Thanks to tcmtech and Sceadwian at electro-tech-online (is.gd/57Evy), my ancient pen plotter is working again. It didn't have a power supply, requires two unusual DC voltages, and a cloverleaf connector. They correctly suggested that I could use approximate inputs as the supply is regulated internally. I did, and now the things works! (Courtesy also of a plot driver from Roland DG Japan - is.gd/57EHV)