PPG 314 Sequencer

PPG 313 & 314

Introduction The PPG 314 analog sequencer dates back to 1976 and the beginnings of PPG when they were a modular synthesizer company. The first PPG sequencer in the 100 series was a straight copy of the Moog 960, but the 300 series sequencer was a more advanced design. It was still only 8 steps and 3 CV’s but it had dedicated controls for step timing and for moving around the sequence itself. Accurate switched step timing was also used in the Roland 717 and the Synthanorma around the same time.

In 1976 I was about to go to the University of Essex and study electronic engineering, whilst I had heard analog sequencers on records by Klaus Schulze and Tangerine Dream, I had never seen or owned one. I just had my trusty PE Minisonic synth! The PPG 314 which sold for £2,000 was way out of my reach, it was a years salary!

How Does it Work The arrival of CMOS logic chips in the 1970’s really opened up sequencer circuit design, thanks to its low power consumption compared with TTL (ARP 1050). The PPG 314 uses a decade counter at its core (4017) producing 8 steps, to drive 3 sets of CV’s that are summed using 741 Op Amps.  The value of each of the 3 CV’s is set by a potentiometer for each of the 8 steps.

The single sided Controller PCB is hard wired into the Main PCB and contains the 4017 counter chip and five 4011 NAND logic gate chips that buffer the 8 step positions into the CV’s and provide the logic control. There are also individual gate output for each step, which goes directly to the jack sockets.

The Main PCB is doubled sided (with no silk screen) and it holds all the potentiometers, LED’s and rotary switches, with the 6.35 mm jack sockets mounted directly to the panel.

Voltage Controlled Clock This circuit lives on the main PCB and is based on another 4011 configured as a VCO with variable pulse width. Whilst Emu Systems and Roland were using analog VCO’s in their sequencers, PPG opted for a simple CMOS version. There are Rate and Pulse Width (0 -100%) potentiometers and a LO/HI Range switch, as well a switch to turn the external control of sequencer rate OFF or ON. A green LED shows clock rate, and there are START and STOP push buttons which are mirrored as trigger input jack sockets. This enables remote control of the 314.

Step Timing The timing control for each step is preset via a 6-way rotary switch, with these timing divisions; 1, 1/2, 1/4, 1/8, 1/16 and 1/32. This is a great feature for setting up accurate rhythmical sequences, as the Moog 960 uses a potentiometer which makes timing accuracy difficult. With separate timing control the 314 retains three rows of CV control, without scarifying one row for timing. The timing is controlled via a precision voltage patched by the switches into the clock.

Programming The lowest row of 6-way switches are for programming how each step works, and provides another level of real time control. The six positions are;

  • LAST – previous step values are used for this step
  • GO NEXT – skips the step
  • GOTO 1 – moves to step 1, and starts again
  • STOP – stops at this step, and repeats
  • SEL TRIG – sends a trigger to the SEL TRIGGER output
  • NORMAL – sequences step by step

On the front panel the lettering is a bit cryptic with a solid circle as STOP and an arrow as SKIP. The GOTO 1 is not marked as available on Step 1 and 2 for obvious reasons, but its still wired up but it means you never move off Step 1.

AM314 Panel Mockup

AMSynths 314 As part of my “Big Moog” project I wanted to replicate the PPG modules that Klaus added around 1976/77 to his Moog IIIP. These consisted of a set of analog voice modules (VCO, VCF, VCA, etc.) and a pair of 314 and 313’s. I was lucky enough to have been given some photos of the inside of some of these modules including the 314, which meant I could reverse engineer most of the circuits.

The 314 clock is based on a CMOS chip but I have upgraded the design to use an analog VCO from the Roland 104 sequencer. I could not accurately trace the CMOS circuit (which is very basic) and an analog VCO is going to be temperature stable. The rest of the circuits are the same as the original . The Timing switches use precision resistors and feed a voltage back into the Clock VCO, just as in the Roland 717. I have recreated all of the six function modes including LAST, SKIP and GOTO1.

The Euro Rack AM314 consists of 5 PCB’s rather than the original 2, as it makes prototyping easier – as well as the final build. There are three Panel PCB’s (Clock, Sequencer and Output) and two Main PCB’s (Clock and Sequencer Controller). The prototype PCB’s were laid out in Spring 2021 and ordered in March. A panel was also mocked up to ensure all the switches and pots would fit and the lettering was in the right place.

 

 

Jupiter One Synthesizer

Overview The Jupiter One project is an original Jupiter 4 Voice Card turned into a complete single voice synthesizer, with the addition of a replica Controller PCB and the addition of a new Pot PCB that holds the switches, LED’s and potentiometers. The project started in 2018, but was paused for a year whilst I worked on Behringer projects, and restarted over the Xmas holidays of 2020. This post describes the work to get to a completed and full working synth.

