Jupiter 6 Waveshaper

Original Roland Chip

Introduction The Roland Jupiter 6 (and MKS-80) use a custom hybrid chip (EHM-S226W83S ) for transforming the CEM3340 oscillator waveforms to a consistent level of 5v centered around 0V (-2,5V to +2.5V). The CEM3340 creates waveforms that are at different levels (Pulse 0 to +12V, Triangle 0 to +5V, Sawtooth 0 to +10V) which need level shifting for mixing into the VCF. This is implemented by a mixture of SMD resistors and transistors.

The chip also contains a transistor based sync circuit, as Roland chose not to use the inbuilt CEM3340 sync (weak and strong), possibly because they wanted to control the sync on/off with a +5V programmable signal from the micro-controller and didn’t want use an analog switch.

Roland took this approach to reduce the footprint of the waveform translation to a minimum using a 12-pin SIP package. This enabled the Jupiter 6 PCB’s to be more compact than the Jupiter 8. An alternative approach is to use resistors and Op Amps but this is 5x the PCB size in the days before SMD components could be used.

Roland EHM-S226W83S

Pin Out The EHM-S226W83S pin out can be easily determined from the schematic in the Jupiter 6 Service Manual:

1.  +15V
2. -15V
3. Pulse Output
4. Pulse Input
5. Sync Input from second VCO sawtooth
6. Sync ON/OFF from microcontroller
7. Sync Output to CEM3340
8. Triangle Input
9. Sawtooth Input
10. Triangle Output
11. Sawtooth Output
12. GND

Tightly Packed!

How it works There are four separate circuits; one for each waveform and one for the sync. The triangle wave generated by the CEM3340 is the 0V to +5V and is level shifted to -2.5V and +2.5V by a PNP transistor (Q1) and a resistor network (R2, R5, R6).

The 0 to +10V sawtooth wave is reduced and shifted to -2.5V to +2.5V using three resistors (R1, R3, R4).

The 0 to +12V pulse wave is reduced and shifted to -2.5V to +2.5V using a more complex circuit of six resistors, an external capacitor and a NPN transistor (R13 – R17 and Q4.). The SH-101 service manual uses a similar circuit without the final level shifting. So it is easy to determine what most of the values are without measurement.

The sync circuit is also more complex. The sawtooth wave from the source VCO is switched off by grounding the input to a NPN transistors (Q2), or going high to enable the Sync. The sawtooth is then pulse shaped by a NPN transistor, 2 diodes, a capacitor and 3 resistors, The narrow pulse resets the VCO integrator via its capacitor (Q2, Q3, C1, R7-R12), creating hard sync.

Tear Down For my Jupiter 6 VCO clone I wanted to replicate the EHM-S226W83S circuit, along with the RA3A resistor array connected to the CEM3340. The array resistor values are easy to determine from the schematics and values in the MKS-80 service manual. The Jupiter 6 makes use of all 6 resistors in the array, whilst the MKS-80 only uses five of them.

The EHM-S226W83S was harder to replicate, as although the schematics are in the manual, the values are not printed and are a mystery. I bought one in April 2018 and removed the plastic encapsulation with acetone (took 2 days of soaking), so that I could then measure the various printed on resistors using a multi meter, some components had to be desoldered to be measured.

Prototyping Once the values had been obtained the full VCO schematics were laid out using Eagle CAD. Roland has implemented the VCO with some changes to the original CEM3340 datasheet  which have also been captured.

A prototype PCB with THD components was ordered in April 2022. The AMSynths Jupiter 6 VCO will operate with 12V power rails, so the resistor values will need changing anyway, as the original uses 15V power rails. I plan to keep with THD components rather than building SMD replicas.