MICROFORUM

In the wanted / sales section I have seen an offer CMOS vintage digital clock circuits from National Semi. All the circuits have 7-segment output, multiplexed (MM5311) or non-multiplexed (MM5316). I would like know whether is possible to use the circuit to drive nixie tubes.

The solution is easy. You need to convert 7-segment-code either to BCD or to decade-code. If you convert the 7-segment code to decade code, you drive nixie directly. If you convert 7-segment code to BCD code, you need change voltage level from 12V to TTL with a resistor divider and use 74141 nixie driver.
I suggest you to use MM5316 with non-multiplexed output. If you need 6-digits, add coup of gates CMOS 4000 to get 10-th second and seconds.
Another, probably more simply way, is to use MM5311. this circuit has BCD multiplexed 6-digits output. In this case you need only six 4-bit latches to save data from multiplexed BCD output. Outputs from the 4-bit latches drive six non-multiplexed nixie drivers.
You don't need a MCU. Just CMOS 4000 gates or latches.