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Early Version Of The SCI Model-800 Sequencer

Early SCI Model-800 Sequencer

Early SCI Model-800 Sequencer

Early SCI Model-800 Sequencer

Early SCI Model-800 Sequencer

A client of mine sent me a Sequential Circuits Model-800 Sequencer for repair, with the incredibly low serial number of 003. I've written an article on repairing a Model-800 in the past, but this unit was special.

Serial Number 003

Serial Number 003

Serial Number 003

Serial Number 003

The early units had a high resolution DAC, which gave them compatibility with Hz/V synths (like the Korg MS series, or the Yamaha CS series). The later units were only compatible with 1V/octave synths.

A High Resolution DAC (for 1977)

Zeltex 10-bit DAC

Zeltex 10-bit DAC

The early Model-800s used a 10-bit DAC from Zeltex (ZD431). This monolithic DAC comes in a rather large package (almost 6 cm x 6 cm), and was probably one of the most expensive components in the sequencer. At some point in the production process, they switched to a much cheaper 6-bit DAC, built from a resistor ladder, transistors, and an op-amp.

The switch to a 6-bit DAC saved in production costs, but also limited the resolution of the pitch CV information that could be recorded. Paraphrasing my previous article:

"A 6-bit DAC has 64 distinct steps. If it's calibrated so that the first 61 steps sweep between 0 volts and 5 volts, the pitch CV should be accurate over a range of 5 octaves. Of course, this only works for synths using a 1V/octave scaling."

Here Comes (A Lot Of) Mathematics

If you are not into the mathematics of music, scroll down to the section named Additional Differences. Otherwise, continue on...

With the 6-bit DAC you were forced to quantize to the semitone. However, with the original 10-bit DAC it is possible to record much finer changes in pitch.

Here is a bit a math to calculate the minimum pitch change that can be recorded.

Minimum Pitch Change Calculations

  • The total range is 10 bits (2^10 = 1024 steps).
  • There are 5 volts in 5 octaves, assuming 1V/octave.
  • 5 volts / 1024 = 0.00488 V/step
  • There are 0.08333 V/semitone, assuming 1V/octave.
  • 0.08333 / 0.00488 = 17 steps/semitone
  • There are 100 cents per semitone.
  • 100 / 17 = 6 cents/step

Therefore, with the 10-bit DAC you can record pitch changes of only 6 cents. Not bad, considering that the threshold of human detection is around 4 cents.

Hz/V Synth Compatibility

One of the shortcomings of the 6-bit DAC version of the Model-800 is that it doesn't have enough resolution to record CV information from a Hz/V synth like an MS-20 or CS-15.

With the extra resolution from a 10-bit DAC, would it be possible to record pitch CV from Hz/V synths as well? The answer turns out to be, yes! Rather than state this as fact, I'll do the calculations to prove it.

Let's make some assumptions...

Assumptions For Hz/V Calculations

  • The highest C we want to record is referenced to 4 volts.
  • Each lower C (going down in octaves), divides this voltage reference by 2 (which is how Hz/V synths work).
  • We still want to record a 5 octave range.
  • The voltages corresponding to each C are: 4 V, 2 V, 1 V, 0.5 V, 0.25 V and 0.125 V.

Notice that the voltage difference between notes gets smaller and smaller as we go down the keyboard. The two questions to answer are:

1. What is the difference in voltage between the lowest 2 notes (C and C-sharp)?

2. Does this difference exceed the capabilities of a 10-bit DAC?

Yes, Even More Math

The ratio between the frequency of two semitones varies by a factor of the 12th root of 2. This is the basis of the equal temperament scale.

This factor is 2^(1/12) = 1.05946

If we multiply 0.125 V (corresponding to C) by this factor, we will get the voltage that correspons to C-sharp.

