TAC Scorpion Overhaul Odyssey

[Potato Cakes]

the concentration required by  the visual elements subtracts from my ability to use my ears , I find .

All I used to do is tweak virtual knobs in tiny amounts for hours focused on the pretty GUIs and I thought it was doing something, but in reality I was just wasting time. That is not the case any more. I turn an actual, physical knob, I hear the sound being changed, and then I'm done, with little to no second guessing afterwards....

Monday I will be ordering all the caps, chips, and some other parts for other projects and hopefully the day after my fans for my PSU and super fancy fader lube/cleaner will arrive and I will get to finish cleaning faders and reinstall them. I also need to pull all the knobs and wipe down the channel face plates and soak the knobs in some hot soap and water or something. I'm certain someone at some point used some sort of cleaning spray on the pots and because when I pulled a couple of the pots there was a tacky residue around the shaft and inside the knobs.

I still need to find some replacement knobs or just find some spare channels online for cheap. Also, thinking down the road, I would like to replace the black side panels as they look a little beat up. I thought I saw some desks that had fancy wood panels but I never found where they came from. I could take the ones I have off and trace them and cut my own, though my desire to pile on more DIY work is wanning most rather quickly. I have some ideas...

Thanks!

Paul

 

[Tubetec]

Theres a special sticky grease used in the pot shaft , its crucial to the smooth  feel ,it sometimes oozes out a bit .
over enhusisastic used of cleaner sprays can wash this stuff out , I always spray a pot with the shaft vertical and pointing upwards otherwise the solvent can wash the the niceness  right out of it . I'll only spray out  pot  as  last resort , often moving it back and forth a few dozen times can help clear any debris that has settled , if you can  pop the back off a quick wipe with a q tip moistened with alcohol is my preferred method , usually you'll pick up quite a bit of loose carbon particles on the cotton bud ,the alcohol may even start to partially dissolve the track in some pots , thats why Im not  a fan of 'douche spraying' it with ipa.

 

[Potato Cakes]

Thanks for the tip. I have been one of those guys. I'm going to be using the Caig Labs Deoxit Fader F5. It seems to be a cleaner and lube in one. This was recommended to me by a trusted source so I'm going to give it a go.

Thanks!

Paul

 

[jensenmann]

Now, that I'm more focused on this project and paying more close attention to the schematics and wiring, I've noticed that pretty much everything on this desk is an unbalanced connection except mic inputs and line inputs. According to the manual, this was to be able to accommodate +4 and -10 gear by which pin on the connector was used. Very smart idea for the time, but now that everything thing is pretty much balanced, the question is how much do I want to make balanced and how much of a pain will that be? I'm going to be using Jeff's ACA board in this project, so that will take care of the L/R outputs, but I would like the bus outputs to be balanced as well. I could add a simple circuit with a DRV134 and trim the level going in to the chip so I would have unity gain on the output. Or I can strap in transformers to do the job and not have to think about the additional load of adding all of those extra ICs.

Anyone here who has gone through this whole procedure have any tips or thoughts regarding the unbalanced outputs?

Thanks!

Paul

In my ex-Scorpion I addressed the unbalanced problem this way: I had removed the voltage dividers which creates the -10 output from the +4 Signal. Then I installed a resistor from pin 3 (-) of the XLRs (or ring of the TRS) to ground, the value being the same as the output chip has installed as current limiting/isolation resistor at it´s output. That will create an impedance balanced output. I did that for all group outs and direct outs. For all auxes and both master outs I made a daughter board with DRV134 balanced output drivers.

 

[jensenmann]

Here's my approach for bypassing the chips. I had found some 0R resistors that were closer to some of the connections, but I didn't know if there was a chance they would ever burnout or fail, so I decided to go to an actual ground connection. I didn't do electrolytics as well as during my reading on bypass caps, the information I came across stated electrolytics weren't needed at each point but just at the main incoming source. I'm not sure if doing this at the channel or at the console power input would satisfy this statement. I also read a Jim Williams post regarding doing bypass caps in a Scorpion upgrade and he only mentioned the ceramic 0.1uF caps.

I also found a number of 10uF caps that someone had used to replace the 100uF on the channels. I don't know if this was done in error or if there was some intent involved with this decision. These will all be 100uF when I order the new caps.

Thanks,

Paul

In the picture the main reservoir caps are missing. I hope you soldered them back in.

Jim Williams often tells half of the story only. He makes a living out of this modding business, so clearly he has an interest to not tell everything to everybody. Or maybe he´s just too lazy for typing too much which is fair enough.
There´s a thread here about grounding and PSU decoupling with pearls of wisdom from some of our members. The tenor about PSU decoupling was that we want a very low impedance to ground ideally close to every chip. That´s done with those ceramics from rails to ground and can be improved with the electrolytics parallel to those ceramics. In audio (=LF design) people tend to ignore this because it´s a technique mostly used in HF designs. But if you want a good resolution (=linearity upto HF area) it´s a good idea to use HF circuit design practises. Many scientific books have been filled with information about this. It´s not audiophoolery or irrelevant, it´s proper engineering when your design goals enter HF territory.

 

[jensenmann]

Some more thoughts:

My main problem with the faders were not hte faders themselves but the selfcutting Molex connectors. I replaced them all, as well as I completely renewed all ribbon cables for the same reason.

Some insert jacks in my desks were intermittant, so I replaced them all. Colin at AML sells them with gold contacts! Remember that your desk has been used for lifeshows, in the same room with smoke machines. Smoke is basically grease which condenses in all contacts and creates contact problems.

Scratchy pots or faders often have their origin in leaky electrolytic caps. If they pass a small leakage current it will be audible. If you recap your desk then you hopefully have new electrolytics which are not leaky. It´s quite possible that a lot of the scratchyness goes away. If dirt is involved it´s another story. In my desk I cleaned all faders with Caig F100 and F5, but not a single pot. After recpping not a single pot was scratchy anymore. Give it a try berfore you spray your pots to death with cleaner. As mentioned before the cleaner will partially solve the grease and create a mess on the carbon trace. I´d try to avoid that.

 

[Potato Cakes]

Some insert jacks in my desks were intermittant, so I replaced them all. Colin at AML sells them with gold contacts! Remember that your desk has been used for lifeshows, in the same room with smoke machines. Smoke is basically grease which condenses in all contacts and creates contact problems.

Being someone who has seen the world via an all access crew pass, I have very fond memories of the oil based hazers and foggers usually placed in close proximity to my console or my amp racks. And confetti. And sometimes pryo dust...

I do have some gold contact TRS connectors that I can use if applying contact cleaner is not enough. Hopefully it won't come to that.

In my laziness I'm going to leave the old ribbon cables and connectors. They all appear to be in good shape and should be fine. I don't plan on disconnecting anything again if all goes according to plan.

Thanks!

Paul

 

[Potato Cakes]

In my ex-Scorpion I addressed the unbalanced problem this way: I had removed the voltage dividers which creates the -10 output from the +4 Signal. Then I installed a resistor from pin 3 (-) of the XLRs (or ring of the TRS) to ground, the value being the same as the output chip has installed as current limiting/isolation resistor at it´s output. That will create an impedance balanced output. I did that for all group outs and direct outs. For all auxes and both master outs I made a daughter board with DRV134 balanced output drivers.

The chips I will be using are LME49720NAs. I was looking for internal block diagram with the resistor values to find the limiting resistor but it wasn't shown. Is this something that I can measure on the actual physical chip? On the schematic, the IC feeding the buss out has a 100k resistor tied to ground after 100uF capacitor, but to me 100k seems a bit high to be the resistor to use across pin 3 and ground to achieve an impedance balanced output. I'm not sure where to find this information. The data sheet didn't seem to have any other printed information that would have been useful. There is a data line for Power Dissipation but the value is Internally Limited with no numerical accompaniment.

Thanks!

Paul

 

[Potato Cakes]

In the picture the main reservoir caps are missing. I hope you soldered them back in.

Jim Williams often tells half of the story only. He makes a living out of this modding business, so clearly he has an interest to not tell everything to everybody. Or maybe he´s just too lazy for typing too much which is fair enough.
There´s a thread here about grounding and PSU decoupling with pearls of wisdom from some of our members. The tenor about PSU decoupling was that we want a very low impedance to ground ideally close to every chip. That´s done with those ceramics from rails to ground and can be improved with the electrolytics parallel to those ceramics. In audio (=LF design) people tend to ignore this because it´s a technique mostly used in HF designs. But if you want a good resolution (=linearity upto HF area) it´s a good idea to use HF circuit design practises. Many scientific books have been filled with information about this. It´s not audiophoolery or irrelevant, it´s proper engineering when your design goals enter HF territory.

I don't have all of the caps yet so those are where I pulled the old ones. I'm still a couple days out from being able to put in new caps.

For my own bypass cap implementation readings, which are rudimentary and limited, do indeed mention the electrolytic caps parallel with the ceramics, but then later mentions this is not entirely necessary if this is already done at the main power supply source. I am not doubting your statements just explaining my thought process of course of action. How big of a value would be needed for the additional electrolytics used for bypass duties?

For the distance to ground, the closest points I initially found were the 0R resistors, but thinking that it is still a resistor and could burn out rendering the ground connection useless, I found the closest point to be on the main ground copper trace, so I ran everything to that. I could rework them so that the ceramic capacitor body is right next to the IC PSU pin and run to ground from there. To me that essentially what is being done on any of other bypass design as all the ground points still have to travel to a point on the PCB and then to the actual star ground point. I think I will do some tests with this to see if I can detect any difference between the varying cap distances. Mostly because I too am lazy and don't want to redo the ones I've already completed.

Thanks!

Paul

 

[jensenmann]

Those current limiting resistors are 47Ohm, e.g. R64 for the channel direct out. It sits between the ICs output and the TRS, in series with the output coupling cap.

Regarding the decoupling caps next to the chip, theory says at least 10uF/amp, so 22uF for a dual chip as used in the desk - obviously per rail.

The Scorpion´s circuits are (mostly) built around the parameters of TL072 chips. That means most part of the circuist are mid to high impedance. That calls for FET input chips. Otherwise noise could degrade. Esp. in the EQ where everything is DC coupled you don´t want BJT input chips because they have too high bias currents. That´d make the pots scratchy. TLE2072 or OPA2134 work fine in that case.
The spots to use BJT input chips are the fader amps and summing amps.

 

[Potato Cakes]

Those current limiting resistors are 47Ohm, e.g. R64 for the channel direct out. It sits between the ICs output and the TRS, in series with the output coupling cap.

Regarding the decoupling caps next to the chip, theory says at least 10uF/amp, so 22uF for a dual chip as used in the desk - obviously per rail.

The Scorpion´s circuits are (mostly) built around the parameters of TL072 chips. That means most part of the circuist are mid to high impedance. That calls for FET input chips. Otherwise noise could degrade. Esp. in the EQ where everything is DC coupled you don´t want BJT input chips because they have too high bias currents. That´d make the pots scratchy. TLE2072 or OPA2134 work fine in that case.
The spots to use BJT input chips are the fader amps and summing amps.

Thanks for pointing out the resistor. It is clear to see and don't know why I didn't spot it except for that it was late when I posted last in this thread and am juggling a number of projects. I am only going to be doing this on the bus outs as I won't be using the direct outs, but it seems they are using the same value for current limiting. And thanks for bringing my attention to the FET and BJT chips. I'll update the shopping cart I've been building accordingly.

Thanks!

Paul

 

[Monte McGuire]

Those current limiting resistors are 47Ohm, e.g. R64 for the channel direct out. It sits between the ICs output and the TRS, in series with the output coupling cap.

While they will limit current, the typical purpose of a 47Ω resistor in series with an amplifier output is to isolate cabling load capacitance from the driving amplifier's feedback network, so that the amplifier remains stable. While the minimum value can be amplifier dependent, typically anything greater than 30Ω will sufficiently isolate the cable load capacitance. Without this resistor, many amplifiers will become unstable with anything from 10 to 100pF of load capacitance connected directly from the amplifier output to ground, depending on the particular amplifier.

47-50Ω is a good choice since these resistors are easy to find and it works with every amplifier I've seen. Values greater than that do not provide any stability benefit, and if the value is made too large, the HF rolloff point will become a problem with very long cable runs. So, 47Ω is both necessary, sufficient, and ideal.

 

[Monte McGuire]

On the schematic, the IC feeding the buss out has a 100k resistor tied to ground after 100uF capacitor, but to me 100k seems a bit high to be the resistor to use across pin 3 and ground to achieve an impedance balanced output.

The 100kΩ resistor is there to connect the un-driven side of the capacitively coupled output to ground at DC, so that it can be switched or patched without creating any clicks. Without this 100kΩ 'tackdown' resistor, the other side of the coupling cap will float to some random voltage depending on the leakage of  the elcap and whatever else is downstream. With the tackdown resistor, the AC voltage will still be completely defined by what's driving the 100µF coupling cap, but the DC value will be set to the ground connection at the 100kΩ resistor. Again, this is to avoid switching clicks that could happen when patching or switching the bus out signal.

 

[Potato Cakes]

Thank you for the explanation on this resistor. This will be helpful to know in future projects.


With advice of jensenmann, I've gone back through and re-evaluated my IC selection and  which ones needed to be "upgraded." Here's a breakdown of what I've come up with:

S3301 Bus Channel
U1 - Meter - TL072A
U2 - not being used as EQ is being replaced
U3 - Aux Sends/EQ Buffer - TL072A
U4 - LME49720NA

S1000 Input Channel
U1 - not being used as input is switched to DOA cards
U2-4 - EQ - TL072A/TLE2072A*
U5 - Fader - LME49720

S4001 Monitor Channel (master)
U1/2 - Meters - TL072A
U3 - PFL - TL072A
U4 - Slate/OSC - TL072A
U5 - Aux Monitor - TL072A
U6 - PFL/TB - TL072A
U10 - L/R Monitor - LME49720
U11/12 - L/R Mix Buss - LME49720

I had previously counted up all the DIP8 IC's needed and then used that for the number of LM49720's to order, but after being made aware of FETs vs. BJT in this design, I paid closer attention to what each op amp's role was and started to slim down on the number of fancier chips. For the ones that don't (or shouldn't) affect the audio on it's way to the buss outputs, I decided to keep using the TL072's but switch to the A grade for noise and to never have to think about them again, hopefully. On the channel EQ, I am bit torn as I know from earlier in this thread that the TL072 ICs and even the TAC EQ schematic is pretty much identical to the classic Trident designs which I know sound great, but I still have to fight off the former audio self that still believes the hype surrounding more expensive gear/parts whether or not it is true. Also, there is the cost difference of $0.80 for a TL072A and $4.24 for TLE2072A, with the OPA2134 being more expensive than that. I do believe that for certain applications the higher end ICs would have a tremendous performance advantage over the TL072A, but I don't think that application would be this EQ circuit, at least not five times better. The input modules all are fitted with nice sounding DOA modules which in turn will improve the sound of the EQ, so I think using premium grade IC op amps here would not be money well spent. However, I am curious if using one TLE2072 for the MF1 band would be of any benefit, more specifically if this would make boosting frequencies in the 2k-5k range smoother. I realize that other aspects of the EQ would have to be modified to notice drastic changes, but I would like to know if anyone following along with my madness has made IC comparisons in this EQ circuit and what your thoughts are.

I'm still planning on using the ACA-BO for the master mix buss, but in the interim I'm using LME49720s for U10-12.

My fancy fader cleaner has arrived as well as my fans for the PSU so that will be on the docket for tomorrow. Hopefully I will get to place the order for the rest of the parts for this project and have it knocked out by next week. I just want to make sure I'm not forgetting anything when I do.

Thanks!

Paul

 

[jensenmann]

This is swampy territory, I know, but I´ll try to describe what differences I heard from swapping those TLo72. In general the above mentioned TLE/OPAs sounded cleaner and more detailed. In the MF section the EQ sounded a less harsh/aggressive. If that´s beneficial for you depends on what you´d like to hear. I could boost more without getting the signal sound artificial. The LF chip drives the insert if the EQ is engaged. It makes a lot of sense to replace the TL072 with something capable of driving 600 Ohm. Again it will give you a cleaner signal when you drive old 600 Ohm gear (Pultecs, 1176 A/D, etc.).

 

[Potato Cakes]

Swampy territory indeed!

I think for the time being I will change out U2 as HF and MF1 would be the only bands I would be boosting 90% of time and I would definitely prefer less harshness in that frequency range. I can revisit the other bands later I feel the need arises but I have other outboard EQs for the handful of times the lower frequencies need boosting. I also am not going to be using the inserts on the input channels as I would be doing that between the output of the interface and input of the console to cut down on the amount of patch bay space and extra cabling that would be needed.

Thanks!

Paul

 

[Potato Cakes]

I ordered the parts a couple of days ago hoping they would be delivered on Saturday, but it seems that I will have to wait till Monday. In the meantime I did clean all of the faders and got the fader panel back in place. I held off on doing any potentiometer cleaning per jensenmann's advice and will wait to see how the new caps affect their performance. I also decided to do impedance balancing for all of the outputs that are being used. The aux sends will mostly be used for cue and FX sends, which this will be fine. Hoping to do all of this and properly unbalance the Tape Inputs this weekend and then be in a position to start reinstalling channels and do some testing when the parts arrive on Monday.

Thanks!

Paul

 

[Potato Cakes]

Got the back panel off and installed the 47R resistors on the buss outs and the aux outs. To make things easy for the buss out connectors I simply lifted the wire going into Pin 3 and inserted the 47R resistor on the XLR connector. The connections for the meters need to be desoldered to be able to remove the panel from the individual cards as I didn't find a disconnect for these wires. Same for the power connector as it is not rear mounted. I've been pretty impressed at the neatness of how most of these wires were run, especially for what was considered a budget console.

For the tape returns I want to unbalance them properly but it wouldn't be as simple as strapping the ring to the sleeve as doing so would also short the signal fed to U3 from the 68k resistor (R3 on the connector card). Being this is how it is connected on the card, I would have to cut the trace from the ring and tie that to ground unless there is a better way I have not considered. I believe that R3 (68k) and R4 (22k) need to remain in place as I have noted that similar audio connections in this console where 68k was between the positive audio connection and an op amp and on the same connection was 22k between that and ground. If this thinking is in error please let me know.

I had high hopes for getting much more done today but I've not felt great and did some serious sleeping in which was also accompanied by the raining that took place all day. I think tomorrow I'll finish unbalancing the tape inputs and finalize decisions regarding the master connections and then put the panel back in the frame. Then I'll be in a good position when the parts show up on Monday.

Thanks!

Paul

 

[Potato Cakes]

This is the Subgroup schematic for those who want to follow along.

 

[Potato Cakes]

Got the tape inputs unbalanced tonight and it wasn't too bad. I loosened the first and quickly realized that there were too many soldered wires for me to deal with, so I devised a plan to do this with all the cards connected. In this picture, you can see the back of the card with the TRS connectors. You can follow the trace from the ring solder joint from the TRS connection on the right hand side and follow it up and to the left to the next solder connection, which would be R3. What we're going to do is cut the trace below the R3 Connection and then solder in a jumper from the trace to ground.