Prophet 600 Refurb – Part 1

P600 as bought

Overview In June 2020 I rescued a rather forlorn and broken Prophet 600 for way too much money, but I decided it was worth the time and effort to get it back it to perfect working order. Although financially it does not make sense, as I will probably spend more on it than its currently worth, once I have fixed the P600, I plan to implement a number of upgrades.

• Gligli OS upgrade
• New power supply
• Stereo chorus and voice panning
• New Fatar keybed
• New walnut end cheeks

The Prophet 600 was announced at NAMM 1983 and shipped a month earlier in December 1982 with the first MIDI implementation ever (thanks Dave!). It was a reasonably successful product with 6000 (approx) sold until mid 1985, against the heavy competition from the DX7. The innovative use of digital envelope generators and a digital LFO pushed the capability of the original Z80 microprocessor, the slow envelopes and obviously stepped pots put customers off. The budget nature of the P600 was a bit too apparent!

CPU board

Part 1: Getting The P600 Working The first stage is to get P600 tested and working again with no modifications, although dusty the circuits look to be in good condition and better than I expected. I downloaded the Technical Service manual from July 1983, as I am sure this is going to be a constant companion for the next few weeks! My Prophet 600, serial number 1406, arrived at the workshop on 12th June and I logged these initial visible faults:

• Large number (11) of keys missing
• The keybed is in very poor condition, needs to be replaced
• Slightly damaged Mylar control panel
• No control knobs
• One pot missing
• One pot with spindle cut
• OS ROM missing, all other chips in place
• All cabling intact but disconnected
• Pitch and Mod wheels stiff
• Battery disconnected

The front panel was cleaned and found to be a very good condition with only one visible chip down to the metal which will be carefully infilled with black paint. The rear and underside of the casing have not survived as well and there are numerous areas of black paint missing. The white silk screen lettering has survived, so it needs a black repaint feathered into the original. The rubber feet are in place but the wooden end panels will be replaced. All the intact pots rotate ok and need a clean, and the toggle switches work fine.

Power On Whilst a set of OS (Version 8) and Diagnostic ROM’s arrive from Germany, I can test some basics; is the power correct and the microprocessor being clocked. There are no visible signs of power failure, such as blown caps or regulator heat damage. The first step is to power on the transformer at the correct 240V but disconnected from the CPU PCB. I then checked the AC output voltage rails which should be 18 and 36 VAC. Measurement shows all ok at 22 and 44 VAC with slight transformer whine.

Onto powering up the CPU and Voice PCB’s (they are hard wired together) and checking the analog and digital DC rails. I left the keyboard, bender and panel PCB’s disconnected at this stage. The power rails measured good (+4.97V, +15.07V, -4.96V,-14.95V) and there is a 4 MHz clock at pin 6 of the Z80 chip. I ordered new OS ROM and Diagnostic ROM chips from a German eBay seller and they arrived very quickly.

Initial Repairs Two new potentiometers were ordered from Wine Country in the US and the wooden sides measured, so that new American walnut sides could be made by MintCase. The piano hinge for the front panel is missing the six machine screws that attach it to the rear casing. I ordered a pack of ten M3 x 8mm black posidrive machine screws, and they were sometimes a loose fit requiring a M3 nut to be fitted as well. More progress in Part II to follow, when we try and boot up the P600.

OB-Xpander Replica – Part I

Overview The Oberheim OB-Xpander was a prototype analog synth created around 1982, and not to be confused with the later Oberheim Xpander which although similar is significantly different. There is very little information about the first Xpander, just a picture in Mark Vails Vintage Synth book (volume 1) and some text in the A-Z of Analog Synthesizers. From this limited information I have concluded that the OB-Xpander was based on the OB-8 and developed in the Summer/Autumn of 1982 (because the panel graphics reflect an early OB-8 with Page 2).

