Monday, 10 October 2011

KR-55 clock-in mod

The Korg KR-55 (or Rhythm 55) is a great sounding drum machine. Originally released in 1979, it followed on from the MiniPops series of preset rhythm machines and was in direct competition with Rolands CR range. While it is not programmable, it is pretty flexible with lots of variations and some excellent fill-ins. There are no individual outputs but at least it gives some control over the voices with seperate(ish) volume pots. You can hear the KR-55 on many late 70s, early 80s tracks - Mute Records guvnor Daniel Miller made great use of it's trigger output, driving his ARP sequencer and of course the excellent snare drum can be heard throughout Depeche Modes Speak & Spell album.
It's one major drawback is that it has no sync or clock input and this makes it a bugger to integrate into your set-up...
...here's a pair of KR-55s waiting for the external clock-in mod.
It's a fairly straightforward process, lifting one resistor and using a switched jack, but there's not a lot of room on the rear panel to mount the new jack socket...
care needs to be taken when fitting the jack socket so as not to foul the top pcb when the machine is re-assembled.
The KR-55 runs at 8ppqn so a clock divider is required if you want to sync it to midi or sync24. The clock output on the Kenton midi to cv converters is ideal for this (divide by 3).

Here is some further information on the Clock Input mod for those interested…
There are two large circuit boards inside the KR-55, the upper pcb is the voice board and the lower one is the logic board and this is where you need to work…
The upper voice board needs to be removed, there are five screws securing it, remove them and carefully lift the board over… You may also need to carefully unplug the connecting cables to give you more room. You can now unscrew the PCB pillars to remove the lower logic board...

Locate R48 on the logic board (it's just below IC3, see pic below)… this is where to access the internal clock. Lift (de-solder) the right-hand side of R48… connect a wire from the new mini jack "tip" to the lifted leg. Connect a wire from the mini jack "contact" to the pcb pad the leg was lifted from. Connect a wire from the mini jack "sleeve" to any 0v point (gnd).

Now, when a jack is inserted to the new socket, it disconnects the internal clock from R48 and inserts the external clock signal instead… Removing the jack restores the internal clock. You can, of course, use a separate switch to select between internal or external clock but I chose the switching mini jack as there really is very little space inside the rear panel.
The clock input should be 5v positive pulses. Do not use gate or trigger signals that are more than 5 volts without some form of attenuation.
When using an external clock you will need to press the Start button on the KR-55 before sending the clock signal - this puts it in "ready" mode.


Disclaimer: Only attempt this modification if you are confident to do so and understand what you are doing. I accept no responsibility if you damage your machine.