My yamaha p85 keyboard has internal speakers and two headphone jacks. Plugging anything into either headphone jack turns off the internal speakers. I think the intended use is for a student and teacher to both be able to hear while not bothering anyone else. For my purposes, though, it can be annoying. If I'm playing for a dance and I don't have good (or any) monitors, I would often prefer to have the internal speakers remain on so I could hear myself better.
I decided to open it up and and see what I could do to change this. There are a very large number of identical silver screws, one black screw, and then four little black screws holding on a plate. Turns out they all need to come out. There's a little plate on the back, and behind it is a circuit board with two ribbon wires attached. Those need to come off if you're going to separate the top and bottom keyboard halves. Unlike an ide cable, the hard plastic bit at the end is part of the circuit board, not the cable.
I found the circuit board next to the headphone jacks, which I thought was probably the place to make a fix. It looked like this:
The big solder bits are where the two jacks connect. There are six pins. From looking at the traces (sorry about the poor focus) I could see that the middle two were connected together and the top one is grounded. Testing with a multimeter for resistance, the middle two pins were normally connected to the ground, but disconnected when a cable was inserted. Looking into the jack, I think maybe the end of the cable pushes apart two bits of metal to break a connection. If we could short circuit this detection circuit, so that it always read "connected" then the internal speakers would stay on.
I considered short circuiting both detection loops and then adding an externally accessible switch to disconnect them, but I decided against that because of the inconvenience of cutting a hole for the switch and mounting it. Instead I decided to modify only one of the jacks so that I would later be able to choose whether to leave the internal speakers on by plugging into one jack or the other. Some soldering later and we had a new connection:
It worked exactly as I had hoped. I closed everything back up and made sure it worked before putting in all the screws. Then I painted the modified jack with some white-out. Total time: 3hr.