I am trying to finish up bringing a Black Knight back into fighting shape. I am running into some troubling switch matrix issues and am out of self solutions, so am reaching out to the community for help.
Switch Matrix column drive J2J-5 and J2J-6 are causing me headaches. Both columns were not registering during the switch test, but everything else was working fine. When I measured the resistors, I found that the resistors for pin 5 and 6 were measuring a few ohms instead of the 4.6K they were rated. I replaced them, but testing them out of circuit found that the original resistors were fine. The new resistors were measuring only a few ohms as well.
Next I did a diode test on IC18 and found that a few of the pins were not reading properly. I replaced the 7406 on IC18 and then measured all the resistors on every pin. They now all read 4.7k ohms!
When I put the board back in the game, now every time the column is triggered, every switch on that row is triggered. When I measure the column drives, all the colums read 4.5volts except for pins 5 and 6 which are reading only a few millivolts. I am not sure where these pins are getting ground from and I am not sure where to test next. I removed the connectors and jumpered the pins and confirm the issue is on the board, I am just stuck.
Any help is appreciated.