The Roland TR-77 is a really, really cool drum machine.  Its' percussion sounds are passive bandpass filter circuits that are excited by trigger signals;  the cymbal type sounds are white noise filtered in various ways.  It's a simple circuit, so easy to modify.

I had a PCB for a 16-channel MIDI-to-gate converter lying about, so we assembled that.  The first problem was getting power to it - because of the way the power supply is built, adding extra current upsets the voltages it generates.  The next issue was that when I  powered the converter from an external source, the +5V gate signals it put out wouldn't trigger the TR-77's sounds.

After running the trigger pulses through an oscilloscope it became apparent that the trigger pulses were actually negative, with an amplitude of about -12V.  To this end, I built in a separate 12V power supply for the MIDI converter to run off, and connected the +12V to ground so that the triggers were switching between -12V and -7V respective to the rest of the unit.  Then I built a separate board with lots of single-transistor circuits that switch between 0V and -12V according to the trigger signals, and wonderfully enough the sounds on the TR-77 triggered!

As a really neat side effect, sounds that involve the white noise are gated according to the note-on and note-off messages, instead of just brief pulses, so you can get several snare drum sounds from the unit just by changing the length of the note you send it.  It also makes hihat programming quite interesting, as you can control the length of the hihat directly with note length.

There is one sound - the Guiro - that has a different trigger circuit and doesn't respond to the -12V pulses.  I'll need to investigate this more so I can also get this sound to trigger as it's quite a cool sound.
 

Here's the finished product - you can see attached to the lid the extra power transformer, and above that a 12V regulated power supply.  It runs off the mains and is fused seperately.

Here's a close-up shot of the MIDI converter, which I got the PCB for a few years ago from a place called Electronics 4 Music which is now defunct.  I didn't have any assembly instructions, but it was easy enough to figure out what went where.

The board that converts the +5V signals is just a whole lot of single transistor inverters.

Usefully the circuit boards were all labelled!  You can see in the top right of the photo the voice board, along the bottom of the board are all the trigger inputs from the inbuilt sequencer.  The internal sequencer still works, you just connect up your trigger signals to the trigger input pads and hey presto, MIDI triggered TR-77.

I'm really happy with this mod, as it's given this ancient old beast a new lease on life.  :-)