Musical Blackboard

This is one of the weirdest instruments I had on my table for repair to date. 

It isn’t a synthesizer, or at least not the ordinary type, it is a analog touchscreen of old. Educational prop made in Czechoslovakia sometime during 1980’s. This blackboard is made of the large PCB painted over with  black paint. There is a separate thick PCB track for each note and you play touch. However since it’s a old school analog you need to hold the wire (or connect yourself using bracelet) for this to work.

The heart of this “thing” is a “organ in one package” Tesla MHB208. This is a Czechoslovakian clone of Thompson M208. It’s rather basic and foul sounding chip but it’s definitely got it’s use. It’s one of those “so bad it’s good”

Whats different is how keyboard is simulated by the touch mechanism. There is a set of 3 state switches Flat, Regular, Sharp for each note. You select the key you want to play in by selecting respective #’s and b’s  for each note and go for it.


There are high impedance flip flops triggered by the small current carried over your body triggering individual notes. It is fully polyphonic and great fun to play once you get used to the individual notes boundaries.

This thing came without external PSU and I had to figure out the power and voltage requirements. It’s dead on straight forward AC input of anything between 15 and 40 V AC.

There is rectifier bridge and MA7812 which is a TO3 package for of usual 78xx.

I discussed with lucky owner and added separate internal transformer to make it easier to use on stage on mains.

clumsy demo below, enjoy the creepy sound