MKS-40 Analog Synthesizer


Overview The AMSynths MKS-40 Analog Synthesizer is a 2U rack version of the original Jupiter 4, with some improvements and changes. The Jupiter 4 was introduced in the Autumn of 1978 at £1,400 and went on to sell over 5,300 units until the end of 1981. Its sibling the ProMars, sold 2,300 units over the same time period. This compares well with 3,300 Jupiter 8’s which was considerably more expensive.

The Jupiter 4 is a complex and component heavy analog polysynth, which is why we have seen no replicas, even though it is extensively documented in the service notes. The MKS-40 simplifies the Roland design, with the first step being to remove the keyboard and keyboard microcontroller, and replace it with a 4-channel MIDI to CV circuit.

The original 8048 Intel processor has been retained along with most of the analog  and digital circuits. The limitation of just 8 user patches has gone, with the addition of a PIC microcontroller and 64KB non volatile RAM, that deliver 64 user patches, a patch number display and an encoder to quickly go through the patches. The original presets have been discarded but can be programmed into the user patch memory, if desired.

Simplification To keep the project duration and costs to a minimum, simplification is needed:

  • No keyboard or key assigner microprocessor, use MIDI to CV
  • Streamline the digital logic for the VCO RANGE
  • Use the Jupiter 8 LFO, as it contains a S&H circuit
  • This remove the need for a separate ARP clock to drive the S&H

I also want to keep as close to the original as possible and speed up development:

  • Retain the 8048-12 main processor and original software
  • Retain the original analog sound, especially the unique VCO design
  • Keep the R2R DAC using a SIP R2R resistor network
  • Keep the data line level shifting circuits
  • Replicate the voice card, same dimensions and pin out

Additions

  • 64k FRAM chips expands User Patch Memory, replaces the uPD5101
  • PIC Microcontroller to provide 64 Patch Preset control
  • Patch number on dual 7 segment display
  • Encoder for patch selection
  • Arduino based MIDI to 4-channel CV/Gate
  • SMD CMOS chips on Processor board

PCB Design The AMSynths design has the following PCB’s – the 4x Voice Cards fit onto the Motherboard PCB (horizontal rather than vertical):

  • PANEL PCB, replacing Control Board A,B,C and F
  • No Control Board D or E or Key Assigner PCB’s
  • PROCESSOR PCB, partially replacing the Motherboard with CPU, DAC, S&H
  • PATCH PRESET EXPANSION PCB, which holds the PIC micro controller
  • MOTHERBOARD PCB with LFO and Controller
  • 4x VOICE CARD PCB’s, same as the original (BA662 filter)
  • Chorus Ensemble PCB, using MN3004 or MN3009 (Juno style chorus)
  • MIDI and Audio Output PCB

Data Switch Decoding The Intel 8048 microprocessor manages 16 CV’s and 14 data switches. The 0 to +5V CV’s are multiplexed into JFET Op Amps acting as sample & holds, whilst the data switches are latched with 4099 chips and then level shifted from +5V to +15V for CMOS analog switch control. They are not level switched when controlling JFET or BJT’s which are acting as switches on the module or controller boards.

A total of 11 data lines are level shifted using individual transistors, which also invert the signal (except PW/PWM), similar to the Jupiter 8 that uses +10V and diodes to do the job. I could have used 4504 level shifters and 4069 inverter chips, but I decided to retain the original design as it can be done in the same PCB space and reduces a potential source of error.

Simplification Detail 

  • Reduce the amount of CMOS decoding of data lines.
  • Remove the transpose switch and go with a 4 octave RANGE.
  • Replace the LFO waveform selection with a 4052 chip.
  • Add the extra power rails for the 4052’s.

The Elephant in the Room The most complex part of the Jupiter 4 are the ADSR’s which are spread across 5 PCB’s with a lot of CMOS logic to create one of the first voltage controlled envelope generators. I have built a replica which works well but doesn’t have quite the envelope times stated in the Roland manuals. The envelope curves are the usual exponential ones, so the real difference is the timing. After careful consideration I decided to retained the original design and use SMD chips.

The 4-way Switch Challenge Finding suitable DP4T slide switches has proven difficult, so I have replaced them with 4-position slide potentiometers just like in the Juno 106. A small 8-pin PIC reads the pot and converts it into 2-bit binary which interfaces straight into the 8048 via the 4051’s. This is needed for VCF Key Follow and VCO Pulse Width, the LFO waveform control uses a rotary switch to drive a 4052 analog switch.

Development Progress at May 2022 The overall physical design is complete with a large 250 x 400mm MOTHERBOARD supporting the four VOICE CARDS, with the CHORUS PCB above Voice 1,  and the MOTHERBOARD containing all the CONTROLLER circuits. A ventilated 2U case with 350mm depth has been selected to house the MKS-40.

The VOICE CARD and MOTHERBOARD have been laid out, and the PANEL, PROCESSOR and CHORUS PCB’s are in development. The 64k FRAM chips have been procured as they are difficult to find, and a 8048-12 with the Jupiter 4 OS ordered. The required PIC chips for the 4 step switches and the 64 patch presets have been procured.

Power Supply  The original Jupiter 4 power supply provided +5V at 1A, and +15V and -15V at 1A.  A modern switched power supply will be used; either the Meanwell RPT-60C or the more powerful LPT103-M, depending on the -15V current consumed. A lab power supply will be used during development.

Copyright AMSynths 2022