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[littleghost]

Hey guys. First time poster here. I tried asking about this on Reddit a few days ago but didn’t get much of a response, so I’m hoping someone here might be able to help 🙃

So lately i’ve been modding a Yamaha PSS-80. Recently I discovered that it likely contains an unused drum pad, which i’d like to wire up with some push buttons, but I cant for the life of me get it to work.

So a while ago I ordered a service manual for the PSS-80 on ebay, and while looking at the circuit diagram I noticed an unused pin on the main IC (shown here, highlighted in red).

I compared this diagram to a diagram of the Yamaha PSS-125 (which has the exact same hardware internals as the PSS-80). The 125 has a drum pad, and upon further inspection I noticed that the unused pin on my PSS-80 is the same pin that controls the drum pad on the 125 (diagram shown here, pin and drum pad highlighted in red. Note the extra “SB5” column with the drum sounds in the bottom right).

Now, the first thing I thought of when I noticed this was the Casio SA-2. It also has unused pins and an “older brother” keyboard (the Casio SA-8)that it shares the same internal hardware with, where these pins are active. And people have in fact managed to unlock the missing drum pad on the SA-2. Heres a schematic of someone doing just that.

Now my question for those who know a little more about electronics and circuits than I do; Does this even seem possible? I haven’t had any luck yet, but here’s what Ive tried so far:

I first tried connecting pin B5 to pin C1 & C2, which looks to be how the kick & snare are triggered on the PSS-125 based on the diagram. This didnt work, and seemed to trigger all of the keys at once. This is when I realized that this pin was grounded on the PSS-80, and that all of the grounded pins had this “all keys at once” effect. So I cut the trace connecting it to ground and tried again. The multi-key sound stopped, but still no drums. Then I realized that all of the sound-select pins pass through a 47k resistor array before connecting to the button matrix, so I tried doing the same with pin B5. Still nothing.

Im about out of ideas. Anyone here done anything similar to a Yamaha keyboard? Anyone see anything obvious that im missing here? Any help with this would be appreciated

[littleghost]

well, just an update for anyone who cares:

I did eventually get some good advice from some people over on the synth DIY subreddit.

one person told me I needed to wire  a diode in series before connecting to the keyboard matrix, but we eventually determined that this wasn’t the case, as only the “n” column pins in the matrix have diodes, and not the “s” pins (the rows), which would be the ones containing these secret drum samples.

Then I received a great reply with the actual probable answer; all of these pins connected to the matrix are also connected to “pull-up” resistors. Before this project & reply, I had never even heard of pull-up or pull-down resistors, logic states, or any of that. But basically, the reason this hadn’t worked previously, is because this pin should be  “active-high”, and needs to be connected to VCC via a 4.7k resistor like all of its neighboring pins to work properly.

Well, sadly, I tried this and still, no results. So at this point, ive decided to throw in the towel. It’s possible that even with the probably-correct solution in front of me, I still managed to wire it up incorrectly, and that’s why this didn’t work. It’s also possible that there was simply a software change on this chip between the PSS-80 & the PSS-125, and no amount of fiddling will ever unlock this drum pad ive spent so much time chasing. Honestly, who knows? Maybe someone will come along and succeed where i’ve failed one day, but for now, the PSS-80 remains drumpad-less.

Except… There actually IS a drum pad on the PSS-80, though its not super practical to use. By booting into its secret test mode (holding 2 highest keys while powering on), you not only reroute all of the face buttons to the keyboard, but you also turn the “voice select” buttons into a real, working drum pad.

The only issue; Because all of the buttons are rerouted, all of the keyboards usual functions are disabled with test mode enabled. And they stay disabled, until you turn the machine off and back on again, disabling the drum pad. Ive done lots of experimenting with this test mode to try to unlock aspects of it in normal boot up mode (it also has a cool reverb voice only accessible during normal operation) and re-route these drum buttons, but I haven’t had any luck there either.

Funny how a functioning drum pad on this device always seems to be so tantalizingly close in so many ways, but also completely unachievable 😭

Anyway. Thats where I leave this project. Maybe someone will pick up this torch some day and continue on, but for me, it’s the end of the road ✌🏼