Hp-70 incorrect key registering

[aj04062]

I am trying to fix a HP-70 for a fellow enthusiast. The symptoms are:
Several keys such as /, O and . Do not register correctly. For example the decimal key is interpreted as the change sign key.

To rule out corrosion or something else on the keyboard, I paired up a 45 motherboard to the keyboard. With this combo it works as it should.

So, this leads me to conclude it is the 70 motherboard. But what chip has failed? A common one or one unique to the 70?