I decided to replicate the Roland Jupiter 4 Controller functionality, which translates the 0 to +5V  potentiometer voltages into the CV’s required for the voice circuit even though I am not planning to add in the preset memory capability at this stage of the development. It also contains the LFO, Noise and half the ADSR’s, so I had to build at least 50% of it anyway! The Voice Card tends to use +15V on/off signals whilst the panel and DAC of the Roland design uses +5V. There are various level shifters on the main PCB which I have simply omitted and used +15V on the Panel PCB where needed.

A future project could be a replica Pro Mars but the Roland design is incredibly complex, very inefficient in terms of components and space and requires a lot of trimmers. The OS is locked up in a 8048 chip, so it not easy to modify the code to add more presets. A replacement microprocessor was available as the IO product but that appears to be history. I am replicating the Oberheim OB-1  preset monosynth instead, using a new microprocessor (already programmed) to reduce the complexity.

Controller PCB This new PCB holds the LFO and Noise circuits, plus the CV control stages for the VCO, HPF, VCF and VCA and six clock generators for the two ADSR’s. The LFO of the Jupiter 4 and Jupiter 8 are very similar and I have used the later design as it uses a CMOS analog switch to select the waveform. Initial tests proved the LFO worked well with a minor resistor value correction to get close to the original frequency range, but with a higher top frequency of 200Hz (not 70Hz) which I have retained.

The Noise circuit worked first time and uses the same 2SC828R transistor from the legendary TR808, which delivers a wide noise spectrum. In fact the noise level was too high and I had to trim in back to 2V p-t-p, as it would bleed through the JFET on/off switch at higher output levels.

Oscillation The VCO based on the ua726 chip worked first time, with the correct sawtooth and square waveforms, and frequency range selection, but the pulse width circuit needed some work as its uses a DP4T switch which I have not be able to source. The PW on/off switch was stuck on LFO modulation, and needed +15V as the reference voltage not +5V (level translation issue). Pulse width selection now works, with 50%, 40%, 20& and 10% widths, although the 10% is very narrow more like 5%!

The LFO bleeds slightly into the VCO frequency even with the modulation turned down, so this needs some attention.

Filters The HPF and LPF control voltages were initially too low and too high, although the filter did resonate correctly. The HPF control circuit works well in my 8104 filter module, so I correctly suspected a mistake by Roland at R111, which should be 47K not 100K. The LPF control voltage is being driven too high by the non existent Foot Pedal input which I have now removed. Both filters now work well and trimming is good.

The original filter S&H circuit uses the digital arpeggiator clock as a sample source, this is not available as it needs the 8048 microcontroller. I have used the LFO as the clock and S&H circuit from the Jupiter 8, with the front panel slider controlling the level of S&H into the VCF using a BA6110 VCA, so this could be preset in a future version with patch memory. My first attempt used an expo transistor pair which made the slider only responsive at the very top, I changed this to a single transistor linear response (as used in the JP-8).

Envelope Generators The original Roland design is unique and a very complex way to get to a voltage controlled ADSR. It uses CMOS clocks and switches to get the voltage control and was replaced by a single IR3R01 chip in the Jupiter 8. The voice card contains half the CMOS circuit, so I have added the other half to my Controller PCB. I have proven the design by building the AM8007 Env Generator module, so I am confident I can get this working.

The filter ADSR worked immediately, whilst the amplifier ADSR was dead. I replaced the buffer Op Amp at IC21 which is a 4558 chip.

VCA and Output The voice card contains a BA662 VCA with a volume control on the front panel, and its own dedicated ADSR. This was easy to get working properly. This is also a final analog volume control before the output signal goes to the rear mounted Output jack socket.

LFO Modulation The original Jupiter 4 has a rather nice modulation panel to the left of the keyboard. I have replicated this using a thumb joystick that provides vertical pitch bend and horizontal LFO modulation depth to the VCO, VCF and VCA using the same three selection switches, and two rotary knobs.

Inputs and Outputs My Jupiter 01 has four 3.5mm jack socket inputs; traditional pitch and gate, and then X and Y inputs which over ride the Thumb Joystick control voltages and enable velocity or aftertouch to be patched into the synth. There is a single 6.35mm jack socket mounted on the rear PCB which also houses a Meanwell power supply.

Pot PCB My initial design used DP4T slide switches for the PWM, LFO waveform and Key Follow selection. But the switches could not be wired as dual four way, so I have replaced them with a CMOS circuit that creates the correct 2-bit binary code, with selection by a push button and the display is 4x 3mm LED’s.

Chorus Options The Jupiter 4 used a BBD chorus circuit to add some much needed depth to the single VCO sound, which I suspect I could do with as well! I will either recreate the MN3004 fixed rate chorus as an internal PCB or a BBD-320.

