There are various approaches for replacing the MM5824 divider chip, prominently used in KORG PS3x00 series synthesizers.

Most are based on the replacement circuit KORG themselves used on their boards. It consisted of a Hitachi HD14520 and some passive components. Many replacements just use a competitor’s 4520 type counter IC and maybe adjust the values of the passive components.

What I encountered when using such replacements: when holding certain notes, the tone eventually became interrupted at random intervalls. The counters occasionally counted mutiplie times during one cycle of the input signal due to violation of the input slope. The original HD14520 allows for 15µs rise and fall time for all supply voltages between 5 and 15 volts, while for example TI allows 15µs at 5V but only 5µs at 10 or 15V VDD, and Nexperia calls for 3.75µs@5V, 5µs@10V and 9µs@15V.

The obvious solution to get rid of any manufacturer constraints was to add a Schmitt Trigger to the circuit, which I did for both clock inputs while keeping attention to the clock edge on which the original counter propagates.

As stocks of working DAC71 series converters are depleted I designed a plug-in replacement for both the current (I suffix) and voltage (V suffix) types, including both CSB and COB coding. While the code is selected by a solder jumper, the voltage type replacement needs an additional SO8 OP Amp, like an OP07, fitted. The “TRIM” pin is not connected, instead of this a 20-turns potentiometer is added to the RPL71 board allowing fine adjustment of the on-board 10,00V reference. Where needed, a buffered output of the reference can be provided by adding another OP Amp and a few resistors.

Shown above is a DAC71-CSB-I replacement board (no I-to-V OP Amp on the upper right side) and without a reference outputs (missing parts on the lower corner) which is equipped with a 14- instead of a 16-bit DAC to save 10€ as this board is tailored for use in Sequential Circuits Prophet-5 synthesizers where the lower 2 bits are not used anyway. The actual use case is shown below.