I recently got this old school drum machine and I want to modify it. I already figured out a way to add seperate trigger inputs for the drum sounds. Next I want to add a pitch control to the block sound, like they did in this video:
It seems few people have done that and nobody has documented it, so maybe someone around here can help?
Since the machine plugs in directly to the wall I don’t want to tinker around the circuit too much, trying to find the right solder points…
Here is the schematic of the block sound. My guess is the 832Hz coming from the master oscillator are the pitch of the sound. So how would I incorporate a potentiometer here? Or am I wrong about that?
I would guess that the 132577 10k FIXED IN POT (top left) trim potentiometer sets the pace of the master oscillator which seems to run at 1664 Hz. Try to adjust that and if that changes the pitch you can put a potentiometer in series with it or replace it with a potentiometer of e.g. 50k.
IC-2 and IC-1 are probably clock dividers, they will follow whatever frequency TR17 is switching at.
thanks, but if that works, it will probably change the pitch of all 5 sounds. see 104Hz at the bass drum and 208Hz at the snare. I only want to change the pitch of the “block” sound.
[Edit] Are you sure the snare will actually change in sound when changing the oscillator frequency? A snare’s frequency is not 208 Hz. Actually a snare is more of a noise burst than a tonal thing in my understanding so then it does not have one specific frequency. But I may be mistaken.
i am not sure that this oscillator is handling the pitch, that’s why i am asking
but if you look at the complete schematic, it seems that the 208Hz are the base frequency of the snare sound and are later mixed together with a noise source, which is also used by the cymbal and brush sounds.
also every sound has it’s own filter circuit, so i think it’s possible the 208Hz are the basic pitch of the snare
as for the block sound: usually in circuit bending, the pitch pot is just a replacement for a resistor somehow, right? so maybe i should try just bridging R47 or R50 with a pot?
The sound of an acoustic snare drum combines the sound of the snares, which is pretty much unpitched, with the sound of the drum head being struck with the stick, which does have some pitch.
It looks to me like IC-1 and IC-2 are probably ripple counters or something of that sort. 1664 Hz goes into pin 2 on IC-2, pin 13 on IC-2 puts out half that, 832 Hz, pin 13 of IC-1 puts out 208 Hz, 10 puts out something unlabeled, 9 puts out 104 Hz, all divisions of 1664 Hz. So, no, I do not believe there is a “pitch resistor” here,* the pitches are all divisions of the master oscillator. In that case to change the pitch of one instrument without affecting the others you’d have to cut the connection to IC-1 or IC-2 and replace it with a new oscillator. If you have a scope you could check the waveform and amplitude at the top of D9, then cut that trace and inject a similar signal from a new oscillator there.
* Or rather, there is a pitch resistor, but it’s that trim pot on the upper left.
I do, because as designed it has a fixed frequency 832 Hz going in. That can’t be changed by the block circuitry, and it must be what’s determining the pitch. And the kick pitch does not change when they turn the knob, so there have to be two different oscillators in the modded version.
It’s not a very complex mod, I would think. Just a simple square wave oscillator, you could do it with an op amp or a 555 and a couple resistors and caps.
ok i will look into that. but the stock “block”-sound doesn’t sound very squarewavy… i think i should finally get an oscilloscope and check what those 832Hz look like first.