Outcomes More work is needed on my PCB design. This set of tests proved many of the circuits but also showed up some issues – the 4053 LFO switcher needs +/-7V rails and the mounting of the Roland voice card needs to be inverted as the gap between the PCB’s is too tight. This means revising the Panel and Controller PCB’s which is quite a lot of work. So this project goes onto the back burner for a few months, before I get a final set of PCB’s designed.

 

Ultravox Custom Mini-Moog

Ultravox Mini-Moog

Overview I came across a picture of the Mini-Moog that was customized by Roy Gwinn for Ultravox and was intrigued by what the changes were for and how they were implemented. The actual Mini-Moog has long since been scrapped, as it had a very hard life touring and then being painted grey. It was in use from 1980 to 1983, until the later songs used Billy Curries bigger collection of synths and Chris increasingly focused on playing bass guitar.

The modifications were aimed at two limitations of using the Mini-Moog live in 1980; reproducing tight bass lines locked to the drums, and the tuning instability. The  modifications were quickly obsoleted by the introduction of MIDI in 1983 and digital synths.

The custom Mini-Moog was a key part of the revised Ultravox band and sound from June 1980 and the Vienna album. It featured in the bass lines of the album and was played by Chris Cross. I cannot be 100% sure about the modifications but I have collated as much information from the Internet as possible. Big thanks to Roy Gwinn for creating these modifications in 1980!

There are three visible modifications to the Minimoog;

  • Left Hand Side – Toggle switches, red push button, input jack and LCD display.
  • Lower Front – Extra potentiometer and a 7 segment display.
  • Env Generators – Toggle switches.

VCO3 Voltage

Early photographs show only two toggle switches on the left hand panel and no jack socket or red button. The displays are Digital Multi Meters that measure voltages and not music values such as tempo (bpm) or pitch (note values).

Trigger Sequencer This was a modification to the Mini-Moog and used to pump out a steady stream of eighth-notes, which could be transposed up the keyboard by Chris. By keying different notes, a bass line was produced with the unwavering perfect tempo of the machine.  This rock solid tempo had a hypnotic effect to it, which was an important part of the new Ultravox sound, which also used early Roland drum machines like the CR-78.

It looks like VCO3 was used as a LFO to drive the trigger sequencer using the external gate input, then switched into the filter and loudness envelope generators by the toggle switches in the Modifier section. The LFO/sequencer was not in sync with the drum machines and Warren simply played along with acoustic drums. Later on the drum machines were sync’d up as the clock source and the control panel further modified. The CR-78 does have a trigger out for this.

The first incarnation has only two toggle switches on the left panel, possibly external CV and Gate on/off. The second incarnation (1983?) has an extra push button, switch and input jack socket, which I presume is for an external clock from a drum machine, with the extra switch and push button controlling the clock in. This setup would have been easier to use on stage with Warren switching on the drum machine and Chris bringing in the clock on the Mini-Moog to create the bass sequence.

VCO1 & 2 Voltage

DDM Readouts There are two DDM displays; the LCD on the left shows the voltage going into VCO3 and therefore the trigger sequencer tempo, and four 7-segment LED displays show the voltage going into VCO1 and VCO2 – with an extra potentiometer (green cap) marked OSCS Find Tune, to the left of the display.

When VCO3 is switched to LFO mode for sequencing, it is disconnected from both master tune, pitch wheel and the keyboard. It is therefore totally independent and not affected by other pitch controls. The extra fine tune control is connected into the Tune CV output which goes to VCO1 and VCO2.

This voltage is monitored by the DDM before the keyboard and pitch wheel CV’s are added. You can see a display of 52, which might be +5.2V! This would be correct as the Tune control varies from 0 to +10V and its usually set mid way.

I wouldn’t be surprised if the 7-segment display predated the LCD display, as tuning would have been a big issue and LCD’s came out later. Early Mini-Moog’s were renowned for temperature instability and even later ones would have struggled in the hot conditions of a live stage. You can see Chris adjusting the tuning and checking the readout at the start of songs, like All Stood Still.

Env Generator Switches These are unmarked toggle switches, which I think switch the gate input on/off for the filter and loudness envelopes, and therefore whether they are driven by the trigger sequencer.

Replicating Today With the advent of MIDI sequences the Ultravox Mini-Moog trigger sequencer is redundant and its really easy to recreate the 1/8th note sequences on a DAW or MIDI sequencer. If you are into old school CV and Gate and have a Behringer Model D, you can patch the on board LFO to the Gate inputs of the VCF and VCA, to recreate the same effect. Unfortunately this is not possible on the Behringer Poly D, as even though there is a 4th VCO which can be set as a LFO it does not have an external output.

