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bontempi system 5 plus GT509

In which i try to circuit bend the toy keys!
bontempi keyboard
I got this keyboard with the intent to circuit bend after i one hit KO'd a sucky biltema-brand one. But then i liked the sound so much that i didn't wanna touch it for a few months! But now the time has come. xoxo, ~sixeye
OBJECT INFO "Tiny" battery powered keyboard thing, cyberyogi has written nicely about it here (archive link). But basically:

First off, my knowledge of electronics is... lacking, so take everything here with a grain of salt. Nonetheless, let's see what i can find out.

Main chip


Can't manage to find any info on it. Here's my attempt to find out what the pins do:

photo of the chip, with pins numbered.
 1: -
 2: ?
 3: ?
 4: ?
 5: ?
 6: keys
 7: keys
 8: keys
 9: keys
10: keys
11: keys
13: ?
14: keys
15: keys
16: keys
17: keys
18: keys
19: keys
20: keys
21: keys
22: keys
23: keys
24: audio out
25: audio out
26: audio out
27: audio out
28: audio out
29: audio out
30: audio out
31: audio out
32: -
33: keys (numbers)
34: keys (numbers)
35: keys (numbers)
36: keys (numbers)
37: keys (numbers)
38: keys (numbers)
39: keys (numbers)
40: keys (numbers)
41: -
42: -
43: -
44: -
45: -
46: -
47: -
48: -
49: -
50: -
51: -
52: ?
54: -
55: ?
56: -
57: ?
58: keys
59: keys
60: keys
61: keys
62: keys
63: keys
64: keys
65: keys
66: - (maybe, leads to unused pad i think)
67: -
68: -
69: ?
70: ?
71: ?
73: keys
74: keys
75: keys
76: keys
77: keys
78: keys
79: keys
80: keys    

First attempt: audio out

The thing lacks any output except a speaker, so i'd like to begin with adding one of those. Should be simple enough IMHO: add a jack parallel to the speaker, right?

circuit diagram

Fucked that up somewhat by wiring it completely different:

photo of my first attempt

What would happen is the speaker would be completely silent UNTIL i put in a cable and bridged the tip and sleeve.

In my defense, i was busy taking care of these noobs too young kittens

Second attempt: audio out II

4.7k ohm resistor

So as i had earlier somehow put the speaker between the jack and ground, i had to open up and remodel a bit, a few new wires and so on... but the problem stayed! What now? I though maybe i messed up which of the two original speaker connections was ground, but no, that was correct... then it dawned on me. I had just assumed which of the pins on the jack was the normal connection and which was the wire. Eh. I'm currently waiting for the iron to get hot so i can fix that.

Yeaa boi. Figured it out. Also, the way i put this together meant i could try different resistors by the line out and find which one gave a nice volume. Turns out, 4.7K ohms did the trick! Incidentally, that might be the lowest value i've got, so 🤷‍♀️

second attempt, working line out, wire nest

You see! Clear as day!

The biggest problem i've had is that the thing relies on pressure to keep the keys spongy, and while at first glance it looks like there's a lot of room in the case, just a few wires can upset that pressure, making the keys stale and unplayable. So some effort has to be put toward making sure the cables are at reasonable places. Piece of shit, super boring and annoying, i hate it.

But yea! At this time, the thing works! 😎🐱😎🐱😎🐱😎

finished circuit diagram

Audio example

line in 2 morphagene w/ chours bullshit

Breaking out the bits

So thing is that the mother brain chip outputs the audio signal on eight pins, which are converted to a nice and lean audio signal in a "resistor ladder" which i've heard is a thing. The board doesn't contain much fun stuff anyway but at least these i can fuck with.
My plan is to put two "patch bays" with eight outputs (from the chip) and eight inputs (to the resistor ladder) so that i can mess with the order of the bits basically. This is done on one of these boards --> Basically like the the DREZNO eurorack module if you're familiar.

So i get to work removing the DAC (resistor ladder). In its place i put all these red wires and it's a bloody mess. The red ones are thinner so i switch out everything in the line out circuit to these as well for a "slim profile" hoping i can get the thing to close correctly. Apart from the eight bits there are two more "mystery pins" on the DAC. One must be the output, and i guess the other is ground or a reference voltage or something? I don't know anything about these things.

Because of the previously described thinness of these wires i need no new holes or anything, i'll just pull everything through the old ones.

So what happens is that i can reorder the bits in the output, and what that means in real life is i can make the sound be distorted, it's a kind of waveshaper (might have said that before but it's true), and that also fucks up the envelope and if i do it right it's just full volume for the entire duration of the sound.

It sounds better some times than others. Well well.



Audio example

wolfys folk songs


Last updated in january 2020 :)