Shadowbox for 7490 Clock

7490 TTL Digital Clock Housing

After a few months of testing ( April – June 2017 ) the 7490 TTL decade counter chip based clock out with the newly added OCXO it was ready to be mounted in a housing.

7490 TTL Clock with OCXO under test
7490 TTL Clock with OCXO under test

Monitoring it against an accurate timebase ( Linux PC with NTP ) while piled up loosely on a small table in the office, it was running good and steady. No flaky counts, no flaky digits that were not lighting or staying lit when they should not be.




Consolidating Power

After the photo was taken above,  the power supplies for the clock were consolidated. Under testing it was powered by 2 wall supplies, 5V for the TTL chips and 12V for the OCXO. For the final version I used a 7805 regulator on the right hand side board to down regulate 12V from a Radio Shack 12V/2.3A power supply to 5V for the TTL chips. I got lucky and spotted the power supply during the last few days that Radio Shack was still in business in April 2017.

Wood Housing

Not having a good set of tools on hand to build a metal housing, a good option was a  wooden one. Micheal’s craft store had both the base that the boards were mounted to and the shadowbox. Both were unfinished and reasonably priced, I believe $5 for the board and $12 for the shadowbox in 2017.

7490 TTL Clock Circuit Board Base
7490 TTL Clock Circuit Board Base
7490 TTL Clock Shadowbox, cover removed
7490 TTL Clock Shadowbox, cover removed. A bit larger than is needed but, it works fine. The glass cover was removed in preparation for sanding and the application of linseed oil.

The base was drilled out to recess fit 10mm standoffs. Holes were drilled for the M3 screws and then countersunk with a larger bit to accommodate the 10mm standoffs. Once again I made out good at the Radio Shack closeout and got a bunch of standoffs.


7490 TTL Clock Mounted on Base
7490 TTL Clock Mounted on Base

The clock mounted on the base in a state of readiness to drop into the shadowbox as a subassembly. This trick is a bit that I learned from working in manufacturing in the past. To make things efficient and serviceable it was common to mount boards on sub assemblies and then mount the subassembly into a piece of machinery.




7490 TTL Clock Mounted in Shadowbox
7490 TTL Clock Mounted in Shadowbox


Finally the subassembly gets dropped into the shadowbox and is held in place by four small wood screws that are on the back.

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7490 TTL Clock Final Housing
7490 TTL Clock, Final Housing



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