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Aaaagh! I can’t believe I missed this!

June 1, 2014

Two of the gates on the 74hc132 are reversed from the 4093.  I didn’t catch iit right off because the oscillator gate is not reversed.  The gates that are reversed deal with the /INP4 signal which is critical to loading the 1802.  Thanks to David Williams on the Cosmac Elf message board for spotting it.

4093 Pinout

74hc132

So, one of the traces I need to change is hidden below the chip which is not socketed:(. Pin 4 of the 4093 is an output and connects to pin 9 on top of the board. On the 132 it’s an input and needs to go to pin 3. So either I clip and de-solder the chip or… maybe I can drill out the one pad from the bottom & then top-wire it to pin 3. I don’t think I’ll wreck anything trying this, the rest of the changes would be much the same if I did pull the chip completely. The other traces I have to cut are accessible either between chips or on the bottom of the board.

The picture shows the 3 traces on top that have to be cut as red dots. the 4th trace from pin 3 to 6 is on the bottom and easy. I’m going to think about it for 1/2 hour before I heat up the iron.
14-05-31 chipclip

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