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.
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.
Model-800 Back Panel
Model-800 Back Panel
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
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.
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.
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
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.
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.
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
