Cheap
Mods

Casio DH-100 Vibrato
Mod

Home |
Casio MT-600 Filter Mods (NEW!) | Casio DH-100 Resonance | Links |
Casio DH-100 Octave Drop | Yamaha PSS-170 FM Scramble | Casio DH-100 Vibrato Mod
(UPDATED!)

Background

UPDATE: In November last year I
got an email from Andreas, who had read my site and did the vibrato mod on
his own horn with great success. He then decided to do the vibrato mod on
his friend's DH-100. He found, however, that his friend's DH had a
differences on the main PCB than the one I show on this site. Andreas
mentioned that my mods don't work on the different PCB (though his friend
also has a DH-200 that DOES have the same board I show here). Andreas
stated, "...After scrutinizing the circuit boards, I found the upper one
to be somewhat different. A slightly different layout (two more
SMD-transistors) and the CPU (IC 1) has a different number printed on -
CM-511 instead of CM-505.
The service manual (downloaded from
Electri-Fire) doesn't mention this..." He sent me an image of the
different PCB which I've added below. It's worth noting that Reed
Ghazala's book also indicates that there are DH's with different
circuitboards. So my thanks to Andreas for discovering this. My
recommendation is that, before you do any of my mods, you verify that your
main board DOES NOT match the board Andreas encountered (see below). I
wonder if the different CPU might be the one from another model, the
DH-500 or 280, perhaps?

Mods will NOT work on
this PCB!!!

I got my DH for pretty cheap
locally (like most of my musical toys) and it had the squeal, no battery
door, and was a little beat up.
I read Dorian Rose’s website and learned that the
squeal was easily fixed, so I did that first.  Then I emailed Ted Keys
and Paul Fox and they gave me clues on how adjust
the VR’s to get the breath sensor sensitivity to my liking.  But the kicker was that Electri-Fire had posted the service manual on the web, so I
downloaded that.  I’m a
circuit modder (sometimes bender) so I started making plans for my
DH.

My first two mods didn’t seem to
take, so I decided to try a little bending to see what was possible.  This is where the vibrato mod came
from.

Personally, I don’t really mind
the vibrato – since I’m not a horn player, it adds a little flavor to my
blowing on the DH.  However,
it’s always nice to have options.
I read on the web (Harmony Central, TedKeys, etc) that some folks
really would like to get rid of it.

The key is the unused pins on the
CPU.  If you read the pin
function table in the service manual, you find several pins that claim “No
Function.”  Uh-huh.  If you then go back to the
schematic and look for those pins you find many of them are tied high or
low.  That’s the tip-off.  Truly unused pins on an IC are
usually left to float, because why would you waste any layout designer’s
time trying to get that pin to a known potential?

So, with a horn key depressed in
BREATH OFF mode I took a jumper and connected one end to VDD or VSS and I
used the other, through a 10k resistor, to tap all the pins on the CPU IC
one-by-one (classic bending technique).  With my jumper to VSS, when I
touched AN6 (pin 34), the sound continued but the vibrato turned off.  Eureka!  Here was a way to modify the
vibrato.

I checked out the rest of
the horn’s functions while holding the jumper to AN6.  Everything seemed to work
fine:  key presses, tone
change, transposition, volume control, etc.  Now the plan was to get this mod
so it could be a usable function on the horn.

The Modification

There were
three levels of the mod possible:

Vibrato permanently OFF

Vibrato switchable ON or OFF

Vibrato switchable ON, OFF (and when OFF,
can be switched ON in real-time with a separate control).

I opted for
mod ver#3, though it’s definitely the most complex, I think it adds the
most control during playing of the horn.  For example, if you switch it OFF,
you might like to have a button near your right thumb that could be used
to kick in the vibrato during a held note or some phrase.

I will only
describe mod ver#3 because that’s the one I did.  The other two are subsets of this
mod; they have fewer steps, components, solderings, drilling, etc.  Even for mod ver#3, there’s only
and one cut trace and 3 wires to solder to the PCB.

CAVEAT:  There
is one wrinkle to this mod.
If AN6 is held low (vibrato OFF) when the horn is turned ON, the
“hold-down-the-Portamento-button-to-activate-the-special-Casio-fingering”
is no longer available.  This
may or may not be important to some players.  And it’s really only a problem if
you choose mod ver#1.  With
mod ver#2 and ver#3, you can leave the vibrato switch that you add on the
outside of the horn to ON and the Casio fingering becomes available as
normal.  (And to be honest,
you could come up with a scheme that holds AN6 high temporarily during
boot-up, and then this mod controls it the rest of the time.  I’ll leave it up to you – too much
bother for me!).

