The Roland SH-101 was produced in the early to mid-eighties.  It's a monophonic analog synth with a simple sequencer built in.  The oscillators are based on the now obsolete CEM-3340 VCO IC.   In it's stock form the SH-101 has sawtooth and pulse waves available, as well as a pulse/saw LFO with sample and hold and a noise source.   There are many simple mods that can be added to the SH-101.  I chose to keep things rather simple, since I already have a modular for creating complex pads and soundscapes - The 101 is used mostly as a bass and lead synth.  If this were my only analog, I'd probably go a little more extreme, creating a synth like the NovaMod-101.

More Modulation:

One of the biggest limitations of the SH-101 is the fact that it only has a single LFO for modulation.  Also, when using the sequencer, the LFO is used as the tempo source, limiting it's usefulness, in my opinion.  I added a second LFO with Saw and Square waveshapes that can be used to modulate most of the parameters of the synth.  LFO-2  is based heavily on EFM's LFO-2A, and can be used for modulating PWM, filter cutoff and VCA level.  Currently I have not provided means for LFO sync, but I intend to add a sync circuit later.  I will use the common FET switch method like the one used in the ASM-1 LFO.

I added a switch  to choose between 2 different timing caps.  The first position is the "standard" LFO range for slow modulations.  The second position is a much smaller capacitor, allowing LFO-2 to oscillate up into regular audio frequencies. Using this audio-frequency modulation on the VCF produces bell-like clangorous noises that sound similar to a ring modulator.

The schematics and routing diagram for the new LFO-2 are available in PDF format.  There are still a few component values missing, since I haven't yet had time to route LFO-2 to the PWM and pitch modulation yet.  Also, there are references to envelope follower 1 and 2 on the schematic.  I plan to add envelope followers to each of the two new audio inputs at a later time.  More details about the external audio inputs are located in the next section.

A second ADSR-style envelope generator is also in the works.  This second EG will be normalized to control the VCF, with other selectable destinations (Pitch and Pulse Width modulation for starters).  This will leave the stock ADSR envelope generator for VCA modulation.

To make the SH-101 even more flexible, I've performed 2 modifications to the original LFO.  First, I duplicated the high / low rate switch that I used on LFO-2.  This is very easy to achieve: remove the LFO's timing cap from the circuit board (C9).  Run two wires from these points to the "blade"  or center connections on a DPDT switch.  Put the original timing cap across one end of the switch, and a smaller cap (I used .0047uF Poly film) on the other end of the switch.   The original cap is the "slow" rate while the new, smaller cap is the "fast" rate.

A second switch was also added to the stock LFO that allows me to switch between white and pink noise (pink is stock).  This lets me to create more convincing cymbal and snare sounds.  This is also an ultra simple modification:   Capacitor C31 helps form a low pass filter on the noise source.  Leaving the cap in will produce pink noise, while removing it allows unfiltered white noise to pass through.  I removed one leg of the cap from the circuit board, running it to one side of a SPST switch.  The other side of the switch connects back to the hole where you removed the cap leg. 

Triangle Wave and External Inputs:

As I mentioned earlier, the SH-101 oscillator is based on the CEM-3340 VCO.  The CEM-3340 offers Sawtooth, Pulse and Triangle waves, but Roland chose not to utilize the Triangle wave in the 101.  It's a rather simple mod to add the Tri-wave to the existing mixer section (Pre-VCF) of the SH-101. 

While I was tampering with this mixer circuit, I chose to add 2 external audio inputs as well.  Since these inputs are pre-filter, you can use the 101 to mutate external sources like drum machines, samplers or other synths.

The schematics for my Mixer Modifications are available in PDF format.

VCA Overdrive:

Overdriving the VCA adds a nice gritty distortion to the 101's sound.  Again, this is another simple modification.  I removed resistor R130 and wired a 100KB pot in it's place.  I tied the wiper to one of the end lugs to help get a more useable curve.  It is also possible to leave R130 in place, and put a 100KA taper in parallel with it.  Please note that as you increase the overdrive level, the volume of the synth also gets louder.  I'm experimenting with dual-gang 100KA pots, trying to find a good way to decrease the level proportionally as you increase the drive.