Apple II Plus Keyboard Not Working Help

Hello, newbie to the forums. Former Apple II user from back in the 1980s. I recently acquired a couple of Apple II pluses. One works fine... on the other, the keyboard (non-encoder card type) does not work at all. The Power Light comes on but no keys work when pressed. I removed the Cable and MM5740 Chip and Cleaned the contacts and sockets but that didn't do squat...Another thing worth noting is that the previous owner installed a Keytronics KB-200 keyboard along with an interface board. On the keytronics keyboard, less than 1/2 the keys would work when pressed. The interface board connects to the MB keyboard socket and has a seperate connection for the Apple II plus keyboard. I've taken that Keytronics KB and its Interface Board out of the equation to directly connect the II plus keyboard to the MB but that did not resolve the issue. I've read thru the various keyboard issue forums on here but I found that with those keyboard some keys worked but others didn't which is not the case for mine. I don't want to make any assumptions or try anything else to avoid damaging the MB. Any help would be greatly apprectiate. Thank you.