Hi, I'm new to the forum.
I got myself a second hand casio px-800 yesterday. I'm impressed with it's build quality. Keys feel a bit too plastic but I just ignored it when I starts playing. Action is simple but good. But I think the sound is the first thing need to be improved.

The speakers are OK so I'd like to have a line-in and feed my software synth to the piano so the speakers don't get wasted. If you want to do it here is the procedure.

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Please take your own responsibility, I can't guarantee everything I said is correct.
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The speaker box is connected to the piano via 15 wires. I'm not sure what all of them do but we only need 3 to get a line-in socket.

I used a 1/4 inch chassis socket 3-way switchable as my socket. It has 6 pins. I installed it between (in series with) the plug and the PCB in the speaker box.

Next thing is find the right wires to cut. I traced the PCB and found the 3 wires. The 15 wires spilt into 2 plugs that plug into the PCB. We need to the one with 8 wires (the one with a brown dot beside it). counting from the top, looking down, above the PCB

1 --- Unknown
2 --- Left +Input
3 --- Power GND (Don't touch)
4 --- Signal GND
5 --- Right +Input
6 --- Unknown
7 --- Unknown
8 --- Unknown

We need to cut wire 2,4 and 5

This is the plug looking towards the wire
.._____..
o o o o o
o o o o o
o o o o o

This is the respective position the the wires
.._____..
5 4 o 2 o
o o o o o
o o o o o

Now wire these to the socket.

L R GND
-..-..-
.......()
-..-..-
L R GND

Beware one side pops up when you plug the plug into the socket. That's the side you need to wire to the 8 pin PCB plug. The other side goes to the 15 pin plug.

It should be done. I found out the volume control does not control the gain of the amplifier. so only way you can control the volume is from your input.
The frequency response curve of the speakers are poor. you probably need to adjust Eq in the software synth.

The sound is definitely better. I'm going to play some music now ^^