Download Blacet 2100 VCO modification instructions and

Blacet 2100 VCO modification instructions and options:
During construction:
 do not install the test / tune switch S2.
 do not install the optional jumper below C2.
 do not install pot R4 (wave) on the PCB.
 for MOTM format installation, do not install any pots.
 Read next paragraph below before installing R57.
 Read paragraph below that before installing LEDs – D4, D5, and D6
At position R4, install a jumper that connects pins 1 and 2. Pin 3 is left not connected.
This connects the open end of R57 to ground providing a fixed gain on the OTA - U8a.
If you wish, change the value of R57 to get the wave amplitude you want. Making R57
smaller will make the amplitude of the shape / wave output smaller. Increasing the value
will make amplitude larger. It does not take much change to significantly increase the
amplitude and start the circuit clipping. Anything over 150K is probably useless. I
decided to leave mine the same. This leaves a wave before shaping at a lower
amplitude than the normal outputs. But, the shaping process does indeed increase the
amplitude of the signal. So, this pre-shaping amplitude seemed reasonable to me. If
you do increase the value of R57, you will most certainly have to omit D4, D5, and D6.
See next paragraph.
Installation of D4, D5, and D6 is optional. Since I already had mine installed before
deciding to do this modification, I left them in for the testing. Basically, no matter what
kind of shaping you do using this VCA method, you will increase the amplitude of the
signal. D4, D4, and D6 introduce clipping at around 10 volts peak to peak. So, they
serve to limit and distort any wave attempting to exceed this threshold. So, if you do
want to limit your output, install these diodes. If you wish to be able to shape waves
that may exceed 10 volts peak to peak without being distorted, leave them out. It is
certainly possible to install a switch to select between limiting and not limiting for testing
or in case you were crazy enough to want to install another switch on the front panel.
To implement that function, cut the trace between the left side of R74 and the left side
of D4. This disconnects the clipping. Install your SPST switch connecting to the left
side of R74 and D4. If you like the clipping, connect these two back together. If you
want the clipping disabled, leave this connection open.
Once normal construction per Blacet instruction is complete, add new S2 (SPDT center
off) by connecting to the S2 switch pads as follows:
 upper right pad (not used – was for switch mounting only)
 lower left pad (not used – was for switch mounting only)
 upper left pad – connects to center ( # 2) terminal of new SPDT switch 2
 lower left pad – connects to bottom ( # 3) terminal of new SPDT switch 2
Add one additional wire to the new SPDT switch # 2. This switch will connect from the
top ( # 1) terminal of new SPDT switch 2 to the left side of R65 (next to C8).
Blacet 2100 VCO modifications (page 2)
Your new switch # 2 will function exactly as the test/tune switch did for check out and
calibration except instead of only having octave-up (+) test, you also have octave down
test. I calibrated my VCO with exact octave tuning on test instead of setting exactly 1.0
volts. A small about of error in the CV summer can cause this number not to be exactly
1.0 volts.
To implement the Blacet method of shaping, simply connected a variable (+) DC voltage
to the WAVE input and you will see that is does no more than vary the gain of the OTA
and hence amplitude of the WAVE signal.
Without any additional modification, you can run any of your output waves into the wave
input for shaping. However, you will first need to run them through a reversing
attenuator, or you will drive the wave directly to heavy clipping. See the section on
dealing with reversing attenuator gain on page 3 as you will want to make note of the
gain on your external attenuator during this testing to select the feedback resistor on
your daughter board.
Essentially, the only modification up to this point is to move the octave switch off board,
and make it work both up and down. Plus we removed the internal control to add
variable bias the OTA. So, I think this is a good spot for checkout and calibration, to
make sure everything is working fine before moving ahead to the shape wave selector
switch.
Once you have completed testing and calibration to your satisfaction and know that
everything is working OK, install the S4 shape selector switch modification as follows:
 cut trace just above the left side of R80. This disconnects R80 from R77.
 connect wire from left side of R80 to new switch S4 – common (output select)
 connect wire from left side of R77 to the “TRI” position of S4
 connect wire from right side of R72 to the “SINE” position of S4
 connect wire from right side of R53 to the “SAW” position of S4
My instructions for S4 are generic at this point. Once I get my SP3T switch, I will make
them more specific to that switch. This completes the shape wave sector modification.
For AC/DC selection of the FM input. Simply connect the two terminals of a SPST
switch to the to “optional” jumper pads just below C2. The open switch is AC, the
closed switch shorts C2 and the input is now DC.
Lastly, a simple reversing attenuator will need to be fashioned on a separate PCB for
the SHAPE input and shape control. This PCB will insert by having its input come from
the SHAPE jack and its output will connect to the J6 wave CV input on the PCB. I am
simply using the simple one amp reversing attenuator from Tony Allgood’s multi-mix
design. On the input, use a 50K or 100K pot and match the input resistor (between the
(-) amp input and pin 3 of the pot to the approximate size of the pot you use. I’ll
Blacet 2100 VCO modifications (page 2)
probably use a 50K pot and 47K to 51K resistor in there. Then, you have to decide the
size of the feedback resistor. I am leaning away from unity gain. Here is how to judge
the size you want:
The feedback resistor sets the gain of the amp in relation to the input resistor which is
fixed in this design. So, make it smaller for less gain and larger for more gain. Here is
how to determine what gain you want:
Hook the VCO up with its own output waves used for modulating the wave signal as we
did in paragraph 3 on page 2 for testing and check out (if you did not take good notes
the first time). While watching your waves on the scope, try out the different modulating
possibilities (each of the 3 waves modulated by the 4 possibilities). Notice the position
of the gain knob on your reversing attenuator as you get the results you desire. You
might as well scale your reversing attenuator so the max setting is the max usable
range that you like. Lets say for example, that 50% gain is what you decided. Then
simply make your feedback resistor 2X the input resistor (and pot) size. In my example,
that would be 100K for that amount of gain. Your gain decision my be influenced by
your decision whether to include the LED clipping.
The last item to discuss is the normalling of the SHAPE input jack. If you want to
restore the original Blacet function, you could normal the 10 volts over to the switch on
that input jack. However, you don’t want to make that connection directly, or you would
be shorting your audio outputs directly to10 volts briefly each time you jacked into the
shape jack. A 10K resistor would be a nice protection. So, you could connect a wire
with a 10K resistor in series from the unused pin 3 at R4 to the switch on the shape jack
to restore the original Blacet shaping function. Or, you could normalize your favorite
shaping wave over to that switch.
Larry Hendry