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DAC (Revised March 1, 2007)
The scan count is buffered by a 74HC541 Octal Tristate buffer. It's actually 2 quad buffers with /Enables for each. Its Tristate capability is not used: it merely buffers the counter bits before presenting them to the R/2R ladder, which is followed by an inverting op amp. A 5-octave keyboard would generate 0 to -5 volts at the output of OA1 which, when inverted again by OA2, becomes 0 to +5V.
All well and good, but if I want to shift up one octave (+1v), or 'bend' up however much, the maximum +/-5V permitted by the 74HC4316 analog switch could be exceeded. So to keep everything within limits, with J1, J2 and J3 removed, the Scale1 trimmer would be adjusted to get the correct range at tp1: 0 to -5 for 5 octaves, 0 to -4 for four, etc. Then install J1 and adjust the offset trimmer to center the output of OA2 around zero: +/-2V for a 4-octave keyboard, +/-2.5V for 5 octaves, +/-1.5 for 3 octaves, etc. Then jumper J2 can be installed and the correct value resistors found for Octave UP/DN and Tune or Bend. Then jumper J3 can safely be installed, connecting OA2's output to the 4316(s). The Scale 2 trimpot may not be necessary.
For this adjustment procedure to work a temporary change must be made to the operation of the counter/scanner circuit, and to do that, that design had to change a little.