Features Here is a probable feature list:

• Part of the Oberheim System alongside OB-XA, DMX and DSX
• Four independent “multi-timbral” analog voices, using one OB-8 voice card
• Four independent LFO’s in the  modulation section
• Keyboard controls, Split/Layer, Unison have been removed
• Probably the same OB-8 CPU board with some minor modifications
• Four sets of CV/Gate inputs as well as the Oberheim Computer Interface
• Stereo and Mono Outputs with preset voice panning
• Cassette storage for patches is retained
• No MIDI, as this did not arrive at Oberheim until Spring of 1983
• Page 2 included, as the Modulation has the additional white silk screening

Modulation Section The Modulation section needs to be per voice with 4 separate LFO’s, to ensure each voice has different sounding patches. The OB-XA has two analog LFO’s for the Lower and Upper voice banks, whilst the OB-8 has two digital LFO’s with the extra sawtooth waveforms that are screen printed on the OB-Xpander panel. So could the OB-8 OS have been extended to generate 4 LFO’s in the OB-Xpander? The limitation is in the Z80 processor speed, but the keyboard scanning workload has been removed, CV/gates added and the envelopes are analog.

The OB-Xpander has a switch in the lower corner of the modulation section, which the OB-XA does not have and was used by the OB-8 as VOLUME MOD. However the labeling is shorter, so maybe this was just VOL.

Master Section This has been reduced down to one column of controls;

• MASTER VOLUME potentiometer
• AUTO switch, labelled as per OB-XA
• MASTER TUNE potentiometer

Control Section This remains as one column but has one change;

• PROGRAM VOLUME potentiometer sets Voice Volume
• PORTAMENTO potentiometer
• OSC2 DETUNE potentiometer and red LED

Oscillator Section The VCO controls are the same as the OB-8 and OB-XA and the panel lettering verifies the OB-Xpander is based on the OB-8 voice card, as it has the lettering underneath the Pulse and Saw switches for Triangle. This is how early OB-8’s were lettered before the Page 2 functions were added on the silk screen.

4 Voices on CPU board

Keyboard Section This has been renamed to three words which are too blurred to read, maybe PROGRAM VOICE SELECT. There are switches to select each voice for programming and patch save/recall. with a fifth switch (assumed) to call up a four layer program (one of 12 available, as in Double mode). The cassette Play and Check functions are retained with the corresponding red LED. The OB-8 VOICE board actually has 4 voice select signals from the CPU board (at A13, A15, A17, A19) but only two are used for Lower and Upper voicing across two PCB’s. This confirms that the OB-8 is the basis of the Xpander and that it would have used 4 voice boards, each with only one voice populated.

Programmer Section The patch selection remains the same as the OB-8 with the familiar row of A-D and 1-8 switches, with 120 patches and a WRITE/RECORD switch at the far right hand side, labelled as per the OB-XA.

Prototype Only The OB-Xpander never made it past prototype stage, probably due to being only 4 voices, and the limitations of the Z80 processor running 4 independent digital LFO’s. Oberheim reworked the idea in early 1984 as the Xpander, with 6 independent voices, lots more digital control, MIDI and CV/gate. The processing power was increased to cope with more features and the Xpander has stood the test of time and is one of the greatest analog synths ever made.

Case Dimensions Peter Forrest states this as; 90cm x 30cm x 12.7cm, the same depth and height as the Oberheim DSX and with a weight of 9Kg, which is half the weight of an OB-8. The Xpander case does not look deep enough for a set of OB-XA voice cards to fit in.

Licio Comisso OB-8 VOICE card

Replica It is possible to recreate the OB-Xpander? It would make a very nice sounding 4-voice multi-timbral synth. All the CEM chips are readily available (CEM and AS), and both the VOICE and CPU boards could be recreated from the schematics. Licio Comisso has already successfully cloned the OB-8 VOICE and CPU boards to recreate the OB-8, but when I contacted him he only had OB-XA boards available.

A rather wide 30″ case would need to be fabricated, with a silk screened front panel. Using smaller width switches might mean the case width could be useful reduced. The power supply would be mounted off the CPU board, and improved.

OB-8 OS Modification I initially considering changing the OB-8 Z80 source code to support the Xpander. The largest change is to increase the existing two digital LFO’s to four, which is no easy task as there is no source code and its quite a large operating system to reverse engineer (16 KB), however this has been done before. The rest of the changes required are smaller. I disassembled the source code but it is a lot of work and time to get the changes made.

In addition I need to find a way to retain the calibration capabilities which use the Bender Board and 1st octave of the keyboard, probably by using a set of miniature slide switches mounted internally. This means retaining the keyboard scanning.

Custom Gligli OS A possibly easier approach is to modify the P600 Gligli code to add the extra digital LFO’s, and at the same time removing the digital ADSR’s, cassette interface, and keyboard scanning. This would also give a strong MIDI capability, but the user interface and MIDI Sysex/cc needs changing to match the Xpander, and then there is all the Page 2 functionality to consider, and the loss of the Oberheim calibration software. The advantage is that the Gligli code is open source and written in C.

