Roland TR-909 w/ Modded Bass Drum

via this auction
"The bass drum has been modded with the Colin Fraser mods which include the following:

EXISTING CONTROLS:
Tune – Extended control over the decay time of the falling pitch envelope.
Decay – Extended control over the length of the bass drum allowing TR-808 like booming kicks.

NEW CONTROLS:
Pitch – Controls the frequency of the bass drum oscillator.
Drive – Allows waveform adjust from normal to square, for hard clipped sounds.
Tune Depth – Varies the amount of initial pitch sweep (decay set by Tune control)."

There's an audio demo at the auction while up.