A Novus 6025 calculator had a missing digit on the display. Poking around with the 'scope, I discovered that the corresponding output from the main IC is present; it is connected via a series resistor to the digit driver chip, and it was the driver that apparently failed for this digit. I disconnected the output of the driver chip (cutting through the PCB trace with a knife) and inserted a 2N2222A transistor. The transistor's base was connected to the digit driver's input; the emitter was grounded; and the collector, via a 10 Ohm series resistor, was connected to the digit input. The calculator works like new and I believe all currents and loads are within acceptable limits.

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