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SCI Model-800 Sequencer

Sequential Circuits Model-800 Sequencer

Sequential Circuits Model-800 Sequencer

Sequential Circuits Model-800 Sequencer

Sequential Circuits Model-800 Sequencer

The Model-800 Sequencer was one of the first products that Dave Smith designed for his company, Sequencial Circuits. It pre-dated the more famous Prophet-5.

Look Ma, No Knobs!

The Model-800 is a digitally controlled step sequencer. However, you'll notice that there are a lack of knobs (other than for tempo). Instead of 'dialing in' each note via a pot, and then being stuck with an 8-note-to-the-bar sequence, it records CV and gate voltages in real time. You can play at any speed, with any syncopation, and with any note lengths. It can record up to 256 notes, and you can edit note pitches and durations. That's right, real time digital sequencing in 1977.

Before I describe the repairs for this particular unit, I'll present some details on interfacing and operation. If you want to see the Model-800 in action, there are some links to demo videos at the bottom of this article.

Interfacing the Model-800

Model-800 Back Panel

Model-800 Back Panel

Model-800 Back Panel

Model-800 Back Panel

Model-800 Rear Jacks

  • External Clock In
  • External Clock Chain Out
  • Trigger In
  • CV In
  • Trigger Out
  • CV Out
  • Foot Pedal

The Model-800 has ample jacks for external clocking, chaining multiple Model-800s together, and separate CV sources for input and output. The jack labeled 'Trigger In' should really be labeled 'Gate In'. The Model-800 records actual gate-on time, and doesn't just record the trigger event.

There is also a foot pedal jack, for hands-free operation of the controls.

Model-800 Interfacing Table

Model-800 Interfacing Table

Model-800 Interfacing Table

Model-800 Interfacing Table

Before MIDI came along, interfacing synthesizers together from competing manufacturers was not straightforward. Each brand used variations on gate voltage and polarity, CV voltage range and scaling, and gates vs. triggers.

The Model-800 has several toggle switches on the back panel to correspond with the most common settings. Here is a table from the original owners manual, showing how to set things up for various synths.

Operating The Model-800

Model-800 Memory Banks

Model-800 Memory Banks

Model-800 Memory Banks

Model-800 Memory Banks

The Model-800 has 16 memory banks, which can each hold up to 16 notes. You can enable or disable the memory banks with toggle switches on the front panel. This allows you to have multiple sequences in memory at the same time, or protect a sequence from erasure. You can also perform minor arranging as a sequence is playing, by enabling and disabling memory banks in real time.

Model-800 Step Switches

Model-800 Step Switches

Once the memory banks have been selected, you can step through the individual notes using the Step and Reset toggle switches.

These functions are used mostly for resetting the sequence for playback or record, or for editing individual notes.

Recording on the Model-800 is fairly simple. You select the memory banks, reset to the beginning of the sequence, press the record button, and start playing.

Playing back is easy as well. You select the memory banks, reset to the beginning of the sequence, and turn the clock on. At the end of the sequence, it will automatically reset to the beginning, and play back in an endless loop.

Finally, there are toggle switches on the front panel that can enable or disable recording of the pitch CV, and also record a single note (with the clock disabled). These can be used to edit the pitch or duration of a single note.

Opening It Up

Model-800 Circuit Board

Model-800 Circuit Board

Model-800 Circuit Board

Model-800 Circuit Board

The unit that I was repairing would light up when you turned it on, and it could single-step through its memory banks. However, it wouldn't record or play back.

When I opened it up, I was surprised to find out that there was no CPU on board. All of the functionality was developed with 7400 series logic chips, static RAM, and rudimentary DAC/ADC circuits. This is quite a feat, considering the number of features that the Model-800 has.

Model-800 Block Diagram

Model-800 Block Diagram

Model-800 Block Diagram

Model-800 Block Diagram

No Service Manual Or Schematics

The Model-800 is relatively rare, and the service manual and schematics are even rarer. However, to fix something, you need to understand how it works. So I started drawing block diagrams, and tracing the circuits.

I started at the system clock (a simple 555 timer), and worked my way towards the logic chips and RAM. I didn't end up making a complete schematic, but I made lots of partial diagrams like this.

Is 6 Bits Enough?

On the photo above, I'm pointing to 5 static RAM chips. Each one holds 256 4-bit nybles. This gives a total word width of 20 bits.

Out of those 20 bits, 14 are used to count the note duration. I later determined that with the tempo knob in the middle, the clock ran a bit faster than 1 kHz. Since 14 bits corresponds to a maximum value of 16384, the longest note value (or rest) you can record is between about 15 or 16 seconds.

This leaves only 6 bits left to record the CV voltage.

