The Taurus is a funny little (actually quite awkward and heavy) bass synth produced by Moog between ’75 and ’81 that is designed with a one octave, organ style pedal board meant to be played with your feet. It has a limited number of actual features… just one sawtooth waveform for both of its oscillators, the obvious 24db Moog ladder filter, portamento (glide), and a simple attack and decay envelope for the VCA and filter. It’s basically meant to do one thing well, which is make bass sounds, and it does it as well as any other Moog synth I’ve played. Though, I will admit it is a fun and unique experience to sweep the filter with my foot using a giant foot-sized slider.
We had a Prodigy in the workshop at the same time, which is the Moog Taurus’s immediate Moog monosynth contemporary. Comparing their sounds when set to equivalent settings, even before comparing the schematics it was clear that even at its highest cutoff setting, the Taurus’s filter, compared to that of the Prodigy, was still cutting a good deal of high frequencies. It was really designed to be used as a bass synth only.
The Taurus did almost nothing when we turned it on, but that was mainly due to the pedal board’s key contacts not connecting to the on and off busses well enough and a bad 5 volt regulator in the power supply. As with many monophonic keyboards, this one uses a low note priority circuit that requires that all of the contacts touch an off buss when they are not being played. If a low note is disconnected from the off buss, it can make it seem like the entire pedal board is malfunctioning, or that the synth isn’t generating any sound.
The 5 volt regulator is used for a number of CMOS and TTL logic chips which are mainly used to switch different settings on and off with the eight buttons on the front panel. I also had to replace a bad NAND gate IC that was preventing the Bass preset from turning on. It would only work temporarily while I held the button down. Once I released it, it would switch over to the Taurus preset.
This project is a nice reminder that a synth can seem completely broken and useless but by checking and fixing obvious areas like the power supply and key contacts, it’s revealed to be quite alright and back to good working order in very little time.
Work Done: cleaned the pedal board contacts and buss bars, regulated the pedal board’s action, recapped the power supply, replaced 5 volt regulator and 74LS10 NAND gate IC, replaced a dead LED, calibrated to factory specs.