SCI Model 700 Programmer - Restoration & Cloning

Overview 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.

A detailed description of the Model 700 is here: more »

In April 2003 I acquired a second Programmer 700 (a Mark 1), this required a lot less work to get it fully operational. It will be recased and restored.. more »

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 rudamentary 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 synthsizers.

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..

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 money.

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.

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 potetiometer, 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 incrementade 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 laided 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.
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 aluminium replaced the beaten up originals
• New slide switches replaced the worn out Switchcraft switches which had started to have a redundant centre 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! So I will add a trimming control, as per the SSM2050 application notes - so that both envelopes can be trimmed to the same times.
• 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. I may replace the set of matched resistors that determine the control voltage quantisation, with 0.1% precision resistors - along with a new LM393.

Blue LED's  One of the nicest visual upgrades on old analog gear is to replace the rather dull red LED's with some new blue LED's. The 700 uses eight 3mm LED's within the front panel switches that select patches. The old ones are easily desoldered and removed. The cap of the tactile switch can be easily prised off for access. New blue LED's are easy to solder back in, but the associated resistors need to be replaced with new ones, to ensure the blue LED's are driven at the right brightness and voltage. The new LED's are so efficient that a 1k8 resistor is a suitable replacement for the original 150 ohm ones. Lower values result in too bright a blue light!

Blue LED 7 Segment Display The "blue" theme continues. After considerable searching I finally found a dual 7 segment display in blue from Digikey in the UK. The original display is a high intensity red display, a Fairchild MAN6740 with 420ucd luminosity and a forward voltage of 2V per segment. The replacement blue display is a LiteOn LTD-5523AB, with 4300ucd luminosity and 3.8V forward voltage. It requires the seven LED resistors to be changed from 150 ohms to 22 ohms, to ensure the correct display intensity. The red arcylic filter has to be replaced with a neutral one.

New Panel And finally a nice new 3mm aluminium front panel, 5U high, 19" across in aluminium 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 duirng 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, but rehoused it into a 19" rack case and added a 240V power supply. The potentiometers are well shot, so they were replaced with high quality Spectrol units (there is not enough space for the deeper Bourns 91A). The control knobs are being replaced as well. They stand 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.