Conclusion Whilst an exact replica of the Xpander is possible, the limitation of 4 voices is a bit too limiting, given the amount of time and cost needed to build a replica. My preference is to build the OB-Xpander but as a desktop OB-8 with 8 voices, Page 2 and MIDI. This means no OS changes are needed and the CPU board can have MIDI built in. I will also remove the Noise circuit and add in two new filter modes (12dB HP and 6dB LP). In Part II we will look at the design of this new OB-Xpander.

Korg Wavestation Refurb

Overview I bought my Korg Wavestation new in 1990 as a grey import with no serial number for £1200, £400 of RRP. It was a big purchase for me back then, and my first digital synthesizer, which I used for many tracks in the early 1990’s. I loved the wave table sounds and the keyboard with after touch. I added a few sound cards, Drums and Performance 3. As my studio grew the Wavestation was rather side lined and then put in the garage for storage for 20 years, where it gathered dust and scratches.

In 2020 I dug the WS1 out to refurbish and possibly sell, but once plugged in and making sounds in the new Garden Studio I decided to keep it. I do have iWavestation on an iPad but it is not quite the same as the original in sound or play-ability. It does have the advantage of displaying the waveforms.

The Refurb My Wavestation needed a new back light, new front panel buttons, keys cleaning and some scratches on the screen removed. The scratches on the panel, created during its long stay in my garage would have to stay. The component values referred to in this post are for the original Wavestation, the component numbers and values are different in the AD rack version. The disassembly of the Wavestation is a complex and lengthy process, I photographed and documented every stage. Here is a visual guide and more information on the new LED replacement process.

New White on Black Display

New LED Display The large 64 x 240 LCD is best replaced with a modern LED display which is nearly plug compatible, it lasts longer and enables the noisy inverter to be removed. I bought my white on black display from BuyDisplay with the pin connector fitted. The display is connected to a 20 pin-header on the main PCB at CH13A, I made a new 20-pin ribbon cable with sockets to connect to the new display. I reused the old 2-wire LCD EL power cable that goes to CN11A (CN4 on early models) on the PSU board, with a minor change to the board (detailed below).

Here is a list of the changes needed:

• Pins 19, 20, 21, 22 should be removed or cut from the header on the LED display.
• Pin 19 needs to be connected to Pin 3 (+5V) using thin wire to ensure the right font is used.
• Add a 3K9 resistor between Pin 2 and Pin 4 to modify the contrast voltage.
• Connect the A and K terminals on the back of the display to the old LCD back light cable.
• Remove EL transformer (T2) from the PSU board.
• Remove C28, C29, R17 and R18 from the PSU board as they are not needed.
• Add a 100R resistor between +V pad of C28 to where Pin 4 of T2 was.
• Replace R17 with 6K8 and R16 with 3K9 resistors, to get the contrast correct.
• I replaced C30 on the PSU board with a modern MLCC 100nF capacitor, as preventative maintenance.

Once all the modifications are done and checked, the Wavestation can be reassembled, and powered on. The display works very well, although not exactly a black background it is very close, and it makes using the Wavestation a joy.

EX Upgrade The Wavestation of 1990 can be upgraded to EX capability by installing 4 additional wave ROM’s (total of 2MB) and the EX OS which is at version 3.19. The EX upgrade was more about drums and piano, but there are additional Prophet VS waves in there as well. A total of 119 waves were added and 8 more digital effects added, most of the new wave sequences are rhythmic.

OS ROM 3.19  The operating system is contained on two NEC D27C1000A-12 ROM chips (the Wavestation AD uses the -15) at IC18 and IC19, they together hold 256 Kbytes of data. They have a slightly different pin out to modern equivalents like the MBM27C1000A-15Z, which need pins 2 and 24  swapped during programming. I have used the original chip type to avoid complications. The OS binary is available from the Synth ROM Database.

