CASIO
Casiotone MT-500		   keyboard with warm timbres, drumpads & versatile accompaniment

This keyboard from 1986 (service manual date) was the direct predecessor of the Casio MT-520. It has quite warm semi- analogue sound, sample based percussion and 4 rubber drumpads for "Super Drums" accompaniment.

Most features and the sound engine correspond to the MT-520, but in opposite to this it has even 20 semi- OBS preset sounds and 4 chord variations, but only 4 drumpads those each can be switched among 3 preset sounds. (Alternatively 4 external pads can be connected through 2 stereo jacks.) The accompaniment sounds warmer than with MT-520.

Important:  Very annoying is that the 4 Super Drums tracks of the rhythm section here can not be muted but only switched among each 4 variants, which severely limits its usability. Thus do not pay too much for the Casio MT-500 if you intend to use it as a drum machine. On eBay people tend to pay crazy prices for this thing while the MT-520 goes much cheaper and is apparently still considered a toy, despite in practise its rhythm section far superior; it also has more drumpad inputs and intro/ ending feature, those all lack on the MT-500.

The original German retail price of the MT-500 in a German Conrad catalogue from 1988 was 799DM (about 400€). Due to the many similarities I only describe here the differences to the Casio MT-520.

different main features:

modifications:

notes:

In comparison to Casio MT-520 the case of the MT-500 has no stair step at the lower speaker rim. The electronics is a very complex multi- chip hardware on 2 large PCBs with many discrete components.

The sound engine of the preset sounds resembles the MT-520, but the stereo chorus effect is narrower and most sounds contain less reverb. Many timbres also sound more squarewave- like - likely by different filters. I will describe the preset sounds in comparison to MT-520 if present. The "piano" here sounds a bit more synthetic than on MT-520 and has no sustain, thus notes stop immediately after key release. The "elec. piano" has a mild attack, a dose of chorus and long sustain; its timbre resembles more a sitar than the MT-520 version. "vibraphone" is more percussive, brighter and octave higher than on MT-520, which makes it more realistic. "violin" otherwise sounds vs. MT-520 an octave higher, duller (less realistic) and has some reverb. "flute" is vs. MT-520 an octave higher, more squarewave- like and has no reverb; thus is it more a wooden recorder flute than a metal flute. "panpipe" is duller, 1 octave lower and has reverb. "human voice" is like MT-520 with longer sustain. The "jazz organ 1" resembles "jazz organ" on MT-520 but sounds more Hammond- like with less buzzy bass range; it also resembles here a sung "ah". "jazz organ 2" is duller, a bit thinner and has reverb. "pipe organ" resembles MT-520 but has more percussive attack. "bells" is a squarewave musicbox timbre with 3Hz vibrato and a dose of chorus; "glockenspiel" sounds 1 octave lower with sonorous multipulse bass range. "harpsichord" has much reverb and a 3Hz chorus vibrato that makes it badly unrealistic; "clavichord" is duller with shorter reverb (but MT-520 "funky clavi" still sounds more powerful and 1 octave lower). "double reed" sounds rather like a french horn with chorus vibrato and much reverb. "synth. reed" is a variant that sounds thinner and thus more like a trumpet or tenor saxophone. (Unlike expected, both have no similarities with a reed organ.) "synth. sound 2" resembles a slightly dull sitar with sustain; 1 is similar but grows duller and thicker during attack like layered with a short pipe organ tone with sustain. The "elec. guitar" sounds duller than MT-520 and has reverb, while "synth. guitar" has a very dry and harsh buzzing timbre that resembles the MT-520 "funky clavi" but sounds thinner, has chorus vibrato and reverb. Unlike MT-520, the MT-500 employs real locking semi- OBS preset sound buttons, those make it easier to see the selected sound but make them harder to trill them for live play tricks. (Like with Casio CT-410V they have no technical reasons since the electronics memorizes the last pushed button anyway.)

