SR-88 Mods

The Sound Master SR-88 is a royal pain to sequence with. Mostly because it only has clock (and trig) out in a nonstandard PPQN(pulse per quarter note).

There are a few solutions:

1. Make it generate a more normal 24PPQN. I like the idea of reworking its clock, speeding it up and dividing down internally. Maybe generate start/stop signals from the buttons then have a full DIN sync.
2. Add a clock/sync in. Slave it to something that makes a better master clock and/or is MIDI compatible.
3. Add trigger ins. The problem with both of the other approaches is that you still have to use its internal, very limited and fiddly sequencer. This avoids the clock and sequencer entirely. Trigger ins cure all ails.

Triggers it is. This is a pretty simple mod: just disconnect the TC5501 RAM from the sound generator and connect your triggers. This has already been documented here, but I found the images unclear. Since I'm performing the same mod, I used the opportunity to take my own pictures.

First cut out the TC5501 that generates the triggers. Just sever its data lines: pins 10, 12, 14, 16.

Then solder wires for the new jacks. Red wires are the original trigger outputs. White wires are the trigger inputs.

Attach switched jacks such that the original triggers are used in the absence of external triggers. Ground to the ring, red to the switch, white to the tip.

Mount the jacks and you're all done. I used the battery area since it had lots of room. Also shown is an added LM7805 regulator that allows the SR-88 to use a standard 9v power supply.

Sound Master SR-88 Schematic

I needed to fix up an old SR-88, but the available schematic was almost illegible. I grabbed the highest resolution copy I could find and tried to touch it up. Some of the lines were very nearly gone, so I had to guess at some of them. There are certainly some mistakes and oversights, but I think it's an improvement.

Interesting to note, is that the schematic very closely follows the board layout as seen from the bottom.

I started to redraw the schematic in eagle, but I quit once I debugged the SR-88. The CD4011 NAND was shot. This seems to be a common problem in these and similar machines.

Anyway, here is the partial redraw and one I rearranged to be more conventional.