Wave ROM The wave data is held in four 512 Kbyte Mask ROM’s (2 Fujitsu and 2 NEC) that have been encoded at the factor, with a total of 2MB. We need to use EPROM’s that we can burn with the wave data and I have used ST M27C4001-12F1 chips encode with the wave binaries available from the Synth ROM Database. Four 32-pin turned pin IC sockets were soldered onto the main board along with four 100nF 2.5 mm spaced ceramic capacitors. The four new ROM IC’s are located in the sockets as follows:

• IC2 – WS4P0
• IC3 – WS3P9
• IC4 – WS3P8
• IC5 – WS3P7

I bought the chips and had them programmed by buyicnow.com for under £50 including postage. This a a very easy and cheap way of getting the upgrade chips and sockets. The original Korg upgrade was £300 in 1992.

The Prophet VS Legacy The Wavestation is a major development of the Prophet VS from 1986 and most of it ROM waves were reused, the Wavestation added wave sequencing as well as many more waves than the VS. The 1990 Wavestation uses Prophet VS waves from 35 to 125, including bells, vocals and some traditional Piano and Bass waves.  The Wavestation documentation does not contain a description of these waves but with some detective work looking at the Poly Evolver waves which are also VS waves, its possible to add the wave descriptions, which are here Korg Wavestation – Prophet VS Wave Descriptions.

The Wavestation EX added in the Prophet VS waves 126 to 155, however the ROM memory in a VS stops at location 127, so these 30 waves are possibly from one of the six Prophet VS RAM cartridges which contain 32 waves. The VS126 to VS155 waves are visible in iWavestation but there are no descriptions.

Korg WSC-3S Cards

Patches The Wavestation comes with 3 banks of sounds, one in ROM and two in RAM. Korg produced quite a large library of patch cards as well as three PCM cards with new waves, and many professional patches were commercially released by companies in the 1990’s. A full list is here. I have a large library of sysex files stored on my PC and my personal favorite patches are loaded onto a MCR-03 RAM Card. I use the iWavestation to preview patches and then I load them up via sysex.

The Wavestation was commissioned into the studio on 8th April 2020 and replaces my S-50 in my active setup (as the W-30 has 240MB of S-50 sample banks on hard disk). The refurb has worked really well and the Wavestation now gets lots of use in creating new songs, the key is to try and avoid all the presets that have been overused in the 1990’s .

Roland RS-09 Restoration

RS-09 As Bought

Overview  In July 2019 I acquired a rather worn out RS-09 Mark 1 Organ and Strings machine. Described as: In need of repair and in a pretty bad way, it does make sound but needs some serious attention. I know the RS-09 is not the most versatile of synthesizers with only one or two classic sounds, but it was local and it would test out my repair skills. It certainly was cosmetically very bad although the keyboard was intact despite the side panels being done up with gaffer tape.

History The Roland RS-09 was launched at the end of 1978 (at around £500) and was refreshed as a Mark 2 model in May 1980 to give it more of a Jupiter 8 look. It went on to sell over 10,000 units and was very popular before the DX7 and D50 arrived as a source of polyphonic strings. My broken find is from May 1979 and is the original Mark 1 version with the big switches!

Glue Damage

Front Panel The worse part of this RS-09 is the dramatic glue damage to the center of the front panel, which simply cannot be hidden or removed. I did clean up the panel with IPA and removed the plastic glue with hot water. The panel had been patched up in places with a paint that doesn’t quite match and there is bare metal in a number of places as well as rust spots and scratches.

I could get the panel taken back to the metal, re-sprayed and new screen print but that would be expensive. My decision is to restore as much as possible with a deep clean and a black T-cut and polish.

Power Up I opened up the front panel and switched on the RS-09 not expecting much to happen and I was right, there was no sound from the MONO output. I suspected that all the hard to find parts were dead, like the master oscillator and 2 divider chips. So I measured the master clock and got a rather odd square wave but at the correct 1MHz as it was transposed down one octave. I then traced all the note signals from the master oscillator and everything was working, even the SAD512 BBD’s!

I plugged the output jack into the headphones socket and yes it plays! The MONO jack was clearly dirty or the contacts needed cleaning. I went on trying out the RS-09 and there were actually only a few major problems:

• The TONE slider does nothing.
• The ATTACK and RELEASE sliders cause all sorts of echoing sounds.
• Crackling and breakup of the audio
• One key not working

PSU Recapped

Full Recap At an elderly 40 years old the RS-09 electronics were in surprisingly good condition with no leaky capacitors or major PCB splits, despite the casing being bent out of shape at the rear. It must have had a big drop. There were some intermittent volume problems which I will need to trace and solve, and the Output board had some minor repairs which I wanted to go over, possibly replacing the MIX .

