This has got a dodgy connection for the 15v supply from the inline transformer, such that it only connects if you apply quite an alarming amount of upward pressure on the plug! So much is necessary that I fear the socket is about to snap, and you can't use the computer while doing that anyway. I think the problem is with the innards of the socket rather than the plug.
Does anyone know how much of a sod it might be to open the case and try and sort it out? As this is a very old (and basically expendable laptop) it could even involve making a direct 15v connection between the cable and the inner wiring, bypassing the plug-in connection altogether!