Quoted from Jimmyhonda:
Sorry, newbie question, how do you “check” a switch matrix? And where is it located? Thank you.
You PM'd me a question about this and the LM339. Here is my response there.
Since this is a bit confusing, I don't think you can check an integrated circuit on a circuit board, number LM339. Which is typically used for switches.
Now in the manual. There is a page with the switch matrix on it. You can download the manual from https://www.ipdb.org/search.pl if you don't have it.
The switches in your machine are wired in a way to reduce wiring called the switch matrix. Columns and rows, usually 8 switches by 8 switches, making a square.
Now on a row or column, if a wire breaks off of a switch, a switch or switches associated with it, won't work past the break.
These videos may help :
LTG : )