The RS-09 deserved a full recap and that’s exactly what I did, with over 70 electrolytic capacitors replaced with the finest Panasonic (power) and Nichicon (audio) plus the 100nF bypass capacitors (Mylar replaced with MLCC).

VCA and Envelope The unusual effects caused by the envelope generator needed investigating as it was making the RS-09 unplayable, with a click at the start of notes as well as the strange feedback effects. I suspected the BA662 VCA chip, and/or the crude envelope generator. Fortunately the RS-09 Service Manual has a lot of detail about what waveform shapes and levels are expected through out the electronics. This makes it easy to check where the failures are.

The VCA and ATTACK control checked out ok, and I replaced a couple of electrolytics in the ATTACK circuit as part of a wider recap. The GATE and envelope driver circuits worked okay. So my attention turned to the source of the release signal which is the GATE board. I replaced all the 1uF capacitors and cleaned up the PCB and checked for damage. There was a PCB crack near the power supply entry which may have caused the crackling noise. I repaired this with solid wires jumper-ed across the PCB traces. The cleaning revealed a few IC pads in a perilously close position of disappearing, so some careful joint re-flowing was done.

Keyboard The keyboard was in a filthy state and needed a complete clean of the plastic key housings and all the keys (in soapy water). The keys are easy to release, just remember to keep the springs in the order they came off. I decided not to bleach the keys back to white, as the rest of the RS-09 is aged, so it would look out of keeping. There had been some sort of spillage onto the metal base plate of the RS-09 and this was cleaned up, along with cleaning some black gunk one of the key actuators and bending the bronze springs back into the correct alignment.

Op Amps The RS-09 is based around the 4558 Op Amp, and for most of the circuits this is a good choice as its used to manipulate the core waves shapes and act as the BBD LFO’s. The only opportunity for upgrading is IC309 and IC313 which are the BBD input and final output buffers, which I may swap out for a TL072 but the RS-0 sounds fine after all the maintenance.

Output Board Whilst I have restored the output board as much as possible, there are major signs of aging with some PCB traces jumper-ed already. I have cleaned up the jack socket contacts whilst I test the RS-09. I will design and have manufactured a replacement and modern Output PCB using Neutrix jack sockets and an improved Class D headphone amplifier in the future.

More Trouble Shooting After the keyboard cleaning, a full recap was completed and some slide potentiometer cleaning and re-greasing (Deoxit F5 and Fader Grease), it was time to turn the RS-09 back on and explore the RELEASE issue and the missing note. I suspect its a divider problem as the key contacts are fine, however I have replaced all the GATE board capacitors, so maybe I get lucky.

Tone Control Initially the TONE control did not work, it failed to change the tone even though it was one of the better sliders. I measured the resistance and it checks out fine as dual 100KA. I then replaced the old Mylar capacitors (C374,C375) in the circuit with nice Wima polypropylene and polyester versions (250pF and 10nF). I didn’t expect this to work, but it did and the TONE control works perfectly once the pot was cleaned and the new capacitors were fitted.

RS-09 End Caps

End Cap Disaster The RS-09 came with plastic side caps, which appeared on many of Roland’s synthesizers, sequencers and drum machines around 1978 – 1982. Mine were done up with gaffer tape, because all the plastic mountings inside were broken off, except one. There had been previous attempts as repairing the plastic mounts with super glue, but this wasn’t done very well and they snapped off again.

The gaffer tape and glue was removed with IPA, eventually. Getting cap replacements is nearly impossible, and its a challenge to repair them properly so they do not snap off again. I used Araldite glue and supported the plastic mounts all the way along. I did the gluing whilst the keyboard was out of the base unit. I am pleased with the result which is very robust.

August 2019 Update After the full recap of the GATE board and key contact cleaning, 3 keys were not producing sounds. Time to check the key contacts are working for these specific keys. Turns out its the plastic bushing on the bottom of the 3 keys that was not protruding sufficiently to hit the gate springs. This was a simple fix of pulling the bushing down the shaft of the key.

The RELEASE now worked perfectly but the ATTACK was intermittent. I took apart the slider and cleaned and greased it, re-flowing the pot pins stopped an issue when the pot shaft was moved causing a break up of the sound. But the ATTACK was still not working correctly, in fact it was now consistently wrong with a sharp “click” attack. There also seems to be an intermittent problem in the ribbon cabling between the two CONTROL boards. I replaced the soldered ribbon cable with JST sockets and plugs to see if this sorted the issue.

