This entire project was to build a synthesizer as it would have been done in the 1980s with no modern microcontroller.
Most chips are from the era and have an 80s date stamp.
Z80 CPU running at 4MHz
8KB ROM (program approx 3.5KB)
2KB Preset storage RAM (battery backup)
All programmed in Z80 assembler
A selection of chips used:
8279 Programmable display/keyboard controller - Front panel
8279 Keybed scanning (sensor matrix mode)
82C54 Interval Timers
Motorolla 6850 UART (the UART of MIDI in the early synths)
ADC0808 parallel 8 channel analog to digital
The DAC board is a series of parallel DACs that are a little more modern, DACs were available in the 80s but more expensive, an R2R could have been used but this was skipped because of the sheer work involved with latches and lots of connections. Sample and hold was avoided because it would have been difficult and really not needed.
The CPU Board has the Z80, an ATMEL 8KB EEPROM and 2x 2KB Static RAM (one is battery backed up for the patch storage) the memory decoder, I/O decoder and all the supporting logic needed to drive the UART, ADC, timers and I/O
The keybed is scanned using the 8279 and the output ports for the digital controls and gate triggers are 74HC374 latching flip flops.
There is a 4MHz crystal oscillator that is buffered and divided down using 4013 flip flops to provide the CPU with 4MHz, the interval timers with a selectalble 1/2/4MHz (for range control) and then 500KHz for the UART and ADC.
The power on reset is a 555 circuit taken from the BBC micro reset circuit.
The oscillator boards
There's 6 voices shared across 3 PCBs, components are shared on each PCB.
This is a DCO based on the Juno 106 with an integrator amp fed with a control voltage to keep the saw wave amplitude stable, a PWM adjustable square wave and a 4013 to provide a sub octave square.
The oscillator is Juno based, but the rest is not, the filter is a CEM clone AS3320.
The oscillator uses a 4066 digital switch along with an op amp VCA for the sub octave level, these are summed into the 3320 filter which has a 3360 final VCA. The PWM modulation is switchable between manual and envelope mod, which creates a nice sound.
There's 2 3310 envelope generators per voice, one wired to the filter via a 3360 envelope amount VCA and the other directly to the final output VCA. Each envelope is then routed to the PWM mod via an MAX333 SPDT digital switch (I used this because it's a quad so I could share between 2 voices)