for the last 3 or 4 weeks i have been liquidating my shop. i have accumulated way too much over the last 3 years that ive lived here. it was a daunting task, but its finally done. its a nice feeling to know exactly what you have and where it is. since my shop had finally been taken care of, my first project would be one that ive been meaning to do for a long time. about a year ago i built a red "novelty synth". its the one with the drum machine and the programmable sequencer.
this keyboard had a few problems that i wanted to fix. i really love the drum circuit in this keyboard and i wanted to do it justice. the 7 step keys that activate each step in the sequence had to be held down continuously for the steps to be triggered. also, the keys themselves were hardly flexible so you had to push down really hard. this eventually caused them to break. i replaced the keys with another set of a similar style but smaller size. i used the remaining space on the keyboard for a small panel. i replace all the momentary tactile switches with latching push switches. push once and the step is on, push again and the step is off. this will make it much easier to adjust the parameters while the sequence is playing. this was all i was going to fix when i started this project, but when i opened it up, i noticed many more things that needed attention. the sequencer was being triggered by the audio output from the lm386 amplifier chip through a chain of signal diodes. the sensitivity switch just bypassed every other one. well its been a year and i can do better than that. i ran the direct output from the drum chip through a comparator and to the sequencer. this makes for much more stable triggering. the output was ridiculous. it was basically a 5k from the lm386 to the speaker and a 5k from the tone generator to the speaker. it was too hot. instead i mixed the signals into the input of the lm386. the drum goes through a 50k to the input of the chip and the tone generator goes through a 50k to the drum 50k. i added a filter to the tone generator with cutoff. i also ran the tone generator through a frequency divider and added a 50k pot to mix between i think 1 an 1/8. it could be mixed better but i think its fine. i also built an attack release generator to modulate the filter. i didnt have enough room for a release pot so i left it closed and just used the attack function. it wasnt until i was finished with this project that i realized i probably could have both on one pot... the A generator is triggered the same way the sequencer is except the trigger goes through the other half of the frequency divider. there is a 3 position switch that will select 1, 1/2, and 1/4 steps. here is the schematic:
here are some before and after shots:
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