Bontempi M40 (simple analogue squarewave keyboard without rhythm)

This ancient Bontempi keyboard has plain squarewave organ tones to the right (3 timbres) and a single finger chord (organ tones) to the left - that's it.

On the back are openings for storing 2 play teaching audio cassettes and apparently a note stand. (Mine are missing.) This instrument was also released as Bontempi MusicPartner MS40 and a midsize key variant as MusicPartner Junior; the MS40 was also released in a very rare black version.

main features:




With distortion and bass cranked up, this initially boring keyboard can now drone really massive and freaky.

circuit bending details

The PCB has only one CPU but many discrete components. Unusual is that the keyboard matrix employs resistors instead of diodes.

additional higher note keys:

The keyboard matrix is divided into groups with each 7 keys. The matrix outputs are CPU pin {9, 10, 11, 8, 7, 5, 6}, the inputs are pin {12, 13, 14, 15, 16, 17, 18, 19}. The additional higher note keys are the rightmost 3 of group output 8 and all 7 of output 7. You need a complex multifold switch to make the additional keys accessible (e.g. to use more keys for the main voice since there is really no matrix easteregg feature to switch the chord section to melody voice play). Also the tuning knob needs adjustment (only possible with increased range?) to achieve standard tuning after switching the key range.

volume control:

The volume control employs a row of 5 resistors (each 10 kOhm) wired in series and connected to the volume slide switch PCB contacts. Wire 2.2 kOhm parallel to the first one to make the first step quieter (or add a potentiometer instead of the row for stepless volume control).

bass volume & distortion:

A 30 kOhm potentiometer across the diode D5 (wiper to right end, other end open) distorts the bass. For bass volume control solder another potentiometer with the ends between the input of D5 and GND, and its wiper through a 39 kOhm resistor to the upper end of resistor R80. 

envelope attack rate:

The electrolytic capacitor C6 controls the envelope attack rate during monophonic play.

increase pitch range:

The pitch range can be increased by changing some resistors at the pitch potentiometer (and increasing its value for achieving lower pitch). When pitch is turned very high by this, the keyboard responds wrongly due to overclocking problems. Wire 22 kOhm pull-up resistors at each keyboard input line (at CPU pin 12 to pin 19) against +5V to reduce the problems.

Attention: I have only very incomplete draft schematics of my modification, thus this description might be partly inaccurate or even wrong since it is based on them.

Technically similar like the Bontempi M40 (but without chord section) were the mini keyboards Bontempi Basic BK32 and BK26.

 removal of these screws voids warranty...    
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