A better approach is to use a 8-step trigger sequencer with toggle switches (e.g. Nosie Engineering – Bin Seq), and drive this from a MIDI clock or analog clock. This delivers the driving 8th notes but with skips and rests, which is probably the way I will go.

Pitch instability of VCO’s is still with us on analog synths, but generally they are more stable than the early Mini-Moog’s. So there is not much call for an onboard fine tune pot and a DDM these days, although your DAW can use tuner plugins to get to the same result, or there are euro rack modules that do the same.

 

 

Lexicon PCM60 Refurb

Overview Back in 1980’s I could only dream of a Lexicon reverb, slim expensive 1U rack FX’s that lived in Pro recording studios. My first FX unit was (IIRC) an ART reverb which was quickly replaced with an Alesis Quadraverb, which served me well for 10 years in the 1990’s. During 2000/2010 I downsized the studio and had no need for FX, even a TC3000 only lasted a year. New synths around this time tended to have in built FX.

In 2017 I built a new studio space with room for some outboard. I was fortunate enough to buy both a PCM70 and PCM80 from a couple of old recording studios that were closing down, one of them in France. These are the back bone of my reverb setup, along with an old Yamaha REV7, which I first saw in a recording studio in Reigate in the mid 80’s. Gated Reverb – ha!

Whilst the PCM70 and 80 have some great reverbs, they are more useful during mix down, rather than for tracking synths. They stay hooked up the the analog console as outboard. For tracking synths I prefer older Roland SDE delays which sound like refined tape delays, a refurbished Quadraverb or my newly acquired PCM60 reverb.

Lexicon PCM60 The advantage of the 60 is you get two reverb algorithms derived from the legendary Lexicon 224 (Room and Plate) with just 4 time delay settings and on/off low and high EQ. No presets, everything is real time button selection, no deep spaces to get lost in – just rooms. The 60 also has an effects send and return which means you can patch in a delay or EG to further change the sounds.

It took me a few years to find one, and its not a perfect example by any means. But I finally have a PCM60 which I use with my “Big Moog”, its patched into the 984 Matrix Mixer, whilst the PCM60 effects loop goes of to a SDE1000 Digital Delay. I prefer to print sounds and FX on the way into my DAW, rather than in post production.

PSU Recap

Refurbishment Outside the PCM60 needed a clean, and there is writing underneath the buttons where someone has recorded their patch settings. Unfortunately this permanent ink will not come off and is embedded in the now faded blue part of the panel. I will have to live with this, unless I can find a better panel in the future.

Electronically the PCM60 works well with only two issues; The input level pot is worn and jumps to maximum causing a lot of distortion as the input stage is overloaded. Fortunately a similar pot is still available from Mouser, so I replaced it. It is not a perfect match; the pin out is further forward and the shaft a slightly smaller diameter. The knob is also a very tight fit, but it works.

The second issue is that one of the signal level indicator LED’s is not working. I suspect this is a problem with the LM3915 driver chip which is socketed, so it was easy for me to replace it. However the problem was a LED failure, so I replaced all the green LED’s, as the modern LED’s have a slightly different colour. Its easy to remove the LED PCB from the front panel and swap the LED’s.

The Lexicon components are high quality and the PCB layout is excellent. The power supply capacitors date back to 1984, so I have replaced them with new Panasonic high quality versions. I am pleased with the sound and noise floor, so I haven’t made any Op Amp changes which would be; FET input and output S&H to AD823,  Bipolar input/output to LM4562.

Outcomes The PCM60 is very well engineered, cheaper and rarer than the well loved PCM70, with two sets of sound that are very nice for vocals and drums. I use it with shorter percussive synthesizers sounds rather than for big ambient pads, which need longer reverb times from my PCM70 and 80.

There is a set of version 2 ROM’s that replace the Plate reverb with an Inverse Room, which was popular back in the 1980’s, but rather a cliché now. The V1.0 ROM’s are 24-pin 2732’s and the V2.0 is 28-pin 2764, they can be kludged together and manually switched but the Inverse Room is not worth the bother and cost.

 

Roland SDE1000 Repair

Roland SDE1000

Overview I bought a broken Roland SDE1000 Digital Delay in November 2020 for just £40, but with the known fault of it not powering on. I already have a mint and boxed SDE1000 which I find to be a fantastic delay unit for using with analog mono synths. The plan is to repair this one and use it with my “Big Moog”, instead of the Revox A77 tape delays that Klaus used.

This particular SDE1000 was manufactured in March 1985. This was two years before I rekindled my interest in synths and a secondhand ARP Odyssey Mk3.  Although the Roland was a lot cheaper than studio FX at the time, it was an expensive luxury for me at the time.