The rhythms employ quite similar woody sample percussion like Casio CZ-230S (that seems to be based on the Casio RZ-1 drum computer); e.g. the "handclap" has the same unusual mechanical typewriter timbre, but the sound set has also differences; e.g. the snare is duller and the bongos have different pitch (likely by changed sample playback frequency). The MT-500 percussion samples are different from MT-520; the individual sounds have changed pitches (some lower, some higher) and also the sample resolution may be slightly lower because e.g. the cymbal sounds thinner and the snare duller. Unlike MT-520 the accompaniment sounds include short sustain and sound warmer, duller and less dry, which gives the bass voice more pressure. (The bass resembles Casio MT-800.) The automatic accompaniment algorithm is programmed simpler than with MT-520; it does not recognize played chords and play different patterns for them but only inserts the up to 4 notes of pressed keys into a given pattern (a bit like the archaic Antonelli Star 2379). Thus the played chord is never split into melody lines of alternating notes but only chopped as a staccato. But the benefit of it is that you can not only play any disharmonic non-chords (the MT-520 still could do them) but also rapidly trill multiple accompaniment keys faster than the pattern plays. And in spite of the simple algorithm the available patterns are still reasonably complex (partly contain arpeggios, walking bass lines etc.) and you can manually switch between 4 accompaniment variations per rhythm. Instead of intro and ending there is only a fill-in pattern (with accompaniment) for each rhythm.

Very annoying is that the individual percussion tracks of the "Super Drums" can not be muted individually. Touching any drumpad mutes the rhythm until any key in chord section is pressed; the "bass drum pattern" switch determined whether the base drum of the rhythm stays audible or mutes also. (Unlike MT-520 the "casio chord" switch doesn't disturb running rhythms here.) Each of the 4 drumpads can be quickly switched to 3 different percussion samples; despite this way 12 sounds in total can be played, the 8 fixed OBS drumpads of the MT-520 are more fun to play. The Super Drums concept and algorithm is described in US patents 4757736 and 4972755.
 

hardware details

The Casio MT-500 is a far relative of the MT-65 hardware family (see Casio CT-410V). While the main voice sound IC "NEC D931C 011" is the identical, the accompaniment CPU "NEC D930G 022" has a different software number and controls the additional percussion IC NEC D934G with the help of 32KB percussion sample ROM "HN613256P CM4" and 8KB accompaniment ROM "NEC D23C64EC 026" (I dumped them). There is also a 4 bit SRAM "LC3514A" (0.5KB) for sequencer and a stereo chorus (single BBD line with emphasis).

The electronics is a very complex multi-chip hardware on 2 large PCBs with many discrete components. The percussion rom is only connected to the D934G and does not communicate with the D930G CPU. Because the CPU data bus is only 4 bit wide, the 8 bit data from the accompaniment rom are split in a TC74HC257 and the active nibble is selected through address line A13 on CPU pin 73. With ROM D23C64EC removed, everything else works (including drum pads and chords) but lacks rhythm and accompaniment. With ROM HN613256P removed, everything else works, but all percussion turns into random digital noise. So it may be possible to replace the percussion samples and possibly accompaniment patterns by installing 27C256 eproms (with diodes in data lines) instead of these roms.

Trimmer VR1 is bit compensation for linearity of the main voice DAC (select 'vibraphone' and play a midrange note to adjust). VR (50k) changes the stereo chorus LFO speed (default 1s +/- 20% at pin 7 of op-amp "4558 (4)").
A fixed lowpass filter (similar but not identical with MT-65) modifies the main voice timbre. These are the filter switch outputs of the D930 CPU shown in MT-500 service manual.
 
preset sound:
IO-1 (F1)
 IO-2 (F2) IO-3 (F3)
piano
H
H
H
vibraphone
 
 
H
flute
H
 
H
human voice
H
H
H
bells
 
 
 
jazz organ 1
 
H
H
harpsichord
 
 
 
double reed
 
H
H
synth. sound 1
 
 
 
electric guitar
 
 
 
electric piano
 
 
 
violin
 
 
 
pan-pipe
H
H
H
pipe organ
 
 
 
glockenspiel
 
 
 
jazz organ 2
 
H
H
clavi chord
 
 
 
synth. reed
 
 
 
synth. sound 2
 
 
 
synth. guitar
 
 
H
The chord voice has a fixed filter with sustain. These are the combinations of rhythm and chord switch.
 
preset rhythm: | chord ->
I
 II III IV
rock
3
4
3
3
disco
3
3
3
3
pops 1
4
3
3
3
swing
1
3
1
3
samba
4
4
3
4
march
4
4
4
1
slow rock
3
3
3
1
16 beat
4
3
4
4
pops 2
1
3
3
1
reggae
4
4
3
4
bossanova
1
3
3
3
waltz
1
3
3
3
 
combination:
filter
sustain
1
close
on
2
close
 
3
open
on
4
open
 

keyboard matrix

I haven't analyzed this keyboard matrix by myself, but only read the MT-500 service manual, thus there may be still unknown eastereggs (due to strong similarities with MT-65 hardware see CT-410V). Locking switches were found in schematics. 
 
