Sequential Circuits Prophet 600 – Gligli Upgrade

Sequential Circuits Prophet 600
Sequential Circuits Prophet 600

This Prophet 600 was brought in for a Gligli P600FW upgrade, which is a CPU/firmware upgrade using a Teensy ++ microcontroller board. It offers improved resolution for all parameters (128 values instead of like, 15 or 7!), a new LFO just for vibrato, arpeggiator MIDI sync and more, which you can read about here. The firmware is generously offered for free by the developer and in order to install it, we procure a Teensy 2++ board, do some modification to the Teensy, flash the firmware onto via USB, and install it in the Prophet 600 in place of the original Z80 CPU. We can do this mod for only about $100 including parts.

Prophet 600 Gligli Upgrade CPU
Prophet 600 Gligli Upgrade CPU

This upgrade is very non-invasive and the old CPU can be swapped back instantly if you don’t like it. Unlike some upgrades, it only remaps a couple of panel controls so you can still basically use the Propher 600 as before while you’re getting used to it. The new parameters are accessed on separate “pages” of control assignments. I strongly prefer this approach to upgrades to situations where every control is remapped, like on the JX8P Kiwi.

Here’s a note on how tuning and scaling of the VCFs and oscillators works with the Gligli, for anyone who is having a problem where their oscillators won’t tune properly after installing the mod. The Prophet 600 uses VCOs and VCFs that are not digitally clocked, but the keyboard control voltages are digitally generated by the CPU. The original/normal Prophet 600 tuning procedure has you scale the oscillators to the CV output of the CPU. The Gligli mod does the reverse: when you press the TUNE button, the CPU scans how the oscillators and filters are scaled, and adapts its output accordingly. However, if the scaling trimmers of the oscs/filters are turned too far in either direction, it may not be able to adapt and get it tuned up right. If any of your oscs/filters “fail” autotune with the Gligli in (indicated by the two digits representing the offending osc or filter flashing back and forth during the tuning procedure), try centering the scaling trimmer for that voice and press TUNE again.

Replacing carbon key contacts in the Prophet 600
Replacing carbon key contacts in the Prophet 600

This Prophet 600 also had some unreliable key triggering. This keyboard uses the rubber-bump/carbon-contact key contact system of most 80s polysynths. I have pretty much given up trying to clean these when they go bad as I have learned that even after cleaning them well enough that the problem is solved, it is very likely to recur. I now strongly advocate for replacing all contacts with these carbon key contact dots, which should be a more reliable long-term solution. They are little self-adhesive carbon discs that I stick to each carbon key contact (after a light cleaning to ensure that they can stick cleanly).

Key contact dot being pulled off the strip

One more thing– always replace tantalum capacitors in Sequential synths!

Work Done: Gligli CPU upgrade, recapped power supply, replaced all tantalum capacitors, replaced all carbon key contacts

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