main features:
49 midsize keys
2 built-in speakers (stereo)
100 preset sounds ("voices")
FM "digital synthesizer" controllable through 7 digital slide switches (spectrum, modulation, attack, decay, release, vibrato, volume) with each 8 step resolution.
synthesizer on/ off button (switches back and forward between modified and unmodified sound).
dual voice button to layer 2 voices (1 of them can be modified by sliders, select twice the same sound = chorus effect)
100 preset styles (i.e. rhythms with very richly orchestrated accompaniment)
2 fill-ins + ending for each style
accompaniment mode switch {normal, single finger, fingered, auto bass}
2 tempo buttons (40..240 BPM)
2 transpose buttons (-12..+12 semitones)
simple sequencer "song memory" with 3 select buttons for 3 independent "songs".
FM sound is up to 9 voice polyphonic and very similar like OPL2 (i.e. like PC SoundBlaster FM, but with grittier envelopes and great digital zipper noise during timbre fades) but can do stereo panning effects.
PCM rhythm (uses sampled percussion)
accompaniment volume slider (6 steps)
volume slider
demo button (a well known latin/ fusion music)
jacks for line out, AC adapter

This is definitely one of the most exciting Yamaha PortaSound keyboards, because this instrument from 1990 features an FM synthesizer that is controlled through 7 sliders.

Any of the built-in 100 preset sounds ("voices") can be modified by them, and also 2 sounds can be layered (one of them optionally modified) by the "dual voice" feature to create thicker sounds or a chorus effect. Although the 7 sliders provide not remotely full control over all parameters of the FM synthesis process, they permit to create a great variety of interesting sounds, including bizarre and harshly distorted tekkno ones, because the remaining parameters are taken from the preset sound that is used as a template, and the sounds can be easily tweaked in almost realtime. This keyboard was first released in 1989 as Yamaha PSS-380 with slightly different case colours (boring black buttons).

FM synthesis is particularly well suited to simulate brass wind instruments, picked strings and metal percussion, but it can also make great electronic organ sounds and lots of freaky bass synth noises. Despite the FM sound generator of this keyboard seems to be technically extremely similar like the one of the Fujitone 6A, the preset sounds are very different. While my 6A has very warm, but rather unnatural programmed timbres, the presets of the PSS-390 sound much more natural, but also colder (likely because a higher modulation level was used to create more overtones but less bass). The PSS-390 also pans sounds in stereo while the 6A is mono. Very interesting is that these instruments use a higher developed FM synthesis engine with more versatile sounds than the more expensive semi- professional MIDI PortaSound keyboards (e.g. my PSS-780), which makes it less easy to answer which PortaSound is "the best".

A unique feature of FM is that (particular in "sustain" mode) the timbre of notes changes depending on how long keys are pressed; thus despite the keyboard is not velocity sensitive, this "duration sensitive" play provides a relatively expressive playability that is far beyond modern sample based instruments without velocity sensitive keyboard. This instrument can also simulate well certain high and creaky distorted C64 SID tones (those used ring modulation?) known from Rob Hubbard's compositions, but it can not simulate well plain squarewave tones over multiple octaves, because depending on the modulation intensity either the low notes become way too dull or the trebles become too harsh (because this simple FM uses multiplied sine(?) waves those can not result in a sufficiently steep square waveform).

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