Letron MC-38 - stereo electronic keyboard     digital squarewave keyboard with complex C64 sounds, accompaniment & songbank

This was the direct successor of the famous Letron MC-3 and one of the very last non-toy squarewave keyboards; later "MC" series keyboards (created by Medeli) of this size already used samples or FM instead of this archaic electronic sound. The timbres still have that great classic Commodore C64 home computer appeal, but are programmed slightly more complex than in MC-3 and partly include simple pitch envelope or chorus effect by layered subvoices (which halves polyphony).

With accompaniment it can now play up to 3 main voice notes, rhythms feature intro and ending, and the "vibrato" and "manual bass" features of MC-3 are gone, but the rest is extremely similar. Unlike its predecessor, the MC-38 has no OBS sound and rhythm select buttons anymore, but uses cipher buttons like modern sound bank instruments. It also features a 20 song bank with simple lesson function (mutes melody). What in 1990th appeared rather cheapish and silly as a kind of imitated wannabe product, is nowadays enjoyed for its unique polyphonic lofi synth style.

This instrument was also released as "MC-38" by Medeli, as CCE EK-55X and as Intersound PK-380. A black version with funky orange speakers was released as Thompsonic TS-38 (seen on eBay). A black version with black speakers came out as Passenger's MC-38 (got photo by e-mail).
"MC-38" version (eBay photo)
(Note: This keyboard sounds great, but don't buy one of these so far your only intention is to get a keyboard with faithfully imitated natural instrument sounds. Remember, this is a squarewave instrument and though many of its sounds sound not even remotely like what is written on its buttons, thus bought with wrong expectation it may disappoint you.)

main features:

At the top you see the fake plastic heatsink and the plastic handle hole.

eastereggs

notes:

Despite the outer case layout of the Letron MC-38 resembles in top view very much the Letron MC-3, it is also in detail much more elegant since it has stylish curved side edges and a slanted control panel, a bottom with handle hole and even a fake plastic amplifier heat sink in the middle of the back (which rather sharp edges relativate the value of that handle hole). Otherwise it lacks the zillion of red rectangular LEDs those were the eye catcher of its predecessor. (The new green silicone rubber drumpad circle assembly was also used in the Fujitone 6A (MC-6), which has an even more stylish case shape and great FM sounds.) The cipher and "play/space" buttons sound a blip, which disturbs live performance. Unfortunately they often were assembled very poorly (see below), and many broken ones have been likely thrown away, so they are getting rare now. The MC-38 sound awesome and are definitely worth to keep.

The main voice preset sounds partly include vibrato, pitch envelopes or chorus effects, unfortunately the manual "vibrato" button of the MC-3 is gone. Like with the MC-3, many sounds are not remotely realistic (even within the limited capabilities of the sound chip); e.g. the "trumpet" is still an envelopeless plain squarewave toot ("jazz organ" and "synth 1" also). The "violin" and "clarinet" contain fast vibrato. The saxophone is just a louder version of the violin with a slower vibrato. The "mandolin" rings and the sounds "brass ensemble" and "synth brass" include a chorus effect (half polyphony). Also the sounds "harpsichord & jazz organ", "organ & string", "electronic piano & oboe" half the polyphony despite they don't sound at all like 2 different layered sounds - at least not with independent envelopes (a program bug?). The "synth 2" and "synth 3" have a pitch envelope, i.e. the quickly howl 1 note down and up again at the begin of a note and fade silent (decay envelope). The 3 "human voice" sounds don't sound at all like voices - they are also just made from a squarewave timbre which starts 1 note lower and quickly howls up to the final note (resembling bagpipes), and these all have a different pulse width and version 1 and 3 fade silent (by decay envelope). The "telephone" is simply the chorus version of "mandolin". The "ghost" is a thin timbre with fast, strong vibrato that fades silent. The pitch envelopes of the sounds are very coarse and grainy, thus they do rather a portamento than a glissando. (Possibly this was done to save ROM space by omitting intermediate pitch lookup tables.)