The circuit
you’ll be implementing is this:

How it works:  when the DH is scanning for
keypresses on the notes, it probably also looks at AN6 (I haven’t verified
that with a scope, but must be how it works).  If AN6 is high, the DH activates
the vibrato.  If AN6 is low,
it disables the vibrato.  Our
goal here is to switch AN6 either high or low, turning vibrato ON or OFF,
respectively.  So, AN6 is tied
to the central terminal of the SPDT slide switch.  If the switch sets AN6 to the 1k
pull-up resistor, then AN6 will be high and vibrato will be turned
ON.  If the switch sets AN6 to
the 22k pull-down, then AN6 will be low and vibrato will be turned
OFF.  Now, in this OFF
position, if the user, during playing, presses the SPST momentary, then
the DH will turn the vibrato ON while the note is playing (and there’s no
retriggering or gaps to the note or anything!).

Parts
List

SPDT slide switch

SPST momentary switch

Six 6-9” strands of 30Ga Teflon-coated
wire-wrap wire with stripped ends

~1kOhm resistor

~22kOhm resistor

Personally, I
hate SPDT slide switches, I much prefer toggles for their tactile feel,
but the profile of slide switches is typically lower and the DH is a very
contoured instrument.
Besides, the DH uses slide switches extensively.  The new slide switch would look
not TOO out of place on the horn.  The SPST momentary is your
choice.  I grabbed the most
suitable one I could find at Radio Shack (not the best selection, believe
me).

Electrical
Installation

Solder the 22k resistor to one terminal of
the SPSTmom and the 1k resistor to the other terminal of the SPSTmom.

Solder 6-9” lengths of wire to the central
and one outer terminal of the SPDT slide switch (let’s call this
term_1k)

Solder more 6-9” lengths of wire to the
SPSTmom

One at each terminal of the switch

One at the other end of the resistor on
each terminal of the switch.

Solder one more wire to the
still-unconnected outer terminal of the SPDT slide switch (let’s call
this term_22k) and the other end to junction of the SPST terminal and
the 1k resistor.

Now open the horn and remove the 3 screws
for the top circuit board (as you’d do when you are replacing the squeal
capacitor).  You need access
to the bottom side of the board for this mod, so tilt the board back.

Solder the end of the 22k resistor of the
slide switch to VSS/GND (I used a corner via of the PCB).

Solder the end of the 1k resistor to VDD (I
found a via on the PCB “above” R49).

Cut the trace that ties AN6 to the pull-up
surface-mount chip resistor R49.
Make sure that you completely open the trace; you can verify this
with a DVM at the via and the edge of the chip resistor.

Solder the wire on the central terminal of the SPDT slide switch
to the via on one side of the newly-cut trace (this via goes to AN6 pin
on the CPU).

That should be all the soldering you need
to do.

Turn the horn on and test all the functions
to make sure it all works.

Mechanical
Installation

This part is usually the messiest
– I’m still learning how to do these things cleanly.

I placed the SPDT slide
switch on the side of the horn just above the volume pot.  There’s definitely enough room,
though it’s a little awkward drilling and cutting in this area with all
the circuitry nearby; I didn’t remove any boards fully, but while
installing I did leave the top main PCB loose.  I used a drill for the mounting
screw holes and to start the main hole for the slide switch.  I carved out the squarish hole for
the slide switch with a utility knife and a pair of diagonal cutters.  The picture shows that my job
wasn’t precise, but it does work fine.  (Square holes is ANOTHER reason I
hate slide switches)

I placed the SPST momentary
just below, and as close to the bottom of the battery compartment as I
could.  The closer the better,
I think, because your right thumb does have to travel some distance to
activate it (unlike the left thumb’s octave key).

My own comments about this
mod:

Performance:  it’s definitely nice to have the
vibrato switchable.  I think
having it off really gives the player a blank canvas for expression.  But sometimes it’s kinda cool to
have it on.

As for the vibrato button,
I’m only partially satisfied with the way I did it.  Again, I’m no horn player and the
more I play this thing, the more I understand the gentle and clear design
of the melody keys.  I think
I’m gonna search for a really really nice, and right-sized SPST for the
vibrato ON button.  It’s still
fairly cool to kick it in when holding a note.

Further mods?

I’ll bet there are more eastereggs
hidden in the DH, accessible through changing the states of the other “No
Function” pins of the CPU, such as AN1,2 and some others.  I leave these up to future
benders.  I’m not gonna chase
them down because a lot of the circuitry on the DH is surface mount and
the component density is fairly high.

For me, this makes it a bear to
cut traces just to explore things.

Besides, I wouldn’t want to
kill mine now.  The DH is cool
for all the analog that’s in it (rare for Casio), and it’s the most
expressive Casio I’ve ever had.
And some of the prices you see on ebay for these things makes
bending a little more prohibitive.