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
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.
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."
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.
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.
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...
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?
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.
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
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.
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
