Below are the schematics for the 7490 TTL Chip Clock. After 10 years of not doing anything with the clock, I had to spend some time reverse engineering it a bit to understand it’s design. The original hand drawn schematics ( not published on this site ) were a bit off target as I deviated from them. These new ones that are published are accurate. They were hand drawn then scanned and imported into Gimp for cleanup.

Clicking on the images on this page will bring up a full sized version.

Gimp Drawing Cleanup

In Gimp basically the process is to use threshold to clean the “noise” out from the scan. Then cut more noise out using the cut tool and eraser. Then go over the lines with the pencil tool where they are faint. Finally, add text in. Yes,  there are some CAD tools that could do this easier but, since I don’t do this kind of thing to often,  Gimp works out nicely as I have had a lot of hours on it and can work quickly in it.

Slave Oscillator and Prescaler, dividing from 10MHz to 1kHz on the right hand board

This schematic shows all connections, including power and ground and is laid out to represent the component placement on the board. Looking at the 7490’s the pattern of the circuit is just like the example from How it Works, dividing frequency by 10 every time. The difference between the clock that I built and theirs is that I used a 10MHz source and they used 60Hz line frequency. Therefore I needed to use more divider stages. There is a shortcut, the 74390 chip. It is a BCD dual divider/counter. It could eliminate half of the stages as it divides by 100. I happened to have quite a few 7490s at the time that I built this clock and went with that approach rather than order 74390s. I would recommend using the 74390s though, less stages, less soldering, less power consumption being the benefits.

1kHz Divided down to Hours, Minutes and Seconds on the left hand side board of the clock

This schematic DOES NOT show all connections, power and ground are omitted, also no connects on the chips are omitted, plus the 7447 chips are there to show there location but the connections are not shown. Refer to the page that covers the How it Works information on how to connect 7490s to 7447s. It is laid out to represent the component placement on the board. However the board is inverted on the clock and the digit LEDs sit on the bottom of the board but inverted in order to read them.