Modification of a Hohner PK 250 midi (synthesizer/keyboard)

By manipulating the circuit, it is possible to freely adjust parameters such as filter frequency, resonance and the envelope (ADSR), an option that was not originally intended.

The built-in CEM3391 chips make it possible.

The additional potentiometers serve as voltage dividers which influence the values, and the switches can be used to toggle between expansion and factory settings.

A project in cooperation with Eduard Brandmair, who also provided the device; October/November 2023

Photos: Eduard Brandmair



Housing opened, the level on which the two CEM3391s are located exposed:


1 of 2 ICs carefully removed:


Both ICs on a secure foam base, the pins carefully bent outwards:


Wire jumpers used to tap the signals from the existing circuit at the base of the IC:


Solder fine wire to the pins of the CEM3391 and secure with heat shrink tubing:


Place the ICs back on the sockets. The protruding legs secure the underlying wires in the socket:


Branch off the power supply for the potentiometers at these points (+5V/red; GND/blue):


Drill and mount the U-profile:


Connect the rotary control (10k Ohm, lin) and toggle switch (SPDT; ON/ON):


Wires coming out of the housing are connected to those of the new components:


A second U-profile forms the lower half of the cable duct:


The connecting cables are wrapped in mesh and secured with cable ties:


Finished device with labeling:

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