Decoupling the negative black lead (not ground) via a 10u polyester cap to
the scope ground the electronic trace was fine at all frequencies and
powers. Semi-permanently blocked off the reflex/bass port or whatever that
hole is called and the intrusive 50 to 70 Hz induced flutter has gone. Just
the normal low level flutter , for low frequency, high power output due to
vortexing/cavitation or whatever the normal distortion due to bulk shifting
of air mass by a cone is called.

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/