Calibration Penrose - not behaving quite linearly

Am calibrating a penrose for someone else and it is not behaving as I would have expected, hope someone can shed some light on what the issue could be?
I get how to do rubber banding calibration and have done it before a lot with other gear. I've tuned a perfect c and fed a steady 0v into the quantiser's input. I've increased the input voltage to jump to the next octave. I've trimmed to get another perfect c an octave higher than the first. Returned to the first octave and adjusted tuning again so is perfect and then gone up and retrimmed so is dead on.
Now, the problem is that there seems to be a drop off after the first c, c# is like 20 cents flat - but this flatness gets slightly less as you go up the scale until it becomes sharp after the next c, which to me suggests an offset somewhere. Nb. The c0 seems to be giving out 0v as expected, plugging and unplugging it to an oscillator does not change the pitch. Obviously I can redo the trim procedure starting from another note, say d0 - which is is where I am now, every note is within a few cents of perfect across the whole range apart from c0 which is sharp by 17 cents). Any ideas how to flatten this c or alternately add an offset to every other note so I can trim properly? What could be causing this, maybe a resistor somewhere out of expected tolerance range? Would be nice to go back to him with an explanation of the problem or, even better, a solution if possible.
Hope that explanation is clear enough and tia for any help!


  • Should add he assembled this from a thonk kit and is a qualified EE - am doing him a favour as he's away with work atm.
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