Second only to the MIDI to CV circuit, this is the most complex module of this synthesizer. Most of the complexity is due to the fact that it’s a 16 step sequencer. That means to control each step there must be pots and switches. The original design of this panel had linear steps. The LEDs went left to right as one would read of book. I thought that was rather boring and decided to make a rotary design instead. I also thought that its operation would be a bit more intuitive for the user. The user will immediately understand that the step pattern is cyclical and the various controls determine what happens to each step in that cycle. The first two photos below are the incomplete layout. As shown in later pictures, two more jacks, two more pots and the ring of LEDs were added.
There was no way I was going to install freehand all of these components and not have it be ugly. I decided for the cardboard layout and especially the later metal layout, it would be best to make a drill template. In Adobe Illustrator I’ve created to-scale symbols for things like pots and jacks. This enabled me to accurately position components on the artboard. I then made circular patterns to get the nice looking rotary pattern for the potentiometers and holes for the LEDs. The holes for the rest of the components I also included on the drill template to make everything easy for drilling. I made the module 12″ wide (8 3/4″ tall), but I might make it more like 13″ or 14″ since I still need to find spots for the buttons for each jack. (The extra buttons are for the pre-patch panel.) 
I started by setting up the wiring that only goes on the panel. As with previous modules, I used solid core wire for this step. The wires going to the circuit board were obviously stranded cable. Some components like diodes and resistors were mounted directly to switches and pots on the panel. This freed up room on a rather crowded circuit board. 
There were four pages of schematics for this thing. The first three concerned the digital section. Basically, this meant interpreting the input from the switches on the panel and generating the correct counting and patterns. These patterns were fed to the analog section (fourth schematic) which cycles through all of the pairs of potentiometers on the panel. A common and stable reference voltage is fed to all of the pots at once. The 4067 chip is what is called a multiplexer. It lets through one input at a time depending on the state of the digital input feeding it. The voltage fed to each pot is reduced by voltage divider action depending on how the pots are set. The output is fed to a scaling amplifier that feeds the portamento control and the control voltage (CV) amplifiers. The CVs are what feed the VCOs and tell them what notes to play. The trigger/gate voltages used for triggering the ADSR/VCAs are generated in the digital section.
This is the finished circuit board before the ICs and wires were added. Bear in mind, the original design of this circuit called for two different PCBs of similar size to the perfboard I used here. I managed to fit both analog and digital sections (4 pages of schematics) onto only one perfboard with my wire-wrap technique. Take that two-layer PCB design! 
I began installing the wires from the bank of potentiometers and switches first. Both were inevitably thick necks of wire that double somewhat as carrying handles. I had six different colors of stranded wire available. I worked out a color-coding system to help things go slightly faster when assembling. Masking tape and a continuity checker were invaluable during this stage. On the large wire bundles, I would label the first wire to aid placement of the other wires.
After the switches and pots were done, I went through and brought over each section of input/output jacks and pushbuttons. I ran a wire from the +Vcc buss (+15V) to three of the four pushbottons. The pushbuttons and the rotary switches were then both soldered into the board. The same process was done with each of the jacks.
There are two rotary switches. One controls the mode of the sequencer, the other controls how many steps it counts to during its cycle. The modes are: 16 step, 16 step select (the other rotary switch controls how many steps are selected), stop step (the sequence goes until the step count is reached and then stops), and reverse (more like forward and reverse). The pushbuttons at the top are reset, run/stop, step back, step forward as read form left to right. 
***The knobs on the rotary switches are temporary.
The bank of switches on the top right of the panel below the pushbuttons controls which steps will sound out during the sequence. However, the switches in no way affect the CV output of the sequencer. For some reason, they only affect the trigger and gate outputs. This means that in order for the switches to do anything, the trigger output must be hooked up to an ADSR unit driving a VCA. In other words, the sequencer will tell the VCA when not to let the notes through based on the switches. Unfortunately, this means that in order to get the full functionality of this module, one must have at minimum a VCO, an envelope generator of some kind like the ADSR and a VCA in addition to the sequencer. However, this does mean that “disabled” steps or notes can be made to be quieter, silent, or the same volume as the other notes/steps. This can make for some interesting possibilities.
For the video below, I used all of the modules I’ve made so far: The VCO, low-pass filter, ADSR and VCA, and of course the sequencer. The filter added some more pizazz and character to the sound coming from the VCO. I labeled the sequencer and made marks on the potentiometer shafts to remind me of their positions.
The Tube Roaster 