Rebuilt Yamaha SK-15 Synthesizer/Organ/String Machine Demo

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Published on Mar 21, 2013 BrassTeacher·3 videos

"Just a quick-and-dirty demo of the Yamaha SK-15 Synthesizer. Please excuse my poor playing skills, I was just making stuff up to show off the various sounds the machine is capable of making. This particular machine I stripped down and nearly completely rebuilt, making more than a few changes in the audio pathway."


via this auction

Pics of the inside below.

via the seller/listing: "If a car were to be rebuilt this way, it would be called a "frame-off restoration"! Sonically and electronically at least, I did the best I could with cosmetics while keeping the original chassis. More on cosmetics later...

When I first purchased this synth (dead) and got it playing again, I was kind of disappointed at what I perceived as a "dullness" in the sound. I had already been thinking of experimenting with a synthesizer, replacing parts in the signal path with audio-grade parts to see if the difference was an improvement or not. So, since the SK-15 was handy, and it was easy to pick out the weaknesses in the signal path, it became my test subject.

Every switch was disassembled, cleaned, and any that didn't respond correctly were replaced. All the sliders were taken apart, cleaned chemically, lubed, then reassembled.

All the pots were also cleaned. On the voice boards (which double as the control panel as well), almost every resistor and capacitor was stripped from the board (the only exception being the power resistors in the audio output section), and were replaced with audiophile grade parts, i.e.: Metal-film resistors, either 1% or 2% tolerance, as well as silver-mica, polystyrene, and either Elna Cerafine or Nichicon Muse electrolytic capacitors. All the opamps were removed, sockets put in their place, and high-end audio grade opamps from Burr-Brown or National Semiconductor fitted. The original transistors and other chips were left in place, as they were either involved in waveshaping or signal generation duties. By the way, the SK-15 uses the same filter, envelope generator (operates the filter contour only), and VCA chip as the famous Yamaha CS series of analog synthesizers!

When reassembled and calibrated, the results were astounding! The synth's sound was MUCH improved, with better high and low-end frequency response, and a much more vibrant tone.

To get the synth working initially after purchase, it became necessary to bypass the relay that connects the output just a few seconds after power is applied, as it had gotten too weak to make a reliable connection, and a replacement was nowhere to be found. So, turn the synth on just before the amp, and off just after turning the amp off, just as you should with any electric/electronic musical instrument, and you will avoid a mild speaker pop. Also, it became apparent that all the circuitry was working fine, but the SK-15 still didn't make any sound. So, the keyboard mechanism was checked, and it turns out someone had removed the entire rubber keyboard contact strip! The correct replacement part could not be found. In fact, typing just the part number into google brought up the message telling you that no documents matching your search could be found! So, the only alternative was to fashion a set of j-wire keyboard contacts! Yes, from scratch. Phosphor-bronze key contacts were cut, soldered in place, fitted with felt padding, and coated with silver conductive paint to prevent corrosion. The new bus bars were made from plastic strip, copper foil tape, and coated with the same silver conductive paint. The good news is, the keyboard triggers FAST!"