I have this old Casio piano (CTK-500) which I have taken apart before. Last time, it had to do with one of the switches. This time, it was the AC adapter input jack. It would still work, but it was flaky.
The first time I took it apart, I had an egg carton (which I still have) with numbers on each hole. Whenever I take something apart that I'm actually planning on reassembling, I draw a rough sketch of the chassis including the screw holes. When I take out screws, I write a number next to the hole on the sketch and then put the screw in the corresponding egg carton hole. And, I kept the sketch for future reference if I wanted to take it apart again. This turned out not to be as big a deal with this particular keyboard, because it didn't have a lot of different screws, and they didn't look similar enough to put one in the wrong hole. But, extra information doesn't hurt as long as it's correct. 
I assumed it would be a loose solder connection. However, on closer inspection, the metal contact for the barrel plug's sleeve appeared mildly tarnished or corroded. In principle, I might have been able to reuse the existing connector by using some contact cleaner or a sharp pin to remove the corrosion from the contact. However, the circuit board layout and the cutout in the chassis is such that an exact match isn't critical, so it would probably be easier to just replace it. All I need is to get at the solder side of the board.
Unfortunately, there are no solderless connectors to speak of in this piano! There is a ribbon cable going to the actual keyboard, which is soldered on both ends. The speakers are soldered as well. Considering how long the piano has lasted with no major problems, this isn’t all that frustrating to me. The ribbon cable is long enough for me to flip the board without removing it, which would certainly not be a fun thing to do. So be it, I’ll work in place. The speaker wires, however, won’t let me invert the board. Last time I took it apart, I unscrewed the speakers and got enough access to the other side of the board to work on it. So I did it again.
In retrospect, I probably should have disconnected them instead, to avoid the speakers falling in just the wrong direction and damaging the circuit board. However, I got lucky for the second time in a row.
Out of curiosity, I gently wiggled the jack while looking at the bottom of the board. I don’t see any movement, but it doesn’t really matter, since I’ll be soldering here anyway to replace the corroded jack, which I now further suspect as the source of the problem.
I have a temperature controlled soldering station and one of those rubber bulb type things to suck up old solder. I desolder two of the three pins without hassle. I plan to gently prod the pins with a screwdriver to make sure they are not still partially soldered to the pads, lest I damage the board trying to remove the old part! I get to the third pin and – oops – the pad just came right off! Fudge!
Well, at least the power supply isn’t all that complicated layout-wise, so I can run a wire to the new jack. Following the traces from that hole to another pad takes just a few seconds. New jack installed with no further collateral damage. I looped a small wire around the pin, to an adjacent, appropriate connection, and solder it on.
I do a quick look over for solder bridges. Everything looks fine. But, what if some extra work is needed? It’s a low voltage circuit, so there’s no danger (to me) if I plug it in while it’s taken apart like this. However, the switch contacts, which are attached to the top chassis, mate with contacts on the circuit board. Thus, it is not possible to even power it on until a number of the screws are re-installed into the board. Well, I suppose it wouldn’t be a good idea to play the piano until it is completely assembled ?
I put some extra hot glue around the new connector as a bit of added insurance, then I put the circuit board back. What do you know? The cutout in the chassis is almost a perfect match! Since this was my keyboard that I don’t imagine would have much resale value to begin with, I would have been prepared to modify the case slightly to use the connector that I had on hand. I screw the circuit board down, and then the speakers. Well, the bottom cover has nothing critical on it, so I’m going to go ahead and apply power. Sure enough, the keyboard’s power is now stable.
I suspect that the solder pad that came off was already partially lifted from the board. If this were the case, it might have been the main problem all along - it wasn't making consistent contact with the trace on the circuit board.
I would like to mention that this keyboard must have been designed pretty well since it has lasted as long as it has with only a few minor repairs (the switch problem I suspect was a result of abuse (not by me) rather than wear) Although it could have been made more serviceable, it would have cost more to do so, and possibly lost it’s competitive price with other similar keyboards as a result.