i got my Soundmaker at a flea-market for a shockingly low price.
It had several problems when i got it, some of the hex inverters in the switch latching circuits were broken, a few LEDs were dead, The keyboard contacts needed a good clean and there was a missing note in the poly section.
The sliders were all scratchy, but they cleaned themselves up with a bit of exercise.
The missing note in the poly section was a dead Tantalum capacitor which had gone open circuit.
Now my Soundmaker is in perfect working condition, but for how long is it going to stay that wayt?
Tantalum Time Bombs
The soundmaker consists of three distinct synths, a monophonic synth, a polyphonic Divider synth and a string synth.
The monophonic synth is based on an m110 keyboard scanner/oscillator and a ca3080 based lowpass filter.
The polyphonic synth and the string synth are based on a top octave divider, and a series of trigger circuits, one for each note. these are made with 62 small pcb modules ,the PA902, the PA-902/1 and the PA-794 they each contain 4 tantalum capacitors, for a total of 244 capacitors. Tantalum capacitors don't age well, and one of their common failure modes is explosion. for this reason it's best to replace *all* of them. that's a lot of soldering and a lot of time.
There are still a dozen or so other tantalum capacitors to change on the other pcbs.
A sloppy back of the envelope calculation showed that it would probably take less time and cost less for me to design replacement PCBs with modern surface mount Tantalum capacitors, and have them manufactured in china than it would to simply replace the THT tantalum capacitors.
after counting the capacitors (244) i decided that this would be worth doing, and it turns out that not only are they cheaper than replacing the THT Tantalum capacitors by hand, but it's also a lot less work. the design part took ages though,it was my first time designing something to be assembled by a robot.
The golden ears
They sound and work identicaly to the originals, if you can measure the difference with test equipment i will be very surprised and buy you a coffee. SMD work exactly the same as THT components.
original Farfisa modules and my modern clones:
there are three types of modules: 901, 901/1 and 794
901 and 901/1 are used for piano, harpsichord and honkytonk voices, the left half the keyboard has 901 modules and the right half has 901/1
794 Modules are for Brass and string voices
794 module installed:
this is a prototype module with a few messed up traces that i needed to fix with bodge wires, the pcb design has now been fixed.
the original pcbs, laid out by hand, soldered by hand and probably even drilled by hand.
in 1978/9 this was modern!
Future Proof your own Farfisa soundmaker
This is a wheel that nobody should have to reinvent, if you are working on a Farfisa Soundmaker, get in touch. i can have a batch made for you.
The Farfisa Soundmaker Service Manual, this copy is fixed up a bit compared to others you might find online, i was going crazy with all the split and rotated schematics.
It's a beautiful manual, with a really in depth theory of operation, it even has a tutorial about how to design a filter based on a ca3080 transconductance opamp.
The M110 datasheet, this chip is both really cool, and really frustrating, since the same chip takes care of keyboard scanning and the oscillators for the monophonic section, there's no easy way to add a midi interface or CV/Gate input to the soundmaker.
After the filter PCBs are done, i'd like to make a midi interface for the Soundmaker, it'll be an MCU and a ton of opto isolators (the poly and mono sections on the soundmaker have separate keyboard contacts) and it'll probably be useful for other synths and organs too.