MKS-40 Motherboard PCB


Motherboard PCB The MKS-40 design is based on the same sized voice cards as in the original, but horizontally attached to the Motherboard rather than vertically. This means I can fit the whole design into a deep 3U rack case and go with more reliable connectors. The new Motherboard holds these cards and has space for the functionality from the Module Controller, and the analog circuits left behind by moving the digital circuits and S&H onto a separate Processor PCB.

The Motherboard PCB is large and expensive at 400 x 250 mm and the area under most of the voice cards needs to be kept clear for the inter voice PCB traces. The design also needs to ensure the trimmers are accessible and they have been placed down the left hand side of the PCB.

JP-8 LFO

Low Frequency Oscillator  The MKS-40 uses the Jupiter 8 LFO design  which is very similar to the Jupiter 4, except the JP-8 uses a CMOS 4052 chip to select the waveforms rather than a set of JFETs. It also has a control voltage for adjusting the LFO Delay time that uses a linear front panel slider rather than the impossible to find 2M reverse  log on the JP-4.

Whilst this slider cannot be read and stored in the Jupiter 4, it means a 10K linear slider can be used in the MKS-40 design. It has a 6K8 resistor in parallel at the top of the slider to provide a log like feel, the same as in the Jupiter 8 circuit.

The LFO waveforms are all bi-polar +/-5V, which means the 4052 demux needs to run from a +/-8V supply to ensure its power rail specs are kept within. The data logic only needs to supply +5V to select the channels on the 4052, so no level shifting is needed.

VCO, VCF and VCA Control The original Module Controller board contains a set of BA662 and Op Amp based circuits for controlling the the oscillator, high and low pass filters and VCA. It also contains the clock circuits for the ADSR’s, and a noise and S&H circuit. All of these have been replicated in the MKS-40 with the only change being to use DG41x CMOS analog switches rather than JFETs. This also means we can keep with +5V logic with no translation to +15V needed.

BCM857DS matched transistors have been used in the OTA control circuits, they are SMD and quite cheap. The Noise circuit uses a R version of the 2SC828 transistor, this was used in the TR-808 and provides a very good spectrum of white noise.

ADSR Clocks The Jupiter 4 uses a very unusual approach to voltage controlled envelope generation by using CMOS clocks for each of the 3 time stages (A,D,R), which drive the generation circuits on the voice cards. This is component intensive and means the voltage control is exponential (check!) and not linear. In 1977 Dave Rossum had invented the first envelope generator chip – the SSM 2050, which became part of the Prophet 5. Roland watched the development of this and the CEM3310 and decided to make their own version the IR3R01. However this was not available until 1981 and the Jupiter 8, so the Roland engineers used CMOS chips.

The MKS-40 retains the ADSR design, as its part of the Jupiter 4 sound. The controlling CV for the CMOS clocks  is reverse exponential. A resistor is put in parallel to each of the ADR pots on the panel PCB’s to create this response. I have replicated this in the MKS-40, although my own ADSR prototype a log taper pot did a better job.

Matched Transistors

Early models of the Jupiter 4 used a matched transistor pair in the ADSR clocks. I have retained this by using low cost SMD transistor pairs (BCM847DS). The timing capacitors are high quality MLCC ceramics and the clock speed of the Attack oscillator has been carefully adjusted to ensure the Roland stated fast attack time. Multi-turn trimmers have been used, which makes setting the ADR timing a lot easier.

The accuracy and stability of the -15V power supply to the matched transistor pairs is critical, so the MKS-40 has precision +/-10V voltage rails for the ADSR clocks and  some of the resistors have been adjusted in value accordingly. This was carefully prototyped before completing the design of the Motherboard.

Oddments The only other circuits on the original Motherboard that needs replicating are the LFO LED driver and the voice output summing Op Amp. This audio signal goes to the Chorus PCB before going onto the balanced audio output jack XLR’s.

LFO Bleed Fix The MKS-40 implements a fix to the LFO bleeding into the voice card outputs via the LPF modulation signal. These two signals are right next to each other and in the JP-4 the traces run in parallel. The post VCA 47k resistor at R97 on the voice card is shorted and moved to the input of the audio output summing Op Amp (IC30B). In my PROMARS this is an easy change with a 0R resistor at R97 and a new 47k resistor on the PANEL PCB.

Outcomes This is a tricky PCB to get right as it contains a lot of circuits and its expensive to get manufactured! The power rails are connected straight into the PSU via a 4 way terminal block (GND, +5V,+15V,-15V). The main power traces are kept as wide as possible to feed the Voice Cards (50mil) and the smaller traces that are used with the SMD chips (10mil) are based on a second set of supply traces. +/-8V is needed for the LFO wave selector 4052 and +5V is needed for the logic levels in the DG418 analog switches.

 

 

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