MIDIBass 303 Mark II Installation

Sync Mods


In order to allow the MIDIBass sync connections to safely over-ride the internal 303 sync signals, two resistors must be removed, and replaced with a pair of series resistors.
The junctions between the two new pairs of resistors are used as connection points for the MIDIBass sync signals.

The MIDIBass sync connections are bi-directional. They switch between input and output mode depending on whether the 303 is being slaved to MIDI clock, running from its internal tempo clock or an external DIN sync input.
Using the revised connection method, MIDIBass will now output MIDI clock while the 303 is slaved to an external DIN SYNC source.
It will also output DIN SYNC (using the plug-halfway-in method) when slaved to MIDI clock.

Resistors R10 and R16 must be removed. They are found in the top left corner of the main PCB:

Here are the pads to de-solder on the underside of the board:

The original resistors are 100k values. They are connected between the DIN SYNC socket and the CPU inputs for the DIN RUN and CLOCK signals.
For MIDIBass, an extra 1k resistor is added between these resistors and the SYNC socket. This allows MIDIBass to listen to the internal or external DIN SYNC signals, or to over-ride them for MIDI clock sync.
The 1k and 100k resistors supplied with the kit should be connected together, and also to one of the wires for the RUN and CLOCK connections.
Pay careful attention to the resistor colour codes - 1k is brown/black/black/brown, 100k is brown/black/black/orange.
Connect them by twisting two resistor leads and connection wire together as shown:

Then solder the twisted connection, and trim the excess with wire cutters:

The resistor pairs should then be fitted to the locations for R10 and R16 as shown.
The RUN connection is R10. The CLOCK connection is R16. Both of the 100k resistors (brown black black orange) should be fitted towards the front of the board:

An extra capacitor must be added between R3 and ground.
This is required because a bug in the way the 303 CPU handles its sync signals means that sometimes the 303 will see the clock pulse before the RUN line goes high as the first clock pulse. You don't notice this using a 303 alone, but if you generate midi clock from the internal sync signals, your synced devices can end up one clock tick behind the 303. The extra capacitor delays the start of the RUN signal so that the 303 knows the previous clock pulse has ended before it starts running.
The extra capacitor is soldered to the pads shown here, which are close to the pads for R10 and R16:

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