It seems to be common for people to lose the power supplies for the Seiko DS-101, DS-202 and DS-250. They use a DIN connector with no pinout indicated on the keyboard itself, so there’s no readily available replacement, or even an obvious way to build one. But they’re not really special or irreplaceable. It’s just an ordinary 13.8 VDC, 1000 mA unregulated supply with a 4 pin DIN plug. Pins 1 and 2 are positive and pins 3 and 4 are negative. At least on the DS-250, the positive terminals are toward the rear of the instrument. There’s a photo of the power supply here, including the pinout.

There are several possibilities for replacement. The best option might be to replace the DIN jack on the keyboard with an ordinary barrel connector so that it doesn’t need a special power supply. I don’t think it’s really possible to solder a barrel jack in place, but it would work well enough to use a female barrel connector with a lead. It would similarly work to attach a DIN plug to an ordinary adapter. Here’s the DS-250’s jack board with the DIN jack highlighted and the terminals labeled:


There’s also a simpler workaround: Alesis and Digitech used the same 4 pin DIN plug, with the upper and lower pins on each side connected together in the same way. Alesis and Digitech used 9VAC power supplies, so these can’t be used with the Seiko, but there are adapter cables available to convert a standard barrel connector to a 4 pin DIN plug. These will work with a center positive power supply.