When I turn on my Prophet 600 the program LED numbers add a 0.0 (dot) between the 2 numbers. It should only do this when I’ve altered a knob or switch, but it is doing this when I immediately power it on or punch in a new set of numbers. When it adds the (dot) it also slightly alters the sound of my written patch.
I’ve had the P600 worked on by David Wilson from New England Synthesizer Museum just before his untimely passing, but the work he did was not related to the screen issue.
Any help in where to start to diagnose the problem would be appreciated. Thanks
Why might a parameter value on a digital synthesizer fluctuate, even if the corresponding knob hasn’t been adjusted? The original Prophet 600 uses analog potentiometers wired into a multiplexed analog-to-digital converter (ADC) system, which is then read by a Z80 microprocessor running firmware stored in EPROM. So a few things line up here:
- Noisy Pots and ADC Instability
Very likely. The Prophet 600 is known for somewhat jittery pot readings, especially in aging units:
Pots may have oxidized tracks or mechanical wear.
The ADC circuitry itself is slow and minimally filtered, so it tends to pass even minor voltage fluctuations.
You may see the parameter value fluttering ±1 or more even when untouched, particularly if you leave the synth in parameter edit mode.
- No Hysteresis or Deadband in Original Firmware
The original Sequential firmware doesn’t implement value hysteresis (i.e., ignoring small changes unless they pass a threshold). So tiny analog jitters are interpreted as parameter changes.
- Gligli Firmware Consideration
If your Prophet 600 is running the Gligli mod, that changes things:
Gligli’s firmware includes software filtering and soft takeover logic.
However, even with this improvement, aging pots can still cause subtle jumps.
If the synth is in a live editing mode, the Gligli firmware will constantly update the screen or patch buffer with the knob’s value—even with slight voltage noise.
- Thermal Drift or Power Instability
The Prophet 600’s power supply isn’t regulated as tightly as in modern gear. Thermal variation can subtly affect voltage references feeding the ADC.
Diagnostic Ideas:
Wiggle Test: Slightly tap or nudge the pot without turning it. If the value jumps, the pot is mechanically unstable.
Heat Test: Warm the area with your hand or a low heat source. If the value drifts slowly, thermal drift is a factor.
Firmware Behavior: If you’re on stock firmware, consider Gligli’s mod—it greatly improves parameter smoothing.
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Thanks mykindofstudio for the detailed response 
I’ll first try cleaning all the pots and switches. Then see where to go from there.