Casio MT-400V Filter Tweak


Let your Casiotone MT-400V get squinky with a simple resonance mod

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The Casiotone MT-400V may not be a serious, professional synthesizer, but thanks to its true analog lowpass filter section, it is seriously fun to play with. Having realtime sliders on a "toy" synth is completely unheard of in recent products and rare in 80s-90s products as well.

That said, the MT-400V's filter falls a bit short on the resonance side; it just doesn't make it into the "squinky" realm of self-oscillation when the resonance control is pushed to the max. But, with a simple tweak, it can!

About the MT-400V Filter

The MT-400V's filter has trimmer pots for sustain level and resonance. You'll find, however, if you turn up the resonance trimmer you'll get nasty self-oscillation many times louder than the sound input. We want it to oscillate, but at about the same level as the sound input, not blow out our eardrums.

Sound Samples

Though it isn't programmable, the accompaniment variations are quite good. The bassline is a squarewave-based and responds well to the filter.

This one starts out with the 16-beat rhythm and bassline with no resonance, then slowly turns it up to about "stock", then proceeds onward to "acid".

Same 16-beat rhythm and bassline, but this time I switch around between the variations, while varying the cutoff.

Not bad for something you can pick up at the flea market for $20! All we need is a way to add "glide" and "accent" and we're on the way to TB-303-space.

How to do it

Stage 1

Here we're going to allow the filter to self-oscillate at high resonance. We'll keep it simple and use just a capacitor. This gives a good punchy, nwerpy sound on the filter attack but does not allow you to have a controlled, sustained self-oscillation. Give it a try. If you don't like it, it is very easy to unsolder the capacitor lead to the ground pin of the nearby 4049 and move to stage 2.

This is not a hard tweak in that you only have to remove the back cover and are able to adjust the on-board trimmer from the back of the board. I took everything apart in my case because my keyboard's power LED would only light if you pressed on it and I wanted to fix that (with a blue LED, of course!).

If for some reason you want to get at the front panel board, I urge you to be careful. All the momentary buttons are membrane switches and the membranes come off their little locating posts very easily. It's a pain to get them back into place properly. Also, don't forget you have to pull off all the slider caps after you remove all the screws or the board will not come off.

See my "Swept-Tone Noisemaker" project for info on soldering if you're unsure. By all means, ground yourself with an anti-static strap or use an anti-static work mat. You don't want to turn your precious gem into worthless junk.

You'll need to solder a 39 nf metallized film capacitor to the pins shown below:

Next step is to turn the external cutoff slider to about 1/4, external resonance to maximum and the volume to low. Then turn on the synth, select a bassline and adjust the resonance trimmer until it chirps loudly (watch out if you are using headphones). Next back off on the resonance trimmer until you get a controlled self-oscillation. Yeah baby! 

You may want to try different variations because some of them may have more or less high-frequency content which might cause the self-oscillation to get out of hand.

Don't sweat it if you sometimes do get a bit of a loud "chirp". You can always back off a bit on the master resonance slider once you've closed up your synth. Or I suppose you could bring the resonance trimmer out to the front panel. I don't see a use for it really, since outside the "sweet spot" is either a dull sound or out-of-control tweeting.

Notice I've blocked the adjustment hole with the capacitor. I got too excited and didn't plan well. If you don't block it, you can adjust without removing the board by putting a small screwdriver through the hole.

Here's the overall view so you know exactly where it is:

Here's a view of the trimmer on the other side:

Stage 2

If you want even more squinky and tweety sounds from the filter, or you're bothered by the occasional LOUD chirp you might get from a high-pitched accompaniment sound, move to stage 2.

All we do here is add a 10-20 k pot in series with the 39 nf capacitor:

If you use a low-profile pot as I did, there is plenty of room for it to fit in the case when it is closed.

To adjust is similar to stage 1. Turn the main volume way down (to protect your ears), turn up the master resonance to max, adjust the resonance trimmer until you get nasty chirping and then adjust the new trimmer to reduce the chirping to a more reasonable level. You will want to try different sounds and different cutoff settings to make sure you're not still getting nasty loud self-oscillation at some settings. 

When you get it sounding good enough, turn the master cutoff down to minimum and adjust the trimmer to the right of the resonance trimmer to get the sort of sound you like from the minimum cutoff setting. I like a tiny amount of bass to leak through. You might want total silence.

You're done! Close it up and enjoy your new super-squinky Casiotone sound!

 Further Ideas

I've noticed that main sounds do not seem to be affected as greatly by the resonance control as the accompaniment and bass sounds. I'm not sure if it's a waveform thing or a difference in levels before the filter.

It might be worth pursuing one of the distortion mods to get a more "barky" sound out of the filter.


What a simple mod for so much sound! Thanks to the Casio engineers for designing a product with such hidden potential.

There are more gems hidden in this and other Casio keyboards. If you want some further mods on this and other keyboards, head over to Warranty Void. They have all sorts of great information on similar casio models and other types of toy keyboards.

Check these links for more ideas: