Unlike most portable keyboards, this 1983er Yamaha PortaSound perfectly represents the sweet sounding early 1980th pop organ estheticism. Particularly the "duet" and "trio" mode with layered sounds and "stereo symphonic" pseudo- leslie would fit perfectly into the environment of the famous WERSI Helios or Yamaha Electone organ concerts by Klaus Wunderlich or Franz Lambert of that era. The thing also has nice sounding synthesizer functions, although these are very limited because they only combine some preset envelopes with preset waveforms. Unfortunately it has not the organ volume pedal jack of the earlier PortaSounds, but this could certainly be upgraded if necessary. There is also a sequencer and a realtime programmable drum computer with bizarre electronic blip percussion that may be great for tekkno.

main features:

• 49 mid size keys
• mono sound, stereophonized by "stereo symphonic" leslie simulator (2 speeds)
• polyphony 8 notes (drastically reduced when duet/ trio or melody mixer active)
• 12 semi- OBS preset "orchestra" sounds {organ, trumpet, saxophone, piano, harpsichord, synthe 1 | piccolo, violin, oboe, music box , guitar, synthe 2} (selected through 6 locking OBS buttons + bank switch button)
• 12 semi-OBS preset rhythms {disco, rock'n'roll, 8 beat, rhumba, swing , march | 16 beat, shuffle, bossanova, samba, slow rock, waltz} (selected through 6 locking OBS buttons + bank switch button. Each rhythm has each 3 switchable bass and chord variations.)
• locking push button switches to select sounds, rhythms and sustain
• sustain {off, depth 1, depth 2} (selected through locking buttons)
• vibrato
• fingered & single finger accompaniment (manual chord mode with rhythm off)
• rhythm fill- in button (left, next to the keys)
• sliders for master volume & tempo
• bass & rhythm volume slide switches (each 5 steps)
• transpose knob (-5..+6 semitones)
• "custom drummer" (all rhythms and accompaniments are re- programmable!)
• "multi-menu": a row of 12 buttons + mechanical 9 page scroll menu to select functions.
• synthesizer: all "orchestra" sounds can be re- programmed; every sound consists of 2 sub- oscillators of those each can select 1 of 4 preset envelopes and 1 of 4 preset waveforms (timbres). The waveform and timbre sets of both sub- oscillators are different and not editable. Every sound can have vibrato on or off.
• every rhythm and "orchestra" sound can be switched between edited and preset version without overwriting the edited one.
• "melody mixer": To the highest held note can any of the other sounds be layered.
• bass and chord accompaniment voice is selectable from any of the "orchestra" sounds
• multi- track sequencer (melody, chord, bass) with cassette saving
• sound generator based on static digital waveforms + envelope; at low tones sample aliasing gets audible.  The drums sound like squarewave blips, the hihats sound somewhat metallic and may be shift register noise or a very low res waveform sample. The mixed digital sound is outputted mono through 1 common resistor array DAC (8 bit?) and then post- processed by the analogue leslie simulator. The PCB of this instrument is quite large and has at least the complexity and component count of a C64 homecomputer.
• battery backed up RAM
• jacks for line out, headphone, AC adapter and data cassette.

This is the "Multi-Menu"; the menu pages are printed on a black plastic prism that rotates by turning the right knob. Menu items are selected by pressing the buttons below. LEDs above this button row indicate which functions are currently selected. This kind of menu was also used in Yamaha Electone home organs (e.g. ME-50, seen on eBay). Later Yamaha abused the term "Multi-Menu" on Yamaha PSS-470, despite it had no plastic prism anymore but only ordinary OBS buttons and slide switches those have nothing to do with a menu.

eastereggs:

• Likely the sequencer data from the cassette jacks could be read into PC and converted into MIDI files and vice versa so far anybody finds out the data format (possibly it might be similar like the PlayCard data of my Yamaha PC-100).

modifications:

• memory reset button added (shorts the electrolytic cap when pushed).

notes:

Warning: Hmm... The thing stores all settings in battery backed up RAM; with no batteries inserted the RAM is backed up by a large electrolytic capacitor for a few days(?). When the cap runs empty, this messes up the data badly and even causes things to subtly malfunction those normally were expected not to be RAM dependant. E.g. sometimes particular preset sounds plays too silent or certain parts of them refuse to be editable, or their LEDs show mess or sustain doesn't work or even the chord volume slide switch refuses to change volume at some positions (e.g. only 2 of the 5 positions have different volume). These flaws can drive you crazy and make you take the entire thing apart for hours to successlessly search for dirty switches etc. etc. and even in the manual I downloaded from Yamaha there is no reset procedure for this keyboard explained to prevent this. (In manuals of later Yamaha PortaSounds with battery backed up RAM stands usually that certain simultaneous button presses reset the thing.)
 

As a fix I installed a memory reset button that shorts the electrolytic cap when pushed. I wired a 100 ohm resistor in series to the button to prevent damage of the charged cap or power supply when accidentally pressed with power supply connected or batteries inserted. This way the button needs to be pressed for about 20s to take effect, but this also helps to prevent accidental memory deletion. I mounted the button in one of the bottom screw holes to protect it from accidental operation. (It would certainly also be possible to place an "opener" button switch into the RAM power supply line, which also could be useful as shitshot, but due to opener switches sometimes fail by oxidization, I yet didn't dare this to avoid accidental data corruption.)

The MK-100 sounds very much like when Yamaha attempted to replicate the timbre of their in the same year released digital DX7 FM synthesizer on cheaper non- FM hardware, thus this instrument sounds very different from the rather muffled analogue timbre of my first PortaSound generation's PS-2. The preset "organ" sound is rather a clicking Hammond than church style and also the "piano" sounds more like an electric than acoustic one; all sounds are very FM- like (except there are no "dwellngs" or other drastic timbre changes in them, but the layered envelopes and sounds still permit slowly fading timbres). The locking push buttons were likely taken from the previous analogue PortaSounds where their multiple contacts had been technically necessary. The sustain of the "music box" sound stays always at depth 2 and can not be switched. The rhythm of this Yamaha uses a row of 3 green and 1 red LED as optical metronome; my Fujitone 6A has a similar LED chain, which may have been inspired by it. While the main sounds are best suited for harmonious pop organ chords or sweet disco music, the strange sounding and programmable drum section can also be interesting for tekkno or other gunk structured music, not least because the custom drummer patterns can be played in live performances by deleting and re- entering a percussion sound while the others keep playing in a loop.

If you want to know more about details, it is a good idea to download the manual from Yamaha.

In 1985 a very simplified variant of the MK-100 without the multi- menu synthesizer was released as Yamaha PSS-450 (seen on eBay). The same sound hardware was likely also employed in the fullsize Yamaha PS-55 (stereo) and its simplified variants PS-35 (stereo) and PS-25 (mono), those had similar looking controls (but no Multi- Menu) and also came out in 1983 (as successors of the Yamaha PS-30 and PS-20). Similar sounds like the Yamaha MK-100 (trio mode etc. but no synth, 49 midsize keys) has also the Amstrad Fidelity CKX100.
 

removal of these screws voids warranty...
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