The accompaniment section behaves much like on MC-3, but an improvement is that although this instrument employs the same 4 note polyphonic DSG sound chip like the MC-3 (also see there), it provides in accompaniment and chord mode up to 3 notes main voice polyphony instead of only 1, because it generates 2 envelopeless plain squarewave organ tones in the CPU itself and so only occupies 1 DSG channel alternatingly for sawwave e-bass and 3rd squarewave chord channel. In organ chord mode (without rhythm) every new chord starts with an e-bass sound. When in fingered mode with no rhythm (i. e. organ chords) multiple keys of a chord are pressed, the chord continues sounding until all of these keys have been released or a different chord is played; this prevents strummed/ chinked chords and other more advanced play tricks. 7th chords can't be played due to the limited polyphony. Interesting is that the DSG seems to be identical with a Yamaha YM2163 sound chip that I found in the Testron CL-60910 and several other similar keyboards. By behaviour and hardware features the MC-38 is a cross between the old MC-3 and the toy-like Elite MC2200 (which also employs cipher buttons, a CPU with integrated tone generators, song bank and combined power & tempo LED). So even with accompaniment volume turned fully down, the 2 squarewave chord channels from the CPU stay audible, thus their volume slider rather changes the mixing ratio and dynamics than muting chord and bass entirely.

The rhythm has the same great "custom drummer" feature (stepwise programmable drum pattern) like the MC-3, and now every rhythm even features an intro, fill-in and ending pattern (with accompaniment). Unfortunately the MC-38 has no OBS rhythms and sound select buttons anymore and every sound/ rhythm select buttons press always makes a blip noise, which limits their realtime use. But as a compensation there are now OBS memory buttons (M1,M2) for each 2 sounds and 2 rhythms those can be assigned from the currently selected ones by holding "voice" or "rhythm" and then pressing one of these buttons. Pressing M-buttons alone selects the stored sound or rhythm. This is a quite unusual feature for a midsize beginner keyboard and only existed on very old Casio instruments (like MT-30 and MT-60). The sound memory buttons however stop notes of held keys, which makes them far less versatile than the great OBS sound buttons of the MC-3. Also the MC-3 custom drummer trick with selecting different rhythm and accompaniment pattern lengths doesn't work anymore, and even entering a pause during rhythm programming now makes a blip noise. This button click is produced by the CPU on an own squarewave tone channel that works independently from the limited polyphony of the rest (and may be safely disconnected). At least the annoying battery alarm tone of the MC-3 is gone now - perhaps the manufacturer wanted to give the thing a more professional appeal by preventing the thing from outing itself every minute as a cheap squarewave tablehooter.
 

circuit bending details

The Letron MC-38 is based on the CPU "MC-38" (variant of MC-3DX) with squarewave sound IC "DSG-MC-3".
Unlike the predecessor MC-3, the CPU is crystal clocked at 4 MHz because it additionally outputs 3 envelopeless squarewave tones for chord channels 1 and 2 and the button click. The DSG produces the 3rd accompaniment channel (squarewave, pin 4) and bass voice (sawtooth wave, pin 5). To generate 1MHz clock for the DSG, the logic IC "74HC74N" (14 pin DIL, 2 D-flipflops) is used to divide the master clock by 4.

All polyphony channels are mixed through resistors, so you may install potentiometers to tweak their ratio.

The hardware of my MC-38 was assembled very badly and looks quite prone to short circuits. E.g. screws were driven through cables and a torn off ribbon cable lead at the keys PCB was patched by soldering a 30cm long piece of bare solder(!) wire between ribbon cable and the very end of that PCB. Also some keys respond unevenly. Someone e-mailed me that in his MC-38 a resistor burned out, which likely was result of such Chinese piecework mess.

disable button click

The button press blip sound comes through an electrolytic capacitor from an own sound channel at CPU pin 30. You may disconnect this nuisance if you don't like it.

keyboard matrix

The keyboard layout corresponds to MC-3, but the panel buttons strongly differ. The only easteregg is keyboard percussion on the 5 highest note keys.
 
