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There are errors in the circuit, the one who drew this schematic couldn't
count to eight. There are only eight thyristors but there seem to be ten
outputs who are connected to these thyristors. The ROM is an antique
8223, 82S23, 82S123 or similar, see the datasheet.
It is not adviseable to search for one of these old devices because they
will be difficult to find and also very expensive. It's logical to use
a more common and cheaper EPROM instead and change the connection for
inputs and outputs to suit the new device.
You can use a regular 2764 or similar to replace it and connect the unused
address inputs to GND as they did in the original circuit.
You could even connect the unused address inputs to a switch to select from
a number of different sequences for your lightchaser.
This is only one example to replace the original ROM, there are a lot of other
(E)PROM's who can replace it, so use what you can get (cheap).
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