Commonly it is believed that missing print rows are due to dead heating elements in the print head. HP service manuals suggest, and my experience confirms, that the real culprit in the "Topcat" series of printing calculators is often the driver transistor. These are housed in fours in 14-pin DIP modules, but they can be replaced with any NPN switching transistor that can carry at least 500 mA of sustained current. The chip pinout on both sides is C-B-E-NA-E-B-C.
Missing pixel rows in HP "Topcat" printout
