Neon displays in HP5275A timer

[artag]

I've started restoring an HP5257A interval timer from 1965. In it's in pretty good condition, though it doesn't actually work (yet). So far I've found a couple of big o/c caps in the power supply.

I'm intruiged by the numeric displays, though. They're the type with 10 neons, each illuminating a window from 0 to 9. Nice and old but not insane. What's amazing is the conversion from the 4-bit binary counters (transistor flipflops) to 1-of-10 neons.

This consists of 8 neons (two per flipflop, one on Q and one on /Q) and 18 LDRs. The LDRs are arranged near the neons in a particular arrangement such that certain combinations are illuminated by specific neons. And if 3 LDRs are illuminated, one of the front panel neons is lit in turn.

The binary count is slightly peculiar, running 0,1,2,3,6,7,c,d,e,f - this doesn't really matter as long as the decoding and carry work accordingly, and means that digit 0 is illuminated when bits 0, 1 and 3 are off, digit 1 when bit 0 is on, bits 1 and 3 off, digit 4 when bit 2 is on, bits 0 and 3 off, etc.

I'm not sure what I expected - perhaps a 10 point ring counter, illuminating the front panel neons like a decatron. Or a diode array. But a product-of-sums combinatorial logic array built from lamps and light dependent resistors ? Wow.