The Roland SDE-1000 was one of the Japanese company’s first effects unit, launched in July 1983 at £399 alongside the more powerful “studio version” SDE3000 which was double the price. The delay unit is quick & simple to use, with a very straightforward front panel & interface.  Sound quality is surprisingly good with a smooth, very analogue sound. This is in part due to the companding circuits to get the analog data into just 12-bits.

Maximum delay time is 375ms in standard mode (750 ms in x2 mode) and 605/1210 ms using the x1.5 rear panel control.  The sound of the SDE in X2 mode is quite reminiscent of a slow tape delay, and increasing feedback results in a gradually decaying, dulling repeat – again, like tape.

Delay time is displayed on a 4-digit blue fluorescent display with an Up – Down rocker switch to alter delay settings. The LFO has speed & depth controls, which can give deep chorusing effects or a gently shifting delay with phasing.  There are only 4 memory presets and no MIDI but the SDE1000 was a sales success with over 7,000 made, and it remained popular before Alesis entered the market with custom DSP chips.

SDE1000 Advert

Technology The SDE1000 is an early digital delay which uses a Gate Array chip as the main controller rather than a DSP chip, and a 12-bit R2R DAC rather than a dedicated DAC chip. The microprocessor is the familiar 8049 which Roland used in many products during the early 1980’s. The analog signal is compressed into a 12 bit data word with three 64k bit RAM chips used to store the digital data.

Roland used good quality NE5532 Op Amps in the output circuits and a dual transistor input buffer

Changes The SDE100 went through a number of circuit changes in 1983 to improve the headroom and HF response. They are documented in the service manual. This particular unit in from 1985 so it has these changes implemented and a V3 PCB which is not mentioned in the service manual.

The Repair This SDE1000 is in reasonably good external condition and a sound interior, which has possibly had a small amount of repair work (like the rear pot for time adjustment). It is a robust and reliable design, so I am not expecting chip failure but a power supply problem.

The power supply provides many different voltage rails;

  • +/-15V rails for the analog circuits using a discrete voltage regulator
  • +5V for the digital chips using a 7805
  • +12V rail (7812) for the front panel LED level indicators
  • +12V for the DAC voltage reference
  • +20V for the LCD driver chips
  • +1.7V for the LCD itself

Any part of these circuits could be where the short circuit is, so this is going to take some time! I checked the X2 safety capacitor on the mains side of the transformer and it was ok but I have replaced it. I disconnected the power connectors until I was left with the +1.7V rail which was the source of a short circuit.

I disconnected the LCD and switch PCB from the main PCB to eliminate it as a source of a short, however there was an additional short in the 15V rails. In the end I replaced the W02 regulators and power diodes, all the power supply capacitors, power transistors and power capacitors. Even though none tested as failed, this complete overhaul solved the problem and the SDE1000 powered up perfectly and works a treat.

Sounds The original factory SDE1000 presets, which can be overwritten, are:

  • Long Delay: 750ms with feedback and light modulation
  • Doubler: 30ms delay with feedback and no modulation
  • Chorus: 50ms of delay with light modulation and feedback
  • Flanger: 15ms of delay with deep modulation

The sound quality does deteriorate when in x2 mode with a limited bandwidth of just 8kHz. So I keep the delay in x1 mode with the rear trimmer set to 1x or 1.3x.

Outcomes A nice warm digital delay line for £40 and another £40 on component replacements, that will maintain the SDE1000 for another 35 years! I replaced the original old battery while I had the delay apart and I will re-calibrate using the notes in the service manual before it finally goes in the studio. The slight change in power supply voltage rails may not be exactly as it came in the factory back in 1985.

Behringer 904A rewinds to 1967

Moog 904A Caps

The 904A Low Pass Filter The Moog low pass filter module dates back to 1966 and is probably the reason you want a Moog modular! The Behringer VC LPF module is a faithful copy and uses THD polyester capacitors in the filter stages, with smaller values to ensure they fit onto the PCB.

It sounds very nice, but lets make it sound fantastic and rewind back to 1967 and fit the same filter capacitors that Moog used in the IIIP.

Rewind to 1967 To get to the 1967 Moog 904A is tricky, as the larger capacitor values are much harder to locate, and are large in size – if the correct polypropylene types are used. These caps won’t drop into the existing Behringer PCB space, so the original capacitors need to be removed and a new daughter board fitted to the rear.

I developed the daughter board to use a combination of 1.5uF, 390nF and 100nF hand matched polypropylene capacitors. It plugs into where the old Behringer capacitors were on the main PCB, using PCB pins and has a cut out for the existing 10-pin power socket. The PCB also supports the AMSynths 904C, as it puts the jack socket inputs and outputs onto a rear mounted Molex connector.

