SH-101 with Novamod VCF slider range
2015-08-22 by Peter Mörck
Hi,
Anybody got an SH-101 with Novamod?
I'm servicing an SH-101 with Novamod installed and I find that the VCF cutoff frequency is sort of offset down so that the VCF, even with the slider at 10, totally cuts the sound off (ENV, MOD, KEY sliders at 0).
I suspect that the Novamod is messing up the VCF CV at the point where all VCF voltages are summed (R184-R187 junction) since it adds a path to ground via a resistor+pot. You can't just add another resistor to a summer without reducing the "contribution" of the other voltages from the existing resistors (e.g. VCF slider won't open the filter as much as it usually does) so I suspect this is what's happening.
If I use the Novamod settings and set the VCF mod source to "none" and VCF FM Amount to max it aaaalmost opens the filter completely. I assume this must be because the resistance of the FM mod path (100k resistor + 100k pot, pg 10 in PDF) is greatest at that point, thus not lowering the summed VCF CV as much.
Also, the ext audio in to VCF CV (pg 11) would cause the same problem I guess, in particular both of them together as it adds a parallel path to ground.
Increasing those resistor values might work, but it would still affect the VCF and would make the mod affect the VCF less, so I';m not sure how useful it would be...
There is a calibration procedure involving the VCF in the Novamod pdf, but at a quick glance that seemed to be more related to getting the VCF keyboard scaling right so I don't think that's gonna help. I haven't tried it yet (don't wanna mess with calibrations unless needed).
I disconnected the Novamod VCF FM, Ext audio VCF and Ext VCF CV wires from the R184-R187 junction and the VCF slider responds as expected and opens fully (well, like 99%) at max with all other sliders at 0 so I am pretty sure there's nothing wrong with it in its default configuration.
TL;DR:
Is it normal that the VCF doesn't open (at all) when using only the VCF slider when Novamod is installed and all VCF mod sliders are at 0 (and FM Amount knob is 0)?
Thanks.