Nixie Tube Clock

Posted on Feb 5, 2014

A Burrough`s B-7971 Nixie Tube clock that I built back in 1979 when I was a senior in High School. The digits are 2 ½ inches high which allowed me to read the clock from anywhere in my bedroom without needing to wear my glasses. The clock circuit is based on a schematic that had been provided by the surplus parts vendor along

Nixie Tube Clock
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with three Ultronic Lectrascan display cards which were part of an Ultronic Systems Lectrascan. The six B-7971 tubes and SK137 sockets (NSN 5935-00-847-8641) used with the clock came from the Lectrascan cards. The Lectrascan cards also had high voltage switching transistors (2N1302 and SA480) that were used in the construction of the clock. The clock uses a National Semiconductor MM5311 clock chip for time keeping and uses the 60Hz line frequency as its time base which is reasonably accurate. Unfortunately, the original circuit was a very poor transformerless design and blew several capacitors, resistors and Zener diodes during its initial test run (it was a spectacular explosion!). I wound up making my own design changes, including adding a transformer, and it has worked fine ever since it officially went "live" on February 11, 1980. Over the years, I have replaced some parts with newer smaller components but it still uses the original clock chip and high voltage transistors that came from the Lectrascan cards. Back in 1979, I was a cash strapped student and did not have the money to purchase the best parts except for a rather hard to find 160V/10W Zener diode, used for regulating the high voltage, that I obtained surplus for a very low price. I designed the PC board and wooden stand myself. I currently have seven brand new spare B-7971 tubes that I purchased back in the early 1980`s as well as five spare MM5311 clock...

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