51 KI1
50 KI2
49 KI3
48 KI4
47 KI5
46 KI6
45 KI7
44 KI8
 
CPU pin
in 1
in 2
in 3
in 4
in 5
in 6
in 7
in 8
in / out
 
o
C1
o
C#1
o
D1
o
D#1
o
E1
o
F1
 
 
out 1
52 KC1
o
F#1
o
G1
o
G#1
o
A1
o
A#1
o
B1
 
 
out 2
53 KC2
o
C2
o
C#2
o
D2
o
D#2
o
E2
o
F2
C.
casio
C.
fingered
out 3
54 KC3
o
F#2
o
G2
o
G#2
o
A2
o
A#2
o
B2
 
 
out 4
55 KC4
o
C3
o
C#3
o
D3
o
D#3
o
E3
o
F3
P.
select A
handclap
P.
select A
cymbal
out 5
56 KC5
o
F#3
o
G3
o
G#3
o
A3
o
A#3
o
B3
P.
select B
rimshot
P.
select B
agogo
out 6
57 KC6
o
C4
o
C#4
o
D4
o
D#4
o
E4
o
F4
P.
select C
synt. hi tom
P.
select C
hi bongo
out 7
58 KC7
o
F#4
o
G4
o
G#4
o
A4
o
A#4
o
B4
P.
select D
synth. lo tom
P.
select D
lo bongo
out 8
59 KC8
o
C5
 
R.
start/stop
R.
synchro/fill-in
 
 
R.
base pattern
 
out 9
60 KC9
O.
piano
O.
vibraphone
O.
flute
O.
human voice
O.
bells
 
O.
select
R.
select
out 10
61 KC10
C.
chord II
C.
chord III
R.
clap/bongo II
R.
clap/bongo III
R.
hihat/cymbal II
R.
hihat/cymbal III
R.
snare/rimshot II
R.
snare/rimshot III
out 11
71 KC19
R.
rock
R.
disco
R.
pops
R.
swing
R.
samba
R.
march
R.
base/bass II
R.
base/bass III
out 12
70 KC20

The input lines are active-low, i.e. react on GND, thus any functions are triggered by a switch in series to a diode from one "in" to one "out" pin. Because the keyboard matrix outputs are multiplexed with the address bus, they have additional driver ICs TC4050BP. Super Drums and drumpad select slide switches default to "I" when no other place selected. Like with CT-410V accompaniment variations, in Super Drums switches the 4th position closes both contacts.
 

legend:
"o"
 = keyboard key
underlined
 = function needs locking switch (i.e. stays active only so long the switch is closed)
R.
 = preset rhythm
O.
 = preset sound ('orchestra')
C.
 = chord
P.
 = drumpad
orange
background 		 = easteregg (unconnected feature)

Strange is that the drumpad select switch positions in schematics differ from the actual instrument. Particularly "dog" suggests that Casio initially planned some toy-like effect sounds like in SK-5.
 
A:
B:
C:
D:
BD
SD
OH
CH
CLAP
RIM
TOM1
TOM2
RID
DOG
CON HI
CON LOW

The 4 drumpads are not part of the keyboard matrix. Their short lo pulse is lengthened in a capacitor and then buffered in a data latch TC74HC175, which is clocked by CPU pin 26 T1 to output its 4 "/Q" lines through a TC74HC367 to the data bus. According to the service manual, "when more than one pad signal is output from Data latch" (i.e. multiple pads hit simultaneously) a TC4002BP (NOR with 4 inputs) sends an interrupt to CPU pin 42 I-4. Maybe that description was translated wrongly; I think a NOR would trigger it always if any (i.e. "more than zero") pads were hit.

No other keyboards seem to be based on the MT-500 hardware class. Direct successor of this instrument was Casio MT-520. A simpler Super Drums keyboard with analogue percussion was the Casio MT-52.
 

 removal of these screws voids warranty...    
WarrantyVoid
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