This is an add-on board for the TR-707 ROM Expansion, which addresses the shortcoming that it cannot change the last two sounds on the machine (either the crash/ride cymbal on the TR-707, or the quijada/chime on the TR-727). The reason for this is because unlike the rest of the sounds, which are all stored together in one block of memory, the cymbals are stored in their own separate ROMs, with their own address counters and DACs.
Electronically there isn't much to the Cymbals Board. Two large flash ROMs take the place of the two original mask ROMs. A 3-way cable goes back to the TR-707 ROM Expansion board to pick up the bank select signals, so that the selected cymbals bank follows whichever bank is selected on the ROM Expansion board.
As simple as it is electronically, getting it to fit in the machine proved to be a challenge. The two ROMs which it replaces are located underneath the mixer board in the machine, and vertical height is very limited. Not to mention there's a large plastic pillar in the top case which protrudes into this area as well. I think it took about 3 tries to get the board just right and clear all of the obstacles.
The cymbal voices in the TR-707 are for the most part reused from the TR-909, albeit with different samples in ROM and less sophisticated envelopes. When the sound is triggered, a binary counter starts advancing through the address space in ROM, which is latched out to a DAC: a simple 6-bit R-2R resistor network.
6-bit resolution is not really enough to reproduce a convincing reproduction of a cymbal - the signal-noise ratio is just too poor. To get around this, the audio was very heavily compressed (that is dynamic range compression, not data compression) prior to encoding it. As such it has no dynamics to speak of - the volume of the sound doesn't drop as it plays, so the quantisation noise isn't really noticeable. Instead, a decay envelope is applied to the output using an RC network and an analogue VCA, resulting in a reasonable approximation of the original sound’s envelope.
The sounds included with the Cymbals Board all match up with the respective banks on the ROM Expansion. It includes the stock crash/ride cymbals from the TR-707, as well as the quijada/chime from the TR-727, so as before, original sounds for the machine it's installed in are retained exactly.
I had to make a few judgement calls for the new sounds - the TR-808 only ever had one cymbal, which serves as the crash. For the ride, I chose to include the previously-omitted maracas sound. I had tried to put a longer bass drum there instead, but the quantisation noise made it unusable, even with the aforementioned compression trick. The LM-1 never had cymbals, so I reused the LinnDrum cymbals for this. The last 707+727 bank uses the 727 quijada and the 707 ride cymbal.