Circuit bend and Expansion Box for the Yamaha PS-3

This page is a work-in-progress, so things may be added or changed later.

The Yamaha PS-3 is actually a really great little synthesizer, hidden in a tiny "toy" home organ.

That looks can be deceiving is something this little beastie really shows, featuring the same VCF and VCA chips used in the much more expensive CS01 synthesizer and Electone organs. The square wave signals and waveforms are also generated in an analog fashion, making this a fully analog synthesizer. Just an hidden one.

video on it can be found on here.

The guts

The PS-3 uses a big mystery IC as it's main processor and sound generator. It can do 8-note polyphony, sending out two octaves separately.
Yamaha used these two octaves, either mixed or separated, and runs them through several capacitor filters to create the various sounds. For two sounds (Brass and Guitar) the VCF is used to get that 'wah' tone. The VCA is only used for the Vibraphone sound to create a subtle tremolo.

At first I felt like completely changing the signal path, making my own custom circuits and though that would've made something great for sure I also really wanted to keep this thing as original as possible while expanding it's functionality. So I opted to route various signals through a connector to an external "breakout box".

I am definitely planning on getting a second PS-3, gutting that one entirely and giving it a typical analog synthesizer signal path with all the controls available.

Here's a block diagram of the PS-3 for reference:

The diagram also shows the pin numbers and involved component numbers, which was extremely useful in finding modding points.
I know there's a service manual floating around -somewhere-, but unfortunately I haven't been able to get my hands on one. This also meant I didn't have a schematic, so I made my own of certain parts that I modded.

Here's the rough schematic of the VCF and the mods I made:

The PS-3's envelope is a rather simple capacitor-based one. At first I was planning to customize it to allow for setting the attack and decay rates, but I settled for a single envelope-depth knob. I also still need to add external CV to the breakout box, so I can do more fancy stuff with the VCF.



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