The Casio PT-10 is a cute little monophonic + drums keyboard with no headphone jack. It has a speaker and comes with the lovely drums that TRIO used in their famous "da-da-da" song.
I have one and will be careful to modify it in a most case-friendly way.
Step one: add a switch to the speaker and get the signal out on a 3.5mm socket.
Step two: replace internal pitch knob and get the signal out.
The inspiration to look for the separate lines I got from this page. My board is different though, the chip is SMD and not through hole.
seems to be the same board here:
The Casio PT-10 uses the same chip as the more known Casio VL-Tone VL-1, it's the D1867G. This is also in some other keyboards, you can find more info here: