TAC Scorpion Refurb and Enhancement
Hi there,
I am new to the forum in terms of contributing some experience, yet have been reading and taken advantage of information shared here myself for a while.
I went through a complete overhaul of a TAC Scorpion II console, which my son picked up for bargain. It was complete, but not working in any usable fashion, some pots were bent, one broken off, one set of knobs was missing and it was dirty as hell. It came as 24 ch, 8 aux busses, 8 groups, along the way we added 6 more inputs, so now its a 30 ch (43 slot) bay console.
In the first go around I managed to get every single module into working condition, upgrade a pair with AMLs Langley mic amps, replaced several pots, swapped all electrolytic caps, cleaned all input and output terminals and bumped into two „major“ roadblocks:
Those 10pin flat ribbon cables, which were broken at the connector ends. The PCV isolation had gotten so brittle that chopping and reattaching those IDC connectors really did not work well as a fix, especially because there is some inherent stress to the cable when taking a module out of the bay. Making the long story short: New ribbon cables of this style are unavailable, but I discovered ECON CONNECT Board-to-Wire Connectors Pitch 2.54 mm Series CV type assemblies, which make a great alternative.
These are available preconfigured e.g. with 10 leads with open cable ends and I simply spliced two of those back to back, shrink tubing, done. So the entire console got an overhaul with these cables , I also swapped the fader cables, cause these suffered from the same ageing effects.
The second roadblock was the meter bridge. As mentioned in a number of places (also this forum): The TAC meters do break and spares are no longer available. Those chips are long gone.
I disassembled the bridge and spent some time to understand how the meters work. A common problem is, that only part of the LEDs light up, I also had two meters of that kind. This can be caused by a single LED being dead. Why? The original TAC circuitry is using a power saving trick and has 5 LEDs in SERIES feed by a current source and the outputs of the meter chip short out the LEDs. Yet if a single LED is broken this 5x chain is dead, while the other 2 x 5 chains are not affected, which creates „holes“ in the display. I was lucky, I was able to fix 2 out 3 of the „dead“ VUs by replacing a few LEDs. Lesson learned here: If there is some life in the meter circuit it might be a dead LED causing the trouble. Thus NOT the case for the PFL meter, here the meter chip was completely dead. As stated, these AMEK / Plassey devices are unavailable, yet the meter bridge body has plenty of open space to play with. I ran across a 16 LED meter kit PCB based on 3 LMC 324 op amps on ebay, so I decided to deploy this board. I calculated the appropriate dB steps for the resistor ladder to resemble to exact scaling of the original TAC circuits and here we go: The PFL meter tracks nicely all the others and from the outside you can not tell a difference.
In this now working state the unit went into a small private recording studio and provided quite a bit of mileage. The good old mixer sounds great!
But there’s some new desires: My son really wanted to have a decent patch bay to bring in compressors and other outboard gear and „ is there a way I can get metering on a per channel basis, this jumping across with the PFL drives me nuts?“
And that’s where the second round started:
Creating a full bore (all I/O) meter bridge and designing a full resource patch bay.
Building a new meter bridge
Obviously there is not enough space in the meter bridge to accommodate 30+ PCBs like the one I had used for the PFL fix up, secondly those channel strips have no metering rectifier circuit on board, only the group and main out modules have those already on board. And lastly: There is limited power available! This drove the decision to create a full custom meter PCB dedicated to the use in this system. I copied the precision rectifier circuitry (like in service for the group modules) and put it along side to a LM 3915/3916 log scale driver chain. Those two chips cascade very nicely and by picking a good handover point you can precisely mimic the TAC scaling (and drive 15 LEDs) Then I fitted 30 of those units into the meter bridge, perfectly lining up with the input slices, repurposed 8 of the original meters for the groups and had space for another 3 slices to cover PFL, Mains out Left and Right. For power saving purposes I am running the LEDs off of a 3V power rail, which is generated via small switch mode buck converters from the 17.5V console rail. The signal is picked up at the direct out socket at the backplane socket board and brought up into the meter bridge, trimmers at the meters allow individual calibration to 0dB.
A new acrylic display cover got a full custom silk screen print to mimic the original TAC look and feel. Designing the PCB, assembly of the 30 plus units, mechanical integration and electrical integration with the system was a a total of 6 weeks plus some lead times for the PCB and the silk screen.
Building the patch bay
My son was able to get hold of a set of 5 TT phone 96 socket bantam patch modules, which came with a bulk of thick multicore cables directly soldered to the TT sockets. This cable „tree“ was non practical for our system, so we decided for a DSUB25 based wiring scheme, yet had to outfit the patch modules with DSUB25 connectors. Wiring up and soldering 5 x 96 TT sockets to 5x 12 DSUBs took quite some time, likewise building the 30 plus custom DSUB25 to 8xTRS or 8x XLR multicore assemblies is time consuming, but by the end we have a fully modular, easy to maintain and robust full up patch bay system:
48 way line input with 48 way DAC feed from DAW (half normal)
48 way insert send and 48 way receive (half normal)
48 way output with 48 way ADC feed to DAW
2x 48 way send/48 return outboard patch array
This array got integrated into a side rack shaped to line up with the TAC console. The side rack also takes the ADC/DAC multichannel units and the TAC PSU plus a set of compressor units of different kind.
After about 2,5 month of hard work the hole system is now fully functional, has been intensely tested, a bunch of tracks and stems are going across it as we speak.
What’s next? More outboard equipment is knocking at the door.
And someone is whispering into my left ear: Don´t you want to do an OpAmp upgrade to the TAC?
Maybe that will be the third go around, for now I am entirely happy with the vintage sound of this 30plus year old baby as it is.
I put a bunch of pictures into the attachment, so if that peaks your interest let me know!
Kindly
Uli
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