[Help] MXR Phase 100 - Siren/Oscillation noise on Intensity 1 & 3
Hi everyone, I'm having trouble with my MXR Phase 100 and would love some technical advice.
The issue: When the Intensity knob is set to positions 1 or 3 (the high modulation modes), it produces a loud siren-like oscillation noise. This noise occurs regardless of whether an instrument is plugged in or not.
Observations:
- Speed knob: Seems to be working fine.
- Intensity knob: Positions 1 & 3 (high mod) oscillate like a siren. Positions 2 & 4 (low mod) don't oscillate, but the phase effect feels weak or subtle compared to how it should be.
What I've tried: I opened the enclosure and cleaned the PCB with a dust blower, but it didn't solve the issue. I haven't spotted any obvious burnt components yet.
I've attached photos of the PCB. Does this sound like a failing LDR (light-dependent resistor) issue, or perhaps a capacitor that's gone out of spec? Any guidance on where to start probing would be much appreciated!
Posted Jun 13, 2026, 11:40 PM UTC
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