Cheap Mods
Casio DH-100 Vibrato Mod

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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:

  1. Vibrato permanently OFF
  2. Vibrato switchable ON or OFF
  3. 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


  1. Solder the 22k resistor to one terminal of the SPSTmom and the 1k resistor to the other terminal of the SPSTmom.
  2. Solder 6-9” lengths of wire to the central and one outer terminal of the SPDT slide switch (let’s call this term_1k)
  3. Solder more 6-9” lengths of wire to the SPSTmom
    1. One at each terminal of the switch
    2. One at the other end of the resistor on each terminal of the switch.
  4. 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.
  5. 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.
  6. Solder the end of the 22k resistor of the slide switch to VSS/GND (I used a corner via of the PCB).
  7. Solder the end of the 1k resistor to VDD (I found a via on the PCB “above” R49).
  8. 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.
  9. 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).
  10. That should be all the soldering you need to do. 
  11. 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.