The latest obsession with MIDI to CV converters is how many bits are required, to give accurate pitch CV. The debate ranges from 12 bits, all the way to 16. How can 6 bits possibly be enough?

It turns out that 6 bits is just more than enough. However, some assumptions have to be made.

The Model-800 can record a range of 5 octaves. If we quantize to the semitone, and don't record any pitch bends or portamento, this corresponds to 61 distinct values (C1 to C6).

A 6-bit DAC has 64 distinct steps. If we calibrate it so that the first 61 steps sweep between 0 volts and 5 volts, we should have an accurate pitch CV. Of course, this only works for synths using a 1v/oct scaling.

If you look back at the circuit board photo, you'll see the DAC circuit in the lower right corner. It is comprised of a 6 resistors in a voltage ladder, 6 transistors to feed the ladder, a blue trim-pot, and an op-amp as a buffer.

Troubleshooting And Repair

Once I had a handle on how the Model-800 operated, troubleshooting turned into a signal tracing exercise.

It became obvious that the internal clock wasn't running. This was due to a blown 555 timer chip, and some CMOS chips that were used to generate various waveforms and route them to other parts of the circuit.

The sequencer started to operate normally again, however the sampling circuit and most of the output voltages were very odd. I found out that several buffer op-amps were blown, or running very hot. I began to suspect that this Model-800 was hit with a big static shock, or that it was plugged into a bad power source.

Once I replaced all of the blown components, and calibrated the DAC, everything ran correctly.

Model-800 Tech Overview

Model-800 Tech Overview

Model-800 Demo

Model-800 Demo

10 comments on SCI Model-800 Sequencer

joseph karstens
Outstanding! I have the untouched and still functioning model 800...serial# 073...I could not find any info on this device...til now..thanks..excellent tip vids...very helpful...j.k
February 1st 2015 23:47 EST
Keith
Hi Joseph.

I'm glad you found this information useful. When I get something rare on my bench, I like to document it.

You should also check out the ModularSynthesis webpage on the Model 800.

http://modularsynthesis.com/SCI/800/m800.htm
February 2nd 2015 22:16 EST
James
hi Keith, Thanks for all of your info and videos! I am very fond of the 800 and already have a Roland CSQ-600. The 800 is appealing due to its 16 banks that can be toggled, as opposed to the 4 on the CSQ. However the CSQ has two major benefits… The number of notes storable in each bank, and the ... Read More
November 16th 2015 16:17 EST
Keith
Hi James, You can program the Model 800 in step mode, without recording 'live' gate times, but it requires a trick. See my other Model 800 video on syncronizing it to an external drum machine (right at the end of the video). https://www.youtube.com/watch?v=d_2JAXgmk4c There is no easy way ... Read More
November 18th 2015 09:04 EST
James
Thanks Keith! That video explains it perfectly. So, the sequencer functions as normal in that state? One can use all of the toggle on/off sequences and everything will proceed like normal? Would you be willing to do service work to a machine? If i had one that worked perfectly, would you consider ... Read More
November 18th 2015 09:52 EST
Oliver
Hi Keith, Flirts of all congratulatiosn for the amazing work you did with the model 800!!! my question is simple, could you explain me how do you proceed to sync the model 800 with an external clock because i can't do it. The internel clock works but when i plug an external clock the sequence ... Read More
September 20th 2016 17:27 EDT
Keith
Hello Oliver. I don't have a Model 800 in my studio anymore, and it's a bit hard to troubleshoot your issue without having some measurements of the clock you are sending to the Model 800. What are you using as an external clock? I made a video showing various ways to sync up to the Model ... Read More
September 21st 2016 07:57 EDT
Olivier
Hi keth and thank you to come over me so fast! I use a umidi from intellijel. I couldn't see on your video how do you plug your drum machine, vco or external clock. The connector i have on my model 800 has 4 pins 1 is ground 2 is + stop in 3 is - slave select 4 is clock in The sequencer start ... Read More
September 21st 2016 18:48 EDT
Keith
Hi again Oliver. The clock-out on the uMIDI should work. Connect the tip on the uMIDI clock-out, to the clock-in on the Model 800. Connect the sleeve (ground) on the uMIDI, to the ground on the Model 800. Depending on the version of Model 800 you have, there may be a switch on the back to enable ... Read More
September 22nd 2016 07:48 EDT
Olivier ROSSI
Hi Keith, I am so sorry for this very long silence but i had a lot of work to do with my own job. I thank you so much for the time you spend for me. I tried today to sync the model 800 using the uMidi as you told me. The sequencer start and run in perfect sync with my external clock. However, in ... Read More
January 28th 2017 08:40 EST
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