Behringer 904A

Outcomes Testing the upgraded filter correctly shows that the frequency cutoff is lower due to the larger filter capacitors. The frequency cutoff is easily adjusted with the RANGE trimmer as there are holes in the new PCB so that both trimmers can be adjusted.

Listening to the audio output and measuring it with a frequency analyzer, reveals that the filter resonance quality has improved, with resonance down to a lower 40Hz and a 3rd order harmonic peak appearing in addition to the 1st and 2nd orders. This is as a direct result of using matched polypropylene caps.

These are subtle rather than dramatic improvements, but very worthwhile in terms of filter resonance and the overall quality of sound. It is now the sort of filter that you want to use all the time! It is worth noting that the 1967 version of the filter did not self oscillate, and it was only later that the circuit was modified. I have retained the self oscillation as its a key part of the Moog sound.

Outcomes The AMSynths PCB, capacitors and headers take a couple of hours to build, followed by some carefully desoldering of the Behringer polyester caps and the installation of the new PCB. This is an easy project for the competent DIYer. The production PCB’s are populated with matched capacitors and the PCB’s are on sale here, or you can send me your 904A for a professional upgrade.

SCI 700 Programmer

SCI 700 Mark 1

Overview  Back in 2002/3 I bought and restored a couple of SCI 700 Programmers, and here is the “restored” blog page from my old web site. The Sequential Circuits Model 700 Programmer is a rather useful way of adding 64 patch memories to an analog synthesizer. In November 2002 I bought one from the USA that need some repair work.

The front panel had corroded quite badly and it was externally in a poor condition. So I decided to do a full electronic restoration and rebuild it into a 5U 19″ panel so it could sit with the AM Modular cabinets. The picture on the right shows the Model 700 as it arrived from the USA. In April 2003 I acquired a second Programmer 700 (a Mark 1), this required a lot less work to get it fully operational. I sold both of these in the mid 2000’s.

Overview  Back in 1976 Sequential Circuits was a small music technology company based in San Jose, California and run by Dave Smith from his garage. After successfully designing and launching a digital sequencer (Model 800) Dave and new recruit John Bowden (x-Moog) went on to create the Model 700 Programmer. The new product provided rudimentary patch memories for analog mono-synths like the Mini-Moog and ARP 2600. The table top programmer had 64 memories of 3 control voltages to drive the VCO’s and two DADSR envelopes to drive the VCF and VCA of the partner synthesizer. In 1977 this was a major step forward, with only Oberheim providing patch memories on its 4 and 8 voice synthesizers.

Mark 1  Dave launched the Model 700 in 1977 and sold one a week, mainly in the USA. Here is a Mark 1 model, all the knobs are large, there are no CV trimmers on the top left hand panel and the patch switches are engraved with numbers. The record switch is part of the right hand lower toggle switch.

SCI 700 Mark 2

Mark 2 In 1979 the Mark 2 model was released with front panel V/octave trimmers for the 3 control voltages, and a single multi-way socket for a single cable connection to the partner synthesizer. The Mark 2 has revised PCB’s and a slightly different circuit design. The original CA3080 and matched transistor design was updated to use SSM2050 and SSM2020 chips, along with TL072 Op Amp’s.

The high quality sealed cermet pots were also replaced with standard carbon pots. All these changes meant the Model 700 could be manufactured for less cost and Sequential Circuits went on to sell over 200 Programmers, and it was still in the catalog in March 1981.

The Model 700 has a place in synthesizer history and it provided some funding and technical R&D for the legendary Prophet 5. Below is a Mark 2 version with Prophet 5 knobs, and lettering above the tactile switches. The 7 segment display is larger and the patch Record facility has a red button rather than being on the right hand lower toggle switch.

Rick’s SCI 700

Description The Model 700 has 3 independent control voltages for controlling the pitch of the partner synthesizers VCO’s. Each pitch is controlled by a rotary potentiometer, and a single external control voltage from a keyboard can be optionally added into each pitch with 3 toggle switches. The pitches are quantized to semitone values.

Two envelopes can be programmed with the usual ADSR and an initial delay – very Dave Rossum! The envelopes are triggered from gate or trigger inputs, usually from the external keyboard. The envelopes are designed to drive the VCA and VCF of the partner synthesizer directly. Each envelope has its own VCA so that the envelope volume can be controlled from the 700. There is an additional offset control for envelope 2, so that the initial VCF cut-off can be set too.

Memories Once a patch has been set up with the programmer you can store it into one of 64 memory locations, selected by 8 switches (Programs) and a rotary switch (Banks). A red Record button is partially sunken into the control panel, which means it is not accidentally pressed. Patches can be recalled or the programmer can simply by used as a live set of controls.

