MKS-40 Analog Synthesizer


JP-4 Inside

Overview The AMSynths MKS-40 Analog Synthesizer is a 3U 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 and replaced the Jupiter 4 in 1981.

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 replace it with a 4-channel MIDI to CV circuit.

The original 8048 Intel processors have 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 Jupiter 8 style buttons to go through the patches. The original presets have been discarded but are programmed into the user patch memory.

JP-4 Diagram

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

  • No keyboard – use a commercial MIDI to CV converter
  • Streamline the digital logic where possible
  • Use the Jupiter 8 LFO, which has some useful features
  • SMD CMOS chips on the Processor board to save space
  • Remove the pitch bender circuit, use MIDI
  • Remove the transpose switch and go with a 4 octave VCO Range.

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

  • Retain the 8048-11 and -12 processors and original software
  • Retain the original analog sound and voice cards
  • Keep to the original panel layout as far as possible

MKS-40 Panel

Additions

  • 64k FRAM chip expands User Patch Memory, replaces the uPD5101
  • PIC Microcontroller to provide 64 Patch Preset control
  • Patch number on dual 7 segment display
  • Jupiter 8 style momentary switches
  • Commercial JP4- KBD MIDI interface

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 already built a replica which works well (see here) but it 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 but with precision voltage reference for the CMOS clocks. Some Jupiter 4’s used a matched transistor pair in the CMOS oscillators (2SC1583), even though the layout is for two unmatched transistors. The  use of matched pairs seems to have been done in older serial numbers, but no mention of them is made in the service notes. I have implemented dual matched SMD transistors (BCM8X7DS) in the ADSR clocks and in the Module Controller for OTA control.

PCB Design The AMSynths design has the following PCB’s:

  • FRONT and REAR PANEL PCB, replacing Control Board A,B,C, E and F
  • No Control Board D
  • 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 Module Controller
  • ASSIGNER PCB with the 8048-11 processor
  • 4x VOICE CARD PCB’s, same size as the original
  • ENSEMBLE PCB, using either MN3004 or MN3009 BBD chips
  • MIDI and Audio Output PCB

The 4x VOICE CARDS fit onto the MOTHERBOARD PCB (horizontally rather than vertical) which is attached to the base of the 3U rack case. The PROCESSOR, ASSIGNER and ENSEMBLE boards are mounted on top of the VOICE CARDS 1-3. The MIDI and Audio Output PCB’s fit on the rear of the case. The connections between the various PCB’s is via ribbon cables and DIL sockets/plugs. Power is distributed in a star arrangement via thick cables to each of the main PCB’s.

Development Progress at June 2022 The overall physical design is complete with a large 250 x 400mm Motherboard supporting the four Voice Cards, with the Chorus above Voice 1,  and the Assigner PCB above Voice 2. The Motherboard contains all the original Module Controller circuits.

The Voice Card and Motherboard have been laid out, but need further changes. The Assigner, Panel, Processor and Chorus are in development. Procured so far:

  • The 64k FRAM chips
  • 8048 processors with OS
  • PIC chips for the 4 way switches
  • PIC chip for the patch presets and button scanning
  • Jupiter 8 style switches with coloured caps
  • Matched transistor pairs
  • AS662D OTA chips

Outcomes The project was started in early May 2022 and I am aiming to finish it by the end of the summer. This is a huge project in man hours to replicate much of the Jupiter 4 and then augment with new designs.

 

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