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A look inside the original countdown clock!

A project log for NASA Apollo / Saturn V Clock / Countdown Display

Vintage 7 Segment displays salvaged from Kennedy Space Center control room, dating from the time of the Apollo missions

Patrick HickeyPatrick Hickey 07/30/2019 at 22:151 Comment

The seller from whom I bought the displays  (spacemissionmarket / eBay seller: rocket-broker  eBay Store) kindly sent me some photos of a partially disassembled “Type A” clock, showing the wiring and PCBs with 8 digits and +/- symbol. Thanks to Diana and the owner of the clock, these images are a tremendous help to the project. I had not seen these images when I built the LED test PCBs; they look remarkably similar, but of superior build quality, which you would expect from NASA! 

Complete clock assembly
Clock complete with front panel. Image courtesy of Space Mission Museum & Market
Internal wiring of clock assembly
PCBs, wiring and digit separators. Image courtesy of Space Mission Museum & Market

I cannot clearly make out bands on the carbon composition resistors, but it looks like brown/yellow/red which would make them 1400 ohms. A lot of aircraft control panels use ~ 28 Volts supply so perhaps these are current limiting resistors to operate 5 - 6 Volt lamps around 20-40mA?

Close up
Detailed view of PCBs. Image courtesy of Space Mission Museum & Market

There is seriously big loom of wires; they are colour coded Brown, Red, Orange, Yellow, Green, Blue, Violet and it appears that Violet connects to segment “a”; Blue segment “b” etc… Looking at the cross section of the cut cable bundle, I’m assuming there’s a total of 56 wires for the 8 x 7 segments and 2 for the +/- indicator.  

Details of clock housing and internal mounting.
Rear enclosure cover and mounting using spacers. Image courtesy of Space Mission Museum & Market

It appears that the common connection (cathode / anode?) is connected in parallel to all digits, as there are 2 black wires linked on each PCB. From this I would deduce that the displays are not driven by multiplexing, indeed if they are incandescent bulbs these would be too slow for strobe control.

Rear of clock showing PCB assembly.
The +/- indicator is on the right side. Image courtesy of Space Mission Museum & Market

I am hoping to find out more about the types of lamps used, and I am also curious about the colon separators, are these illuminated at all? 

Finally, I have discovered a you tube video from Fran Blanche who has 3D printed a segment display which looks almost identical to the "Type B" units, however there is no reference to their use in the NASA control room equipment. It would be interesting to check the dimensions on the drawing files and compare to the original artifacts. Instead of 3D printing a plastic backing I would probably use a white PCB to help reflect the light through the segments. 

Discussions

Ken Yap wrote 07/30/2019 at 22:41 point

I see them as Brown Red Brown which would make them 120 ohms. I don't think 5V bulbs were that common. I remember owning 12V indicator bulbs. But this is moot since you don't have to mimic the operating current, just the look.

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