places and then these pots have 3 terminals (in, out and signal ground). In the most extreme cases Soundtracs provided ground connections from different paths to both sides of the bracket...redundant and not the best practice anyway. So on every group PCB I had a bit of carving to do to isolate the bracket from the ground trace since it was intermingled with a couple other necessary lands, and then also remove a 0 ohm jumper.

group_PCB_mods.jpg



In the end all the signal grounding references the chassis ground through the shortest point at the point of signal entry (at the jack) and the chassis gets its reference back at a central point as close as possible to where the power enters the chassis...no intermingling here and there and everywhere.

Another fun discovery is that as I was reseating each of the opamps with an application of DeoxIT, I found one pin of a TL074 bent over. Who knows what that used to prevent from working (I didn't take the time to chase that down) but whatever it was it should work now.

So everything is deep cleaned in the master section, new local power filter caps installed, opamps reseated with DeoxIT installed, and grounding straightened out.


I've modded the bussing interconnect ribbon cable too. Removed 10 IDC connectors, compressed them back on in new locations and trimmed off a significant length of excess ribbon cable...now ready to connect up the modules in a 24 x 2 x 8 x 8 configuration rather than 32 x 2 x 8.

Got the new power cable connectors. Time to build up that new power cable.

IMG_9858_2_1.JPG
 
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