Modified Yamaha PSS-140

Modified Yamaha PSS-140 - This keyboard was rescued from a pawn shop, where it had been wedged in the display window beside an old soft-drink vending machine along with a PSS-270 and some other bits and bobs. It was absolutely filthy, both outside and in.

Before I even started the build I took the innards out and washed both halves of the casing and the rubber push-buttons. The keyboard assembly and circuit boards got away with a quick wipe-down.

The holes for the switches were drilled after carefully checking for obstructions inside the casing. There's a generous amount of empty space under this section of the front-panel, and I could've fit a few more switches if I needed to. They were mounted first and wired-in last.

Here you can see the switches from the underside. I've flipped the circuit-board to get access to the solder points.

Here I've cut the traces that connect 8 pins on the large chip (CPU) to 8 pins on the smaller voice chip. This mod is documented elsewhere on the 'net for both the PSS-140 and 270. I also installed a 9th switch to other points on the chips. The scribbled notes were just a guide which I removed later.

Et voila! I always hate this part of the build. 17 wires later...

The board is now screwed back in place, and the switches soldered to the color-coded wires.

I tested the unit on battery power before the build and it didn't work. However, it worked perfectly with a DC adapter. There was a lot of gunk in the battery compartment so I assumed at that was the problem. Wrong! My circuit tester told me power was getting to the board, but beyond that I was stumped, so in the end I added this cheat: an adapter plug wired to the battery compartment. Works like a charm.

Build time was approx. 3 hours. Prep time was spread out across the previous afternoon.


- 8x toggle switch matrix. These connect 8 pins on the CPU chip to 8 pins on the voice chip, which are ordinarily connected by traces on the circuit board. Left on, the keyboard operates normally.
What it does: Turning switches off at random, changing patches, then turning them on again temporarily corrupts patch data in unpredictable ways. It could result in a patch containing elements of two sounds, a patch that changes from one sound to another in a cyclic loop, or a patch with the wrong envelope shape, or ...?
Considering there are 40320 combinations possible with the matrix, and at least 9900 patch combinations (that figure increases real fast with each choice beyond 2 patches), the odds of running out of unique sounds are pretty astronomical.
As an added bonus, the switch matrix also messes with the demo tune, producing very strange behaviour. It may degenerate into vicious white noise which can be manipulated with other controls (such as the volume switches), switch the demo instruments around, warp the sound, or let you play tones that drone on forever, each new note layering over the previous one until it runs out of polyphony.

- 1x low pitch toggle. Connecting pin 16 of the CPU to Pin 11 of the voice chip.
What it does: It lowers most (but not all) of the keyboard by a few octaves. This being an FM keyboard, lowering the pitch also brings out the graininess of the timbres.

Sound clips:

Pitch comparison This is the default patch, 'accordion 1,' before and after the pitch-drop is engaged.
Pitch shift A little pitch-shifted ditty that reminded me of Dudley Simpson's oboe scores for Doctor Who.
Switches on In this example, I was able to play notes while some of the switches were disengaged.
Infinite notes 1 Edited from a 2 minute recording, where each note played layered with the one preceding it until the polyphony limit was reached. From then on the earliest note played would be wiped out by the newest, and so on.
Infinite notes 2 Here I've engaged the pitch-drop switch. The oscillation on these lower notes blends in interesting ways.
Infinite notes 3 This is the end of the recording. I somehow managed to change patches, and the 'infinite note' glitch was overridden by the last few notes.
White noise The demo tune degenerating into white noise.
Aphex noodle The demo tune warped into something reminiscent of Aphex Twin's goofier moments.
Polka nite Demo tune. A little off-key polka insanity.
Polka nitemare At the flick of a switch, the cheesy polka music becomes, well ... deranged.
Electric bagpipes Yes, it's the demo tune again, now rendered unrecognisable.
Harsh modem A symphony of evil noises.

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