Powertran Transcendent 2000
|| External CV Tuner | Gate/Trigger Switch ||
|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:|
|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.|
|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.
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):|
|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!)
|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).|
|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!)