Sunday, May 1, 2011

Monotron-Squared - problems update

Dual Oscillator Problem Fix:
So I found a way to fix the dual oscillator connection, and combined with a 260k potentiometer, will also adjust the range of the second VCO. This should work in a similar fashion to the range adjustment for the monotrons' ribbon "keyboard", except it'll be adjusting the range of the incoming CV signal.

The connections I am using are C5 of monotron 1 (there is no capacitor here, just the terminals, and labeled NU on the schematic from Korg), and the middle bottom pin of Q1 from monotron 2 (which I think is pin 2, but could be pin 4 - have no data sheets on that dual transistor - don't know which way is up on it either!).
Between these connections I put said potentiometer - only had 500k, so added a 550k in parallel to make it about 260k - anything less than 100k has a really widely spread range, and more then 300k is also useless. Still, this will be neat for adding layers of notes.

The pitch knob is still operational on second VCO (monotron 2), but the most exciting thing is that any pitch bending on VCO 1 (i.e., from the LFO, or CV pitch bend) is also received at VCO2!!

CV Mixer Section:
Since I'm still a bit crap at soldering, I often file the gaps between the copper strips on the pre-fab experiment boards to make sure there are no shorts. Since I disconnected and reconnected a few times, I had filed kind of sloppily and had actually divided one of the power rails!! What a doughnut! So I reconnected this using a blob of solder.
Now, when things were not working properly, I still had the polarizing CV mixer section working, but since filing the power rail I also dropped the LM358 op-amp on the floor! I still have not hooked it up again and may need a replacement. I do have a NE5532, but its minimum supply voltage is +/- 5V. The monotrons' Vcc is 5V, but I have measured only 4.67V and have concerns about this operating inefficiently. Therefore I'll probably just replace the LM358 if need be.

LFO Routing:
I have since stripped down most of my re-route switches for LFO 2 (LFO from monotron 2) but have a short between the switches I re-did. I used the soldering iron to shrink the heat shrink, and I got a little solder in this, so I think it'll be an easy fix.

VCO 2 Routing:
Since I'm re-doing so much, I decided to try and flip the output of VCO 2 from before the square wave voicing circuit and after it, so that in the former connectivity the dual sawtooth waveform will get some square shaping, and the latter connection will combine square and sawtooth waves. Also, I have a connection to take VCO 2 directly back to VCF 2 - which is then equivalent to simply mixing two individual monotrons - each being played from the same CV notes.
This should give some really fat tones and harmonics - in fact, with the triggered noise gate output, mixing VCF 2 out put with VCO1, with some LFO and stuff, and lots of resonance, I'm already getting some dark and twisted overtones layering up the single voice. Its pretty awesome.

So, all I have to do is wire/solder CV 260k pot between monotrons; re-do CV mixer inputs and output; check LM358 op-amp is working - if not: replace it; clean up or fix LFO routing switches; and, drill out one more hole in the front panel to accommodate 260k pot.

In the future I may try to reinstate the ribbon controller, as its quite fun to use, and great for wobbly effects and sweeps, etc. But to do this I need to get hold of a ribbon cable connector - I already have an adequate piece of ribbon cable that I stripped out of a busted DVD player, and it is compatible with the monotron ribbon cable connector. But not sure if I can be bothered just yet...

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