Tricks  The 700 has a few neat tricks up its sleeve. The 8 Programs within any Bank can be incremented electronically from a footswitch or a LFO or keyboard. Using an external LFO you can create a traditional 3 control voltage x 8 step sequencer with different VCF/VCA sounds for each step. The number of Programs that are incremented can be adjusted from a rotary front panel control from 0 – 8. When set to 3 the Model 700 will switch Programs as 1-2-3-1-2-3-etc.

Technology The Model 700 is based around CMOS logic chips, there is no micro-processor as the new Zilog Z80 was too expensive to use in 1976. Dave couldn’t use ADC chips either, so he emulated one with discrete digital and analog chips. The control voltage resolution is 6 -bits, which for the VCO controls equates to 64 discrete voltages, 5 octaves from 0 – 5V. However the envelopes also get the same resolution, which is a bit more of a limitation.

Inside there are two neat and well laid out PCB’s stacked on top of each other. The first board contains 16 pots which are scanned and stored as 6-bit digital words into 768 bytes of SRAM storage, which is backed up by a 3V lithium battery. This data is used to drive the 3 control voltages, and to control two DADSR envelopes based on SSM2050’s, along with two VCA’s built from SSM2020’s. This design is very similar to the Dual Transient Generator made by E-mu Systems.

SCI 700 Panel

Restoration Part 1 The first step was to see if the Model 700 would power on, and to do a smoke test! A quick internal visual inspection showed no burn outs, so I powered it on and was greeted by a set of LED’s that worked perfectly but no control voltages or memories. Plus the tale tell smell of burning – a tantalum capacitor was on its way out!
A detailed inspection revealed:

  • The +5V rail was okay – the regulator was replaced as a precaution
  • The +15/-15V rails were dead – both regulators were replaced
  • The comparator reference voltage had failed – thanks to the blown tantalum capacitor. I replaced this with an electrolytic and replaced the scarred 47 ohm resistor on the inbound connection to the LM723 regulator chip – which I also replaced as a precaution.
  • The lithium battery was dead – so it was replaced with a Varta 3V lithium 2/3AA
  • The transformer was in very good condition – so it was left alone.
  • Some IC’s had previously been replaced, as there were some new IC sockets. I added sockets when replacing chips, as Dave Smith had only put sockets in for the SRAM memory chips.

An order for new regulator chips was sent out, and whilst I waited for the parts to arrive I replaced the tantalum capacitors with electrolytic capacitors, and completed the following upgrades:

  • The electrolytic power regulator capacitors were replaced with high temperature versions – they are smaller too!
  • All ceramic disk capacitors were replaced with new dipped versions
  • The rusty old pots were replaced with Bourns sealed conductive versions
  • New E-mu control knobs in solid aluminum replaced the beaten up originals
  • New slide switches replaced the worn out Switchcraft switches which had started to have a redundant center position, as well as lots of surface rust.

Restoration Part 2 With the new power regulators in place and the rails working again, it was possible to power up the 700 and see what else had failed. Quite a list!:

  • Voltage 1 had gone permanently -12V, this was fixed by replacing IC4 (LM348 Op Amp)
  • Voltage 2 was fine.
  • Voltage 3 was correct but modulated at about 20Hz, once again replacing a LM348 Op Amp at IC3 sorted the problem.
  • Envelope 1 was at 0V – the SSM2050 and SSM2020 checked out okay, so it was clear that IC3 was the problem. When replaced the envelop worked perfectly.
  • Envelope 2 was fine, although the release time was over 30 seconds! I will add a trimming control, as in the SSM2050 application notes.
  • The patch buttons didn’t work consistently, so I replaced almost all of the CMOS logic around the scan and clock logic, using new IC sockets. The key problem turned out to be IC14 – it needed an exact replacement MC14163. Near equivalents gave erratic responses.
  • The 12 position rotary switches have been replaced with new plastic ones. The original shafts were damaged when I removed the control knobs, and they were rather clunky.

April 28th 2003 (5 months after purchase!) and everything was working perfectly again.

New Panel And finally a nice new 3mm aluminum front panel, 5U high, 19″ across in aluminum with black lettering. This replicates the old panel exactly, so that the PCB’s can be mounted directly to the panel. The rear jack sockets for inputs and outputs have been moved to the right hand side and are 3.5mm jacks (my studio analog synth standard). The new front panel was designed during May – July 2003, with many checks against a paper print out of the front panel. The panel was finally ordered on 8th August.

Restoration – Again The second Model 700 I bought in April 2003 required a lot less work, I’ve added a lithium battery and replaced all the decoupling and PSU capacitors. It is a Mark 1 model, which means its based around lots of CA3080’s, with no SSM chips, very different to the Mark 2.

I have kept the original red LED’s and 7 segment display, replaced the potentiometers with high quality Spectrol units (there is not enough space for the deeper Bourns 91A). The control knobs were replaced as well. They stood proud of the front panel by 1/4″ thanks to the factory not cutting the pot shafts down to size! Overall the unit worked very well as bought, which is a good job as it cost £300.

