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External CV Pitch & Octave "Tuner"
Problem: When using External CV, the T2k is not in tune. I need to use transpose and fine tune and scale on my Midi-to-CV Converter (a Kenton Pro-Solo, in this case) to make it work properly.
OK, I could save the settings on the Pro-Solo, but if I use the Pro-Solo with other synths, then i'd have to bloody tune it again.
I saw on a Japanese site that a guy has built special circuits for CV-Gate inputs for his synths. At first I thought they're for routing the gate to REPEAT & other functions [so that external gate would work with these onboard functions] (as most CV-Gate sockets kits don't actually cater for this). 
But he also has these circuits for the CV which has a trimmer for fine tuning the CV.
This seems to be what I have been looking for - a CV Pitch Offset tuner. However, I followed some of these circuits and built a stripboard to see if it works on the T2k, but it didn't. This kinda pissed me off.
The circuits are op-amp based, seemed to be just offsetting the gains and inverting etc.
I've long forgotten my Electronics theories, and i realized i needed to understand what's happening if i really wanna solve this T2k problem. An evening with my old electronic books refreshed my memories on how Op-amps work...
Here's what i eventually come up with:

Click to Enlarge!

How It Works...  (<- sounds very ETI/Maplins, doesn't it!)
Well, a dual Op-amp is used, 1st part acting as an invertor gain/adder circuit, the 2nd part is just inverting it back.
The Pitch Offset Pot or Trimmer actually goes to +12V in one end and -12V to the other, so we're feeding +/- voltage to the incoming CV. Normally you'd use 2 resistors of the same value each side. But somehow, when I tested this, the incoming CV is way too high, middle C on my midi keyboard is like 2 octave higher, so i figured it probably needs more -ve voltage (hence the resistor is reduced to around 5k). The other parts and the Octave Width Adjust are just purely fiddling with the gain of the resistance ratios.

Again, I did this on a stripboard:

Click to Enlarge!

I de-solder the GREY wire from the TIP of the Ext CV Input Jack, solder a new wire between the jack and the EXT CV IN on the stripboard, the CV OUT from the stripboard then goes back to the GREY wire.

+12V/0V/-12V are easily obtained from the power supply board on the back panel - they're the Red/Green/Blue wires (which also goes to the VCF Pedal jack, but with resistors on the ends).

Again I used a connector mounted on to the stripboard, so that it's detachable if i need to remove the back panel. 

The T2k handles its own keyboard CV and the External CV at different points on the PCB. Probably because they're calibrated differently (but on my T2k, the Ext CV is still somehow not very much in tune). 

Well, the internal Keyboard CV actually goes to the Portamento Pot, whereas the External CV doesn't.
Since we're re-calibrating/tuning the CV, I thought I might be able to tune it to the same calibration/scale as the Key CV, so I could tap it to where the Key CV comes in and have Portamento as well.

Tested it, tuned/trimmed...  and yes, it could be done. Here's how...

I still use the Ext CV jack, but when it comes to the PCB, i de-solder the resistor [where it joins to] and add a wire to bring it to the Key CV input pin. See diagram:

The EXT CV comes in from the middle 5-pin connector and has a PCB routing to the resistor R40.
So i de-soldered R40, insulate the empty leg, and solder a new wire into it, and run it to the Key CV Common pin on the left.

The KEY CV COMMON pin is the TOP pin of the left 5-pin connector. On the connector, it's the yellow pin. See below:

A more detailed image (with instructions + Gate/Trigger jumper - see next part):

Click to Enlarge!

Then it's time to test the circuit. I don't have any oscilloscope or test/cal equipment, so i just use a pocket Chromatic Tuner (yes, a guitar tuner!)

(If you're not connecting to the KEY CV, then skip to step #3)

  1. Unplug the keyboard 5-pin connector. 
  2. Because of the Trigger issue (see next part) you have to short the -12V pin and the "To +12" pin on the left 5-pin connector, see the above detailed image.
  3. Connect your Midi-CV converter, place the CV Tuner circuit board close to the front panel (I just put a cardboard on top of the panel and place it there), get yourself a small screwdriver ready to tweak the trimmers!
  4. Plug the output to your mixer and also send a signal to a chromatic tuner/guitar tuner. Turn on the synth, make sure the Pitch Bend Pot is at its middle, and Octave Switch is set to middle OFF position. Select the Square wave, Shape 0 and no modulation, also, keep all the Pitch Control knobs down. Then play a C note on your midi keyboard.
  5. You'll probably hear a hi-pitch tone or nothing (because it's too high!). Adjust the Pitch Offset 20k Pot/Trimmer until you get to a comfortable octave (relative to the midi key/octave you're playing).
  6. Now play octave notes (the original C key followed by the C an octave higher) on your midi keyboard. You'll hear either the actual octave range is probably less than an octave or high C is too high. Now adjust the Octave Width 20k trimmer/pot and use your ears to try get near to an octave. Check the tuner to see if it's close to an octave.
  7. Then, hold the original C note. Re-adjust Pitch Offset 20k Pot/Trimmer to tune this to C on the chromatic tuner. Use the Fine Tune (1k/5k trimmer) to help.
  8. Play octaves again and readjust/fine tune the Octave Width.
  9. Repeat until you get perfect tuning AND octave.
Note: Only 1 drawback of this mod: the onboard (-2/0/+2) octave switch would result a slight drift in tuning. But since we're dealing with External CV, you'd probably won't need to use the octave transpose switch (you can always transpose the incoming external CV beforehand).
If all is well, then we can now mount the stripboard. I mounted it on the top underside of the PCB, using the spacer above the PCB screw. See images:

So it'll be like this when turned upside down:



Gate/Trigger Switch or Jumper
Somehow on my T2k, for External CV/Gate to work, i needed to hold down 1 key on the onboard keyboard. It's probably because, the original key S-trigger works when it drops past 0V to -ve voltage.
Short the -12V and the "To +12"  pins of the keyboard 5-pin connector (at the left of the PCB).
Before I planned to remove the keyboard, I actually did this with a switch, so if you're not planning to remove the keyboard, then you should add a switch to short the 2 pins (See Left Pic below).

Click to Enlarge!

Click to Enlarge!

Since I planned to remove the keyboard, and now that Ext CV tuning is sorted out, I simply just put a short wire shorting the 2 pins, eliminating the switch (see right Pic).



Next: T2k Keyboard Removal (Module-fied!)


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