0.125 V * 1.05946 = 0.13243 V

All of this rambling answers the first question. If low C corresponds to 0.125 V, and low C-sharp corresponds to 0.13243 V, then the difference in voltage is:

0.13243 V - 0.125 V = 0.00743 V

The second question was actually answered much earlier, when we were calculating the smallest pitch change for 1V/octave synths. We determined that the voltage difference between steps in the DAC was 0.00488 volts.

Since the voltage difference between low C and C-sharp is 0.00743 V, and there are 0.00488 V between steps in the DAC, then there are almost 2 DAC steps between these pitches. This is barely enough to record the difference between low C and C-sharp. It might be slightly out of tune on low C, but the rest of the range will be in tune.

Additional Differences

Model-800 Front Panel Differences

Model-800 Front Panel Differences

Model-800 Front Panel Differences

Model-800 Front Panel Differences

In addition to the DAC, there are some more obvious differences between the early vs. later Model-800s.

There should be a clock status LED above the clock On/Off switch. This was missing on the unit my client sent me. This is normally only an issue when you are using the footswtich to toggle the clock, as it overrides the front panel control, and there is no physical indication of the clock status.

In the middle of the front panel there should be a control labeled One/All. This is used to select only the first recorded sequence for playback, or to loop through all recorded sequences (similar to a sequence chain mode). This was also missing on the early unit I repaired.

One final front panel control that was missing was the Rest toggle switch. This is used to manually insert a rest into a previously recorded sequence. It is usually crammed in between the Step and Reset switches. Not only was this missing on my unit, but it's rather rare to see on later models as well. This switch is normally not labeled, and was probably a last-minute engineering change.

The later models also had some snazzy blue plastic caps on most of the toggle switches. These are missing on the early models. Perhaps they saved so much money on getting rid of the 10-bit DAC that they splurged on caps for the toggle switches!

The eagle-eyed among you probably noticed that the green cap for the Start Record button looks different on the early version. This is a replacment part, and is not original. My client's Model-800 was missing the red cap from the Stop Record button, and I replaced both caps at the same time to make them look and feel uniform. The original button caps have been out of production for some time, and are not available anymore.

Model-800 PCB

Model-800 PCB

Model-800 PCB

Model-800 PCB

There are two additional differences between early and later models, but they are internal, and don't really change the functionality.

In the early models there are 6 static RAM chips, but there are only 5 in the later models. Each chip increases the memory word width by 4 bits. Since the early models recorded 10-bits (compared to 6-bits in the later models), they needed the extra 4 bits of memory.

The external clock-in jack on the Model-800 will accept a typical 5V peak clock signal, as well as a line-level audio clock source. This allows you to syncronize via tape sync. To handle the level differences between 5V peak, and the output of a tape deck, there is an additional small PCB on early models. It acts as a buffer and level adjuster. On later versions of the Model-800, it is built onto the main circuit board.

Synchronization Demo

Once I repaired my client's Model-800, I needed to check the external syncronization feature. Here are two videos. One is a demo, showing external clock sync, tape sync, and drum machine sync. In the other I open the sequencer up, and go over the repair process.

Syncronizing The Sequential Circuits Model 800 Sequencer

Syncronizing The Sequential Circuits Model 800 Sequencer

Sequential Circuits Model 800 - Early Serial Number

Sequential Circuits Model 800 - Early Serial Number

2 comments on Early Version Of The SCI Model-800 Sequencer

Chris Vermette
Hi Kieth, I just seen your video on the EML 100 that was recently posted on youtube - enjoyed it alot as there isn't much online for the EML products. Low and behold the next video it went to was a demonstration of the Model 800 sequencer. I was somewhat surprised as there is hardly any ... Read More
June 30th 2015 11:53 EDT
Hi Chris.

Upgrading the later version of the Model-800 to use a 10-bit DAC is not really feasible. In addition to the DAC, you would also have to add more memory (for the extra 4 bits in the word), and all of the logic and data lines that go with that.
June 30th 2015 23:02 EDT
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