AMSynths 700 Well its now 2020 and the time is right to reproduce the 700 as a EuroRack module. My “Big Moog” has a set of 3x 921B VCO’s and a PPG303 filter that are ideal to connect up to a new 700 programmer with 64 patches. The availability of CEM3310 clones makes a reproduction 700 viable and the patches (and notes) can be driven by the 960 sequencers, so the timbre of each note changes. The 700 quantizes the VCO notes into semitones, so this is a perfect match for a 960.

A&H Spectrum Mixer Refurb 3

Second Master/Group Section

Overview During March and April 2020 I had more time to complete the refurbishment and was lucky to find a set of two 8 channel Spectrum input channels with panels and frame on eBay. This enables me to refurbish a full set of channels outside the mixer, as well as having a full set of spare pot and switch buttons. The panels were more rusted than my original, so I am only transferring the channel cards in.

In October I located a Master/Group section (made 30 April 1991) with rather nice wood ends, some different coloured pot caps and a rusty front panel. This enables me to restore a set of master and group cards whilst keeping the working Spectrum in the Studio. A previous owner had started to modernise the Left and Right master channels with nice Nichicon caps, OPA2134’s and unusual Roederstein EKU Bakelite Bipolar Caps. Unfortunately cheap IC sockets were used and the Op Amps move about in them, so these will be replaced with turned pin versions.

Master Section Refurb The two master channels have the highest level of noise in the mixer even with the groups and channels off. The L and R signals are buffered by a 5532 Op Amps and then it is TL072’s downstream from there on FX Sends. The 5532 buffer was replaced with bi-polar OPA1612 Op Amps and the FX return TL072 with OPA1642. Both the new chips are SMD and are fitted with PCB adapters and 100nF SMD power decoupling capacitors.

The OPA1642 was chosen due to its low noise (5nV) and low power consumption at 28% higher than the TL072. It also has low THD and reasonable offset at 1mV. The 5532 is also used in the five balanced output drivers where it is replaced by a new PCB and THAT 1646 chips. The usual electrolytic recap was completed at the same time and the slate oscillator fixed. The ten Op Amp upgrade cost was around £45.

This was the original plan, but the second mixer had already been upgraded with OPA2134 and OPA2604 so I will try with the cards initially on the Left and Right channels.

Original EBOS Board

Master Balanced Outputs The Master section has five balanced outputs that use a separate EBOS PCB with Main Left & Right, Mono and Aux 3 and 4 outputs. It is an important part of the mixer and I want to ensure it has high performance.

The circuit uses NE5532 Op Amps and a trimmer to get the offset correct. Whilst this design was fine in the 1990’s, the rest of the PCB needs updating with new caps, 1% resistors and new cermet trimmers. The PCB is single sided and poor quality, with traces breaking, not at all like the main PCB’s. So rather than restore the old PCB, I have designed a new PCB that uses modern THAT1646 chips which provide an uplift in performance and reliability.

The new PCB has RFI suppression added to each circuit, with 100pF caps and inductors on the outputs. The THAT chips reduce the power consumption from 40mA to 30mA, which isn’t much, but any power reduction is worth having! The slew rate and THD also improve, so we should hear a better sound.

Group Refurb The eight group circuits have a high noise floor and a couple of the groups are not working correctly. The design has five sections:

  • Group bus summer
  • FX Sends for the lower and upper channels
  • EQ for the lower and upper channels

The bus summing is done with NE5532’s which have been replaced with bi-polar OPA1612 Op Amps. The FX and EQ originally used TL072 Op Amps which I have replaced with JFET OPA1642’s which reduced noise, and improve slew and THD. The upgrade cost is £22 per Group, with 10 IC’s replaced per card.

Channel Refurb The channels are a lower priority for upgrading due to the reduced cost/benefit, and the priority is to get all 16 channels working. Each channel has 4x TL072 Op Amps and a NE5532 as the combined Mic/Line pre-amplifier. Replacing all the Op Amps across 16 channels would cost £350, and I have to be careful not to increase power consumption. Upgrading the TAPE inputs to balanced is a priority.

With more channel cards available from buying another Spectrum I can experiment with different op amps.

PC Noise My Spectrum lives above a Windows PC used for audio recording, which puts out a lot of high frequency junk onto the mains which the mixer picks up rather loudly. This has been sorted out, but moving the mixer onto a clean power supply did not make much impact. I have added an external mains EMI filter to the PC along with ferrite rings on the PC and Mixer power cables. I also have an EMI internal filter for the RPS power supply and I will check the mixer frame ground is sound.

 

 

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

OB-Xpander

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).

 

 

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