Here are some RD8 schematics (for instruments, these were adopted from the Roland TR808 service manual and are preliminary versions unless declared otherwise).
Except for the newly implemented BD Tune potentiometer, the circuit is a replica of the TR808 bass drum. (Output stage in the schem needs correction; it’s actually like in the 808.)
This is the SD schmeatic transferred with (still some) minor omissions. Some cap values in my transfer may need revision, yet two film box caps on the RD8 read valued different from OG.
The LT/LC circuit is mostly true to the original 808 schematics with two notable differences. Firstly, a 15 ohm and a 820 ohm resistor were chosen instead of a 1k (R269 and R270 for the original R231), and secondly, in the amp stage of the T-Bridge network (IC31-C) the two 31k resistors in the feedback path made way to two 4.7K resistors and a capacitor (value not measured yet).
As regards differences to the original, the RD8 MT/MC also shows different values for two resistors and uses two resistors in series where the OG has one 1k (see LT).
Again, we find the same minor deviations as described for the LT/LC circuit.
As for the majority of the circuit, the original setup was retained in the RD8. One more or less spectacular deviation being the RS attenuating resistor (with its value label 220K missing in the original schematics, first guessed by Behringer at an extremely low value, then increased to 82k). Another change, similar to the toms, is at IC25C with resistor values changed in the op-amp feedback path and a capacitor implemented.
While the Maracas circuit is true to the 808 schematics, the Clap underwent some changes. Firstly the CP offset trimmer is used as a potentiometer with two resisitors between GND and V+ limiting the offset range. The feedback resistor in the CP bandpass filter has a higer value (150k). Two further changes are on accounbt of obsolete ICs. A LM339 is used instead of an AN6912 for the sawtooth envelope, and a VM13700M replaces the BA662 (as of recently manufactued again, by coolaudio) as a 1:1 drop-in replacement VCA. All peripheral components around those two replacement ICs have original values.
//As regrads the basic sound building block for CB, CY, and the hats, however, there are differences in that the hex schmitt trigger (IC35) is driven by PWM from the CPU and cannot be tuned manually by means of trimmers (soon to be described in more detail).//
The RD8 cowbell circuit follows the original setup exactly.
Also the RD8 Cymbal circuit follows the original schematics exactly. (The small cap part numbers around T20, T21 and T26 in my trace are yet incomplete/slightly off. The missing part number for the final op-amp buffer is IC23-D)
Closed Hat and Open Hat
Apart from the differences as regards the Hex-Metal Noise also relevant for CB and CY, there are two changes for the hats. Firstly, the highpass circuits are extended by variable resistors (VR 27 and VR28, values need checking) and a 1k resistor each (replacing the fixed 2.7k resistor in the originaö 808). Secondly, the 47k resistors on the positive terminals of each op-amp buffer are missing.