Apologies for not providing any further updates/ or pending mod descriptions for this project any longer. I describe many mods here (more than the classic mods for the original), including mistakes and spelling glitches, and feel I rather want to spend time on other synth modding/development projects.
Behringer 110 VCO/VCF/VCA Module Modifications
This site covers a set of modifications for increasing the functionality of the 110 VCO/VCF/VCA Module in Behringer’s line of Roland System 100M recreations. While adding VCO synchronisation as known from the 112 Module, I added also some SH101-inspired sub oscillator with a little twist, an additional bandpass filter response and VCA overdrive. The Roland schematics scans on yusynth were helpful for doing that: http://yusynth.net/gear/R100_en.html.
NB that this guide/information is for non-commerical use only.
For implementing these mods, you can either do this on switched jack sockets, so that (most of) the original functions remain intact or replace some jack sockets with switches (as I did first and describe in the PDF linked below). If you plan on using an extension panel, you have, of course more freedom in arranging your interfacing.
Some soud demos here and more to come: https://soundcloud.com/uibkmedan/system-h
You can download the info contained here also as a PDF: SYSTEM-H GUIDE (NB THIS IS ALEADY A BIT OUTDATED).
SUBOSCILLATOR (updated version)
First, i used a SH-101-based design here but just tested a version known from Jen SX1000 mods, and this is easier and uses less components. Basically, use this circuit here as designed by Neil Johnson. As your input waveform, use the SAW signal from the Waveform select switch.
Feeding your sub osc back in: if you don’t use an extension panel, I suggest use the -1oct output from your sub and feed it into FILT IN2, using a switched jack socket. The existing jack socket is a switched type already, yet, unfortunately, the normal pin of that socket is grounded. So, you need to replace teh existing one with a CLIFF FCR1281 or similar. This way, you have your main vco normalled to FIlt IN1 and your sub to FILT IN2. When you plug an external signal to FILT IN2, this breaks the signal flow from the sub, so you keep the original functionality while having a sub osc under “the hood.”
VCO HARDSYNC INPUT
While the 112 Dual VCO Module offers inputs for VCO synchronisation, this option is missing here, so I adapted the sync in circuit from the M-112 schematic. As I did not want an extra “strong”/“weak” switch, I just wired “strong”. Moreover, I ended up using a 1nf cap at the input for the master VCO, which gave better results with various VCOs and waveforms than the 100pf in the schematic.
Note that while the added circuitry is minimal, connecting it to the 110 module itself is fiddly. Best connect a thin wire to IC8, PIN6, which you then connect to your extra components lodged on a small PCB attached to the main PCB with an M2-spacer.
As for locating your sync in socket: I repurposed the VCA LOW output for mine in the end.
6DB LOWPASS and BANDPASS FILTER MODES
This one is easy to wire up and sounds great in my opinion. You wire the -6db stage of the filter to VCA SIG IN2, so that when you turn its level up in parallel to that of the normal filter output (-24db) you create a bandpass response through filter pole mixing. (Since the -6db out signal has a different polarity, phase cancellation subtracts the lows from the -24db out when both are at the same level.) If you just dial up the -6db signal, you have a one-pole lowpass filter resonse. So, with this mod, you can have three different “falvours” of filtering in total. You can tap the -6db out at the source of T18 (Roland schematic “Q7” on the picture below).
On my module I wired the -6db filter signal to VCA SIG IN2 VCA IN2 (via a switch that also controls the VCA overdrive, see below), so by just turning VCA IN 1 and/or 2 you can have 24db lowpas, 6db lowpass or bandpass. So, depending on your choice, remove the VCA SIG IN2 jack entirely or decouple the NC (normal) lug of that socket from ground. Tap the source of T18 (lower right “leg”) and feed it via a 1k resistor to the PCB terminal for VCA IN2 tip lug. If you want a really prim and proper version, go with your -6db signal through an op-amp before feeding it into VCA SIG IN2. Next, you need to match the maximum level of this signal with that of the 24db out present at VCA IN1. If no op-amp is used, wire a 210k-220k resistor in parallel with R68 (180k).
More on pole mixing filters here: https://electricdruid.net/multimode-filters-part-2-pole-mixing-filters/ and here: https://mutable-instruments.net/archive/shruthi/build/4pm/.
Update: I revised the interfacing and use a switched jack socket now rather than a switch. The existing socket needs to go out, and in goes a CLIFF FCR1281 or similar.
Further update: tested a cleaner version than the passive mixing method described above and will describe it briefly here. You need a dual op-amp. Take the -6db out and send it through a simple op-amp buffer (non-inverting amplifier with a gain factor of 1). Take the -12db out and send it through an inverting amplifier (the other half of your op amp) with a gain factor of -1. Mix both op amp outputs with 15k resistors and send them to VCA IN2. Lower the resistance of R68 (180k) by wiring a 100k resistor in parallel. Sounds cleaner than the version above, yet I like both.
VCA OVERDRIVE (FEEDBACK)
I’ve always been fascinated by CYKONG’s VCA drive mod on the SH101 and did a similar thing on the MS-1. This one is easy and sounds close, even though it is technically a bit different. What you do is you wire the VCA out back to the VCA IN. You can do this by wiring the *anode* of C10 via a 33k (or lower) resistor to the VCA IN2 socket. If you want to keep the socket, you need to free the normal pin from all connections to ground. Since I have the bandpass filter mod on my module, I took the VCA IN2 socket out anyways and use a switch to toggle between bandpass option and overdrive option. So, wire to VCA IN2, turn up the level of that input and tear away…
VCO OUT/DIRECT FILTER OUT
I personally don’t see the VCA OUT LOW finding much use in my studio (might be different if you use a lot of guitar pedals and such), so I rewired this as a VCO OUT (see VCO Sync section on why). Even if you don’t need that (extra) VCO out, a direct filter output might be handy.
First decouple the LOW out jack socket by taking out R62 and R63. For connecting VCO out, take the signal from the waveform select switch (see above). For connecting to VCF out, connect the NC (normal) lug of the VCA SIG IN1 socket to the LOW OUT tip lug via a 1k resistor (M-121 has no output resistor, but why not using one?).
Transferred Schematics (incomplete)
If you do and like these mods, make a small donation to a charity helping refugees, and if you have suggestions for improvements, be in touch via the contact form.