MIDIBass 303 Mark II Installation

Component Removal

Once the switchboard is clear, you can locate the components that will be removed to fit the MB303 board.
Here are IC9 (a 4174 latch) and resistor R59 (which carries the SLIDE signal):

And here are the pads underneath you must de-solder to remove the IC and resistor. Once the IC is removed, the 16 pin DIL socket should be soldered in its place.

In place of resistor R59, the 'SLIDE OUT' wire from the 6 pin connector on the MIDIBass board needs to be wired to the pad shown above.
To allow clearance for the MIDIBass board, transistor Q29 and resistor R60 must be gently pressed flat to the PCB - Q29 can only be flattened once R59 is out of the way:

Here are resistors R146 (carrying the GATE signal) and R141 (ACCENT):

And here are the corresponding pads to de-solder for removal of these resistors, along with the wires to attach from the 6 pin connector:

For clarity, here is an image showing the connection points for the MIDIBass wires (except Filter CV, which goes to the rear of the board) superimposed on the component layout of the 303 from the service manual.
Click on the image for a larger version:

mainboard wiring points

Finally here are images of the 6 pin wiring connector in place, with different colour wires to make the connections obvious.
First without the MIDIBass fitted, so you can see the SLIDE connection, then with the board in place.
Note the 303 switchboard is pulled forward here, just to let you see properly. The filter CV connection to the rear of the board is shown in a following page.

N.B. The Mark II board has been revised to a IIb version.
The only difference for installation is that the ACCENT IN and OUT pins are swapped on the board so the wires dont cross.
The original Mark II boards are BLACK, the Mark IIb & IIe boards are BLUE.
This image shows the Mark IIb board.

Click here for the original Mark II board images.

Click on the images for higher-res images:

MIDIBass wiring, no board

MIDIBass wiring, board in place

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