RS-09 and SH-1

RS-09 Tricks The RS-09 has a few tricks up its sleeve to expand its features when used with synthesizers like the contemporary analog mono synth SH-1. There is an external audio input which you can run synthesizer sounds into and get the lush ensemble effect. There is a GATE OUT which is active on every key depression which means you can play a mono synth in parallel to the string and organ sounds, albeit with no CV output. And finally there is a RAW ORGAN audio output which you can run into the analog synthesizer and use its VCF, VCA and ADSR to create filtered paraphonic sounds.

Roland MKS-80 Power Supply

Introduction This is a feasibility study into a replacement power supply for the Roland MKS80, and it is not a project that AMSynths are aiming to deliver, as Guy Wilkinson owns the rights to the MKS70/JX10 re-engineered power supply which this thinking is based on. Thoughts and ideas are welcomed!

MKS-80 Inside

Overview I am fortunate to have a near mint Roland MKS80 (March 1985) and MPG80 (August 1984). This is the second time I have owned an MKS-80, the first one I had to sell in 2001 due to hard times. It is a REV5 with a 110V power supply and therefore requires an external converter, which is all a bit bulky and messy. The transformer in the MKS-80 is not tapped for both 240 and 110V operation, so a NOS 240V replacement transformer is needed.

Rather than going down that route, I have was inspired by the work done by Guy Wilkinson in replacing the Roland JX-10 power supply with a modern switched power supply. Details of the JX-10 PSU are here. This approach greatly reduces the heat from the old linear regulators as a bonus, as well as having 240V operation, and is a great long term solution with no capacitors to replace.

Roland are good at designing high quality power supplies with precision rails in the right places for analog circuits. The MKS-80 is an all analog voice design, where-as the MKS-70 is using DCO’s and then an analog circuit path. The MKS-80 therefore draws more current from the 15V rails and I do not want to fall short of the Roland design specs.

Roland MKS-80 PSU

The New JX-10 PSU Guy’s design provides +5V and +/-15V rails with low ripple after careful filtering of a set of three Meanwell power converters. Unfortunately this PCB cannot be directly reused in the MKS80 as it requires a +10V precision reference voltage for the VCO/DAC and requires a more power to the 15V rails than a MKS70, even though it has 8 rather than 12 voices and the overall power consumption is similar at 35 W.

The low voltage connections are also in a different location, and the mains filter is on board the power PCB. A new PCB needs to be designed with slightly different Meanwell units to meet the demands of the MKS80. There is an Unregulated +7VDC in the MKS80 to power the MPG80, just like the MKS70 design which also powers the display. Guys design uses a Vigortronix power converter to deliver +9V DC at 0.55A which can be used for the MKS80.

+5V Power
MKS80 Rectifier = 2D4B41 (2A) Main Capacitor = 4700uF 800mA at 8.5VAC
MKS70 Rectifier = 4D4B41 (4A) Main Capacitor = 6800uF
Guys Replacement PSU delivers +5V at 1.5A

+/-15V Power
MKS80 Rectifier = 2B4B41 (2A) Main Capacitor = 3300uF 450mAx2 at 21.5VAC
MKS70 Rectifier = 1B4B1 (1.5A) Main Capacitor = 4700uF
Guys Replacement PSU delivers +/-15V at 600mA x2

Replacement MKS-80 Power Supply The MKS-80 service manual states that the +5V rail should be within +/-30 mV, which is a tighter tolerance than the IRM Meanwell converter delivers and the actual +4.94 volts measured in Guys design. The +5V does not get used as a reference voltage, but potentially a more accurate converter/solution is needed as AC/DC Converters typically have 2% initial voltage accuracy, and Roland are stating 0.6%.

There are alternatives to Meanwell and I have gone with the XPPower ECE10US05 which delivers +5V at 2A with 1% initial accuracy and a line regulation of 0.5%.

For the 15V rails the more powerful 20W IRM-20-15, which delivers 33% more current at 800mA per rail, is needed. The 15V rails are at a much lower tolerance of +/-100 and 400 mV, so the Meanwell IRM can easily achieve this accuracy. Other Roland synths require very accurate 15V rails, such as the Juno 60 where the tolerance is +/-10mV which really demands a linear or LDO design.

There is a nominal +8.5V supply to the MPG-80 from the MKS-80, and I have retained Guy’s design, using the 5W Vigortronix power converter to deliver +9V DC at 550mA, which is more than enough to power the MPG-80.

The original Roland +10V power is provided by a 1SZ59 6.5V zener diode, with an Op Amp and trimmer to set this to an accurate +10.00V, the 1SZ59 has a temperature co-efficient of 2ppm/C which is very good! A modern LT1027 precision regulator can deliver the same 2ppm/C. It needs to be buffered with a precision Op Amp on the power supply PCB, although in the voice boards the +10V is further buffered by average performance M5218L Op Amps. The reason to take care here is that this voltage is used in the VCO’s as a reference voltage, as well as the DAC reference voltage.

The rest of the MKS-80 replacement power supply can follow Guys design but with a layout and pin out that fits the MKS80 PCB dimensions. The mains power conditioner on the top right of the Roland PCB can be dispensed with and its taking up valuable space at the rear of the MKS80. The power socket needs replacing with a grounded 3 pin IEC socket with power leads going straight to the front panel ON/OFF switch and then running back to the bottom right hand corner of the new PCB.

MKS-80 Power Connectors There are 7 low voltage power connectors on the PCB to supply the various MKS80 modules:

• P1 Analog +15V 10 pin
• P2 Analog GND 12 pin
• P3 Analog -15V 8 pin
• P4 Digital GND 6 pin
• P5 Digital +5V  6 pin
• P6 Reference Voltage Analog 2 Pin
• P7 Unregulated and Digital GND 4 Pin

These need to remain in the same position on the new PCB to ensure the cables and connectors fit.

PCB Design The PCB size can be as large as a rectangular 100 x 200 mm, with the same mounting holes as the original. The removal of the large internal heat sink means there is more space at the rear of the case. I have mocked up a high level design with the 4 converters and the power input at the front corner.

Next step is to do a PCB layout mock up in Eagle CAD to see if the components and the required isolated tracks can fit. A solution to the +5V DC initial voltage accuracy needs to be found as the best AC/DC converter I can find is 1% accurate.

Roland IR3R01 Design & Replica

IR3R01 Chip

Introduction The Roland IR3R01 envelope generator chip was used in the early 1980’s in three key synthesizers; the awesome Jupiter 8, the Juno 6/60 and the MKS7. The chip is obsolete and impossible to locate as NOS, which puts these synths at risk of never being repairable in the future.

The chip is a custom design by Roland which although looks similar to a CEM3310 or SSM2055, it is not the same design. This post is about researching how the chip works, building a simulation circuit (especially for the Key Follow Jupiter 8 design) and finally developing an accurate PIC based replica.

Description The IR3R01 generates a positive going 0 to +10V output signal (maybe less on Juno and MKS) with traditional Attach, Decay, Sustain and Release phases. The chip has CV control inputs for the ADSR phases, as well as the usual gate and trigger inputs, and it always runs from +/-15V power with an additional ground pin. The Attack, Decay and Release are driven by 0 to +5V control voltages whilst the sustain control voltage is 0 to +10V. The CEM3310 uses negative control voltages, so is a long way from being a drop in replacement.

The chip also has a Reference Voltage pin as well as a Common pin, and finally a Modulation pin which is used only in the Jupiter 8 to enable the level of the envelope to be controlled by a CV. Here is the pin out:

1. Modulation Input
2. Attack Voltage
3. Decay Voltage
4. Release Voltage
5. Sustain Voltage
6. Common
7. Reference Voltage
8. -15V
9. Ground
10. Timing Capacitor
11. Output
12. Current Input
13. Gate
14. Trigger
15. Retrigger
16. +15V

Common Pin The input voltage adjusts the timing of the envelope, and a manual trimmer with a Tempco resistor adjusts the attack time maximum, so that all 6 or 8 envelopes have the same timing. On the Jupiter 8 this is set to 6 seconds (although on Page 1 of the service notes its stated as 5 seconds), on the Juno to 3 seconds and MKS 7 to a very short 2.5 seconds. The maximum timing of the Decay and Release is 10 seconds on the Jupiter and 12 seconds on the Juno and MKS. This variation could be due to the increase in Decay and Release CV input resistors from 19k to 22k.

Reference Voltage This is tied to +7.5V (Juno) or +10V (Jupiter) or +5V (MKS). Typically envelope generator chips use this voltage as the highest voltage the attack phase is aiming to, the actual maximum voltage is typically less. Some real world measurements would be useful.

Timing Capacitor This is 68nF (Jupiter and MKS) and 47nF (Juno), this will vary the basic timing of the envelope stages. The manual adjustment of the envelope timing via Common is required because all voices in a polyphonic synth need to have the same envelope times. The adjustment is needed due to variations in the timing capacitor value and variation in chips.

Envelope Oscillator

Modulation Input This is only used in the Jupiter 8 to implement Key Follow, elsewhere the pin is grounded and a different approach is used to implement Key follow in the Juno. In the Jupiter a higher pitch CV shortens envelope time. The IR3R01 circuit is unique with a 28 kHz saw wave being created by a 555 timing chip and sent to all the IR3R01 chips via a buffering Op Amp and a comparator with the Keyboard Follow control voltage.

The IR3R01 receives a 0 to +15V square wave to the modulation pin, which presumably varies in height or frequency to control the envelope speed. It is not yet clear how this works, but it is an important part of the circuit to replicate.

Initial Conclusions The IR3R01 has similarities with the CEM3310 design with a current Input pin and reference voltage, but the control voltages are negative in the CEM3310. The SSM2055 uses positive control voltages and a timing control pin, but no current input. Neither of these chips have the Modulation Pin of the IR3R01. A strong possibility is that the IR3R01 has some basis in the complex Jupiter 4 ADSR which uses oscillators that vary from 10 kHz to 800 kHz, for each of the ADR phases.

The clue is that the ADR front panel sliders on the Jupiter and Juno are non-linear, just like on the Jupiter 4. They have a 4K7 resistor added between pin 1 and 2 to provide a more exponential like CV, rather than getting the microprocessor to do this job. This could be because the ADR voltages in the IR3R01 are controlling the rate of an oscillator, as in the Jupiter 4 design. The modulation pin uses a clocked input to vary the length (not height) of the envelopes, which also suggests a Jupiter 4 ADSR with oscillators. With no IR3R01 datasheet, a more detailed circuit analysis is needed.

Key Follow in JP8

Replicating the IR3R01 There are two uses case of the IR3R01. The simple ADSR in the Juno and MKS where a SMD CEM3310 could be buffered to get a similar “drop-in” response. However a micro controller based design (like Tom Wiltshire’s PIC ADSR here) is going to be less component heavy and potentially could be adapted to the Jupiter 8 use case with the Modulation Pin being used.

By using a digital design the timing will be perfect and the exponential curves can be adjusted if necessary in the data tables, so there is no need for timing adjustment, reference voltage or timing capacitor and these pins can simply be left unconnected. The modulation input needs to be read and the level of the envelope adjusted accordingly. The PIC would need a +5V power supply and most of the input voltages would need reducing (simple resistor dividers?) such as Gate and Trigger, the 10V Sustain CV as well as the 15V Modulation input level.

Whether all this can be done in a 16 pin DIL footprint remains to be seen!

Next Steps 2020 I am testing a ENVGEN8 chip with a Juno 60 with the intention of making an adapter PCB with the extra components on the underside and in SMD. I will also create a test harness circuit of the Jupiter 8 Key Follow circuit to see what waveform is generated on Pin 1 of the IR3R01 as the Key Follow CV is varied from 0 to +5V.

Replica Design Here are some initial thoughts, I envisage two chips; One for the Jupiter 8 and one for the Juno 6/60, so the envelope timing can be perfectly matched to originals:

1. Modulation Input – create Jupiter 8 test harness to work out what this does – use a resistor divider to read pulse frequency
2. Attack Voltage – read directly, change maximum time in ENVGEN8
3. Decay Voltage – read directly, change maximum time in ENVGEN8
4. Release Voltage – read directly, change maximum time in ENVGEN8
5. Sustain Voltage – check voltage, reduce by 50% using resistor divider
6. Common – ignore
7. Reference Voltage – ignore
8. -15V – use for buffer Op Amp
9. Ground – use and decouple with +5V
10. Timing Capacitor – ignore
11. Output – may need Op Amp buffer to increase output to 10V
12. Current Input – use =%v precision voltage from +15V rail
13. Gate – reduce to +5V using resistor divider
14. Trigger – ignore and connect internally with Gate
15. Retrigger – ignore
16. +15V – use to create +5V power for PIC using regulator