3031 - Modifications
Power supply modification
The standard 3031 power supply design limits the +12V power supply current to approx 120mA. While this was sufficient to run the 3031 itself (~45mA) and the MIDI to CV converter (~25mA), I also wanted to power the JoyKontrol from the same PSU. The JoyKontrol was to live in the rack space above the 3031 and I was being lazy and not wanting to build yet another power supply!
Having an LED display, the JoyKontrol is quite power hungry and needs up to 90mA from the +12V power supply. So I had to increase the current limit of the 3031 - this was very easily done by added another 10R resistor in parallel with R142 and R145. This pushes the current limit up to 180mA, a comfortable 20mA more than what is needed.
The small clip-on heatsink used on the TIP31 power transistor only gets slightly warm even after running for a long time at a full 180mA, so I'm sure the 3031 power supply could be modified to give significantly more current than this if the mains transformer is man enough.
The only other consideration when drawing more current from the 3031 power supply is whether the 1000uF smoothing capacitors are enough to supply the extra current without letting ripple through. With the mains transformer I am using, and drawing 180mA I could see no trace of ripple on the power rail.
Of course your mileage may vary: the AC voltage of the mains transformer makes a difference - too high a voltage and the TIP31 will dissipate more heat, and too low a voltage and bigger smoothing capacitors may be needed. Remember also that electrolytic capacitors degrade over time, so it's good practice to use a bigger value than is needed.
VCO Wave switch modification
This is a simple modification, but well worth it if you intend to use the external input to the 3031 VCF to process external sounds. If an On-Off-On switch is used for the VCO Wave select switch (rather than the normal On-On switch), when the switch is in the centre position only the external input is heard, and the internal VCO is disabled. The 3031 still needs to be triggered though to open the VCA and VCF envelopes. Unfortunately I didn't think of this modification until after I had already assembled my 3031, and since I can't be bothered to take it all to pieces again to change the switch I have to carefully 'balance' the switch in the centre position to get the internal VCO to turn off.
Another useful modification would be to add an opamp buffer on the external input to boost the signal level - without such a buffer the input level needs to be very hot (something like the output level of a modular synth VCO).
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