Some pix and info on adding MIDI control over the Out Slot button on the TX-816

The goal was to gain MIDI control over the button so TX/DX editor librarians could address each slot without the user having to go over to the TX-816 and press the button each time.

Parts list:
- Yamaha TX-816
- Highly Liquid MPA board
- Coto Tech Reed relay w. protection diode. Mouser part number: 816-9007-05-01
- some ribbon wire

You have to make some mods to the MPA board and you don't need to install all the parts. Refer to the pictures below.
The relay is installed with pin 1 pointed 'down' the board, towards the pot connections.
Cut the +5 trace leading to the right leg of C5 (just above where the relay is installed)
On the bottom, cut the looping trace that connects pins 12 and 14.

Connect power (red) and ground (black) wires as shown. Red is a regulated +5 from the TX-816, so the on-board regulator is not needed.
Connect 2 wires for MIDI IN. You will be seeing the messages in parallel to the TX-816, so you don't need Out/Thru connected.
Connect 2 wires for the Out Slot switch control. They are wired to POT 1 A and to the right side hole where C5 is located.

Connect a jumper from Logic 0 to POT 1 B
Connect a wire from Relay Pin 3 to Ground (see MPA board bottom view for that connection)

On the TX-816 board, connect power and ground to the 5 pin connector jack. +5 has 2 pins connected (4,5) and Ground has one (pin 1)
Connect the MIDI in wires to pins 4 and 5 of the MIDI In jack.
Connect the switch wires to the two sides of the switch (note that I moved one wire to another connection along the path to make it easier/clearer to see)

The default configuration for the MPA is to respond to middle C (note on, hex 3C, decimal 60) to trigger logic gate 0.
So if things were connected correctly, you should see the red LED on the MPA board flash and the TX-816 Slot number increment each time you send a note ON for middle C on MIDI channel 1.

MPA board (top view)
MPA board (bottom view)
TX-816 MIDI/Slot board (bottom view)
exciting video showing the mod in action

Note that these instructions/pictures are meant for someone that is an experienced technician. If you've never done this before, save some grief and take it to a tech and have them do the work. I am not responsible for fried MPA boards or fried TX-816's. Be smart about this.