41
40
39
38
34
35
36
 
CPU pin
out 4
out 3
out 2
out 1
out 5
out 6
out 7
out / in
 
o
B4
o
B3
o
B2
o
B1
demo
transpose
+
P.
open hihat
in 12
4
o
A#4
o
A#3
o
A#2
o
A#1
R.
select (M3)
transpose
-
P.
snare
in 11
3
o
A4
o
A3
o
A2
o
A1
O.
select (M3)
tempo
+
P.
conga
in 10
2
o
G#4
o
G#3
o
G#2
o
G#1
'4'
tempo
-
P.
base
in 9
1
o
G4
o
G3
o
G2
o
G1
'3'
sustain
P.
close hihat
in 8
8
o
F#4
o
F#3
o
F#2
o
F#1
'2'
C.
fingered
R.
M2
in 7
7
o
F4
o
F3
o
F2
o
F1
'1'
C.
single finger
R.
M1
in 6
6
o
E4
o
E3
o
E2
o
E1
'0'
C.
off
O.
M2
in 5
5
o
D#4
o
D#3
o
D#2
o
D#1
-
keyboard percussion
O.
M1
in 4
16
o
D4
o
D3
o
D2
o
D1
-
R.
start/stop
demo
chord only
in 3
15
o
C#4
o
C#3
o
C#2
o
C#1
 -
R.
ending
R.
play/space
in 2
14
o
C4
o
C3
o
C2
o
C1
o
C5
R.
start/fill-in
R.
program
in 1
13

The input lines are active-low, i.e. react on GND. Any functions can be triggered by a non- locking switch in series to a diode from one "in" to one "out" pin.
 

legend:
"o"
 = keyboard key
R.
 = preset rhythm
O.
 = preset sound ('orchestra')
C.
 = chord
P.
 = drumpad
orange
background 		 = easteregg (unconnected feature)

  • keyboard percussion
    A button at 16->35 switches the 5 highest note keys into keyboard percussion mode {base, conga, snare, open hihat, close hihat}. Press itagain to exit.

The instrument has a song bank library of 20 demo melodies in minimalistic but nice C64 squarewave sound:

  1. Red River Valley
  2. Camptown Races
  3. Brother John
  4. House Of Rising Sun
  5. Rain And Tear
  6. My Cup Runneth Over
  7. The Old Folks At Home
  8. Twinkle Twinkle Little Star
  9. A Little Brown Jug
  10. Silent Night
  11. London Bridge Is Falling Down
  12. Happy Birthday
  13. Jambalaya
  14. Night in Moscow Suburb
  15. When The Saints Go Marching In
  16. Michael Row The Boat Ashore
  17. Mary Had a Little Lamb
  18. Rowing Boat
  19. O. Du Lieber Augustin
  20. Umterlanders Heimweh
These demo melodies play in a loop and cycle through all available sounds. Each demo includes a matching rhythm and initial sound, but they only use a monophonic main voice with standard accompaniment end end with the special ending of that rhythm. In spite of this I like many of them. These tunes also include the German folk song "Unterlanders Heimweh" (misspelled "UMTERLANDERS HEIMWEH"), but this tune sounds very different from the wonderful music called "Unterlanders Heimweh" on the ROM-Pack RO-551 (which corresponds to the famous demo of Casio VL-Tone 1) and instead corresponds to the Casio MT-36 demo, which sounds just like a rural folk waltz and resembles a bit "Little Brown Jug". (Read more about the unofficial Casio anthem "Unterlanders Heimweh" here.)  Also some other melody names are written in badly misspelled Engrish. When only the "demo" button is pressed, the MC-38 plays all songs in a sequence. When a demo song is selected by its number and then "demo" is pressed, the instrument repeats only that song and cycles through a different main voice at each repeat. The red lesson function button is mislabelled "chord"; when pressed, it mutes the main voice and permits to play on the keyboard to its accompaniment.
A 37 midsize keys variant of MC-38 (odd black case with sideways protruding round Mickey Mouse ear speakers) came out as Thompsonic TS-75 (seen on eBay). 
Thompsonic TS-75 (eBay photo)
The direct predecessor of the Letron MC-38 was (as already mentioned) the Letron MC-3. Another great keyboard with complex squarewave sounds is Yamaha PSS-100.

Note: Do not confuse the MC-38 with the MC 3800; although many technically identical MC keyboards exist in model name variants with either 2 digit or 4 digit MC numbers (like "MC-**" or "MC **00" where ** are the same ciphers), the MC 3800 is an exception of this rule and technically very different. This may hint that it is one of the oldest MC series instruments.
 

 removal of these screws voids warranty...    
WarrantyVoid
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