Several digits were faint and incomplete on an SR-10. Close examination revealed the culprit to be an SN27423 chip, apparently a hex driver buffer. Indeed, when I swapped the two SN27423 in the machine, the bad digits and good digits were swapped on the display. I have no replacement SN27423, but I had enough information to deduce its pinout: 2->3, 5->4, 7->6, 10->11, 12->13, 15->14; pins 8 and 9 are on VDD (we don't care about pins 1 and 16.) I was able to replace the six buffers in this chip with a simple circuit, shown here:
Compared to the circuit solutions recommended by Texas Instruments (essentially, a Darlington circuit of two transistors and several resistors) this solution is a bit simplistic, but it works; and best of all, there is enough room inside the calculator for the six extra transistors and six resistors needed.
