Repairing a Hewlett-Packard 5245L Frequency Counter, Part 2

Usually, when a vintage transistor fails, you can replace it with a modern transistor with the same or better specifications (maximum frequency, breakdown voltage, current capacity, etc.), but when you’re dealing with early 1960s circuity running on the very edge of stability in the interest of sensitivity, you’d better stay with the same type, however hard it may be to find. But that isn’t the only puzzle to solve to get this instrument counting again.

Look at how those clever Hewlett-Packard engineers built a binary-coded-decimal (BCD) to Nixie tube decoder/driver with no logic circuitry using neon bulbs and photoresistors!