The Korg M500 is a somewhat overlooked 1970:s Korg. As the name suggests, the M500 is preset driven and quite small.
This doesn't stop it from being a lot of fun to play! In fact, I like the idea of limited tweakability to enable you to
quickly get to playing. How many settings does your guitar or (traditional) piano have?
There is some user control however, which is nice. For the Synthe 1 and Synthe 2 sounds, you get envelope control and
the famous Korg traveller for filter cutoff. All sounds can also be pitch modulated with the LFO. The brass has it's own
filter which is tweakable with the traveller as well, but no envelope control. Both filters can be modulated with the LFO,
triangle only.
One annoying limitation is that Synthe 1 always have a decay only envelope, no sustaining notes for you! Synthe 2 sets
the envelope generator to attack/release mode which gives sustained notes but makes snappy sounds unavailable. Using both at
the same time sets the envelope to decay. I modifed my M500 with a switch that lets YOU choose between the two envelope modes
for the Synthe sounds. Details below. Now I choose freely between plucky decay only envelopes and traditional A/R.
Another thing is that the Synthe filter is a fixed resonance affair. Which is a shame since it's a Korg35 filter! The
Synthe presets chooses between no/low resonance and higher resonance by grounding a pin when Synthe 2 is selected. I did a quick
mod to manually toggle between the two modes.
The pushbuttons for preset selection are connected to the PCB with two connectors, one on each side. The one on the right
(closest to the Synthe buttons) contains everything we need for both this mod and the resonance mod. For the envelope mod you
only need to cut (or remove the pins) from the connector and attach them to a SPDT switch (also calles on-on switch) as shown below.
Note: The photo is incorrect regarding the envelope selector. Correct is yellow in the middle and green and red on either side.
Those cables should still be removed from the connector. Leave the orange one in the connector. See this photo
For the resonance mod (called bright in the schematics) you need to do two things. First, attach a SPST/SPDT (on-off) switch on
the blue wire. This lets you enable/disable the resonance, but it is still only on when Synthe 2 is selected. To have resonance
selected all the time (with the switch on) you need to bridge two pins on the Synthe 2 button. You can confirm that you are looking
at the right pins by testing continuity from one to ground and the other to the blue wire.
You can of course put the switches anywhere you want to, but there is a perfect space to the right of the keyboard. I used
normal submini switches (about 2 cm deep). If you are using bigger switches, make sure that they do not short to the keyboard below!
Updated 2022-10-09
