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Dunlop produced the MXR Blue Box (M103-U) as an octave fuzz unit within the MXR effects line. The pedal occupied a specific niche: it combined two-octave-down pitch shifting with fuzz, generating what the original product literature described as subterranean tones.
The M103-U operates on a 9-volt DC power supply and draws 4 mA with the effect engaged, dropping to 1.5 mA in bypass. Bypass is hardwire. Input impedance is rated at 1 MΩ, output impedance at under 20 kΩ, with a nominal input level of −22 dBV, maximum output level of −5 dBV, and a noise floor rated above 100 dBV.
Owners sourcing replacement components or diagnosing signal issues should consult a qualified guitar technician familiar with analog fuzz and pitch-shifting circuits.
Nothing in The Vault is for sale. These records exist so the instruments stay documented after the manufacturer moves on — and the most useful part of that record is what owners add to it.
The catalog shot showed one finish, under one light, a long time ago. Yours has been played. Photos from owners are what the next person finds when they are trying to identify a part, match a finish, or work out whether the one they just bought is really what the seller said it was.
Specs only go so far. What actually fits, what breaks first, what the factory got right and what everyone ends up changing — that lives with the people who own them, not in a datasheet. Post what you know and it stays on this page for whoever comes looking next.
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From spec questions to full builds — players who have lived this gear since 2007.
Since 2007
