ICOM IC-720A Rotary Switch Replacement

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Hi, I’m IW9HGS, Gabriele “Asbesto” Zaverio, and this is my ICOM IC-720A Rotary Switch Mod!

I made this mod starting from the F5IKI Rotary Switch Mod. I decided to simplify the mod itself, wiping out the PNP section and the binary count logic, and using just a simple 4017 CMOS counter instead of TTL logic. It works very well! The original mods and other stuff are here, saved in “Various mods” section.

A brief description: the counting pulse from J1-7 is “filtered” using 74HC00 NAND doors; it goes directly into the 4017, that start counting. The 4017 output goes to the 2222 transistor that drive the relay, and also goes to the diode matrix, to form the L1,L2,L4,L8 binary code needed for the 720’s microprocessor. The 4017 is powered directly to 7.5V, so there’s no need to convert this level to obtain the binary code so the microprocessors can use it directly (in fact, the 720 uP need about 7V logic levels. I don’t know why)

I used the F5IKI boards he sent to me, so I haven’t developed the PCB boards for this mod. Can someone build them, so I can add to this mod? Anyway, as a starting point, you will find the F5IKI pcb in the “Various mods” section.

Many thanks to: F5IKI, that sent me the logic boards to do this mod and some components; IZ2LSL for helping and supporting me and for the 1N4004 MELF diodes; WB0GAZ, ON4AZD and udomahr@t-online.de for documentation I found on the Internet, IRC Channel #retrocomputing on Azzurra.net for support and happiness, the Ari Fidenza Forum for support and help, and Acme (olocolors.org) for the LIFETIME TOOLS that really helped me a lot! 😉

You can find all the mod HERE.