Burrough's B-7971 Nixie Tube Clock
These are front and back pictures of 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 2½ 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 (page 1, page 2 and page 3) that had been provided by the surplus parts vendor along 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 New Old Stock Burroughs and Ultronics branded B-7971 tubes with original Burroughs Data Sheet as spares that I purchased back in the early 1980’s for around $5 each (versus $4 for used tubes pulled from Lectrascan cards – the extra $1 seemed worth it for new tubes), one used Burroughs branded B-7971, five spare MM5311 clock chips and one complete Lectrascan card still equipped with its SK137 sockets. However, after almost 30 years of continuous operation (the clock took a break when I was away at college), the original tubes are still working fine and show no signs of needing replacement so the spare parts remain in storage. PLEASE NOTE: From time to time I receive requests to purchase my spare tubes. They are spares and they are not for sale. With B-7971 prices now USD$175 and higher each for used tubes, when they are even available for sale, having to replace them at a future date would be cost prohibitive and they cost me nothing to store. If at some future date I do decide to sell them, I will put them on eBay but I suspect that will be up to my heirs to decide.
To learn more about Nixie tube clocks, visit Mike Harrison's Nixie Tube Clock Page at http://www.electricstuff.co.uk/nixclock.html. Be sure to visit the Nixie Clock Gallery at http://www.electricstuff.co.uk/nixiegallery.html where this clock is also listed.
The IEEE published an article on Nixie Tubes in their June 2002 issue of Spectrum. The article is available at http://spectrum.ieee.org/consumer-electronics/gadgets/new-life-for-nixies.
Last modified April 7, 2020