Usually, if the same segment is dead across all digits in a calculator, the segment driver is suspect. In the case of the HP-67, the segment driver chip is the smaller of two chips located underneath the LED display modules. It is easy to map this chip's inputs and corresponding outputs. This chip contains seven independent driver circuits. When a single segment is dark across the entire display, chances are that one of the driver circuits is dead. Replacing the entire chip is often not an option, since substitute chips are hard to find. However, a single driver element can be replaced using a 2N4401 transistor. The base of the transistor should be connected to the corresponding input, the emitter to the corresponding output, and the collector to pin 15 of the segment driver chip. Using fine wire-wrap wire, you can place the transistor in such a way so that the keyboard/display PCB still fits snugly against the calculator's plastic backbone, and the case can be closed securely. If the driver circuit in the segment driver chip is shorted, it may be necessary to disconnect (cut) one or both corresponding pins in order for the substitute transistor to work correctly.

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