Open Source Mechanic blog

Sometime in the late 1980s or early 90s I bought a Casio SK-200 sampling music keyboard. It has been a lot of fun over the years. I’ve sampled everything from radio static to hulusi’s and harps. The sound quality isn’t brilliant, it’s only 8-bit sampling at 10 kHz, but slightly higher quality than the more popular but older Casio SK-1. I was surprised to learn that 20 years after I bought this keyboard, there doesn’t seem to be a similar “kid-sized” fun sampling keyboard. So I hung onto it and now my kids have fun with it. I think it may have helped encourage my little boy to talk.

Boys will be boys, and the little guy loves playing music keyboards with his feet. So one of the black keys was wobbling like a loose tooth.

I decided to take it apart and try to epoxy glue the key. Wow there were quite a few screws inside! I read that the SK1 had 4 screws holding the key assembly into the case, this one had 14! Ideally I would replace the cracked key or swap it for a seldom used one at the top of the range, but while white keys are discreet, black keys are grouped, about 10 together on a single piece of plastic. The plastic has exactly the right springiness to make the keys bounce well. The SK-200 is full of discrete electronics, many ICs capacitors, diodes, transistors… all work together to make a robust and wonderful keyboard. Here is a photo of the M4135-MAIM board:

Here are some of the parts on the M4135-MAIM board

TC4066BP (3)	TC40175BP(2)
5218/4558?(2)	TC74HC174P
TC74HCU04	8517PX204
TC74HC157P2	MSM6294-03
74HCABP(2?)	7416PX204(uPD4464C -15L)
tM6283-02	HD61702A04

The other circuit board has a few small ICs and lots of transistors, capacitors, resistors and diodes. It appears to be the power supply and audio amplifier board.

I took apart a cheap modern keyboard recently and only found one of those ugly black blobs. You can see why keyboards such as this and Casio’s SK-1 are sought after by circuit benders. There are thousands of points where circuit modifications could be made. I found a point where it would be easy to pitch bend and the trick of piggybacking an extra memory bank and soldering the data-select to a toggle switch wouldn’t be too difficult, but I think I’ll leave well enough alone. With this minor repair the keyboard is just as fun as it was in the 1990s and neither Casio nor Yamaha appear to have taken advantage of 20-years of advancements in memory and sampling electronics and given us a higher quality portable sampling keyboard. So this was well worth fixing.

One word of caution, if you do use epoxy, only use enough to fill the crack. You don’t want the stiffness of epoxy to mess up the key bounce. And